[reactos.git] / reactos / dll / 3rdparty / libpng / docs / libpng-manual.txt
1 libpng-manual.txt - A description on how to use and modify libpng
3 libpng version 1.6.18 - July 23, 2015
4 Updated and distributed by Glenn Randers-Pehrson
5 <glennrp at users.sourceforge.net>
6 Copyright (c) 1998-2015 Glenn Randers-Pehrson
8 This document is released under the libpng license.
9 For conditions of distribution and use, see the disclaimer
10 and license in png.h
12 Based on:
14 libpng versions 0.97, January 1998, through 1.6.18 - July 23, 2015
15 Updated and distributed by Glenn Randers-Pehrson
16 Copyright (c) 1998-2015 Glenn Randers-Pehrson
18 libpng 1.0 beta 6 - version 0.96 - May 28, 1997
19 Updated and distributed by Andreas Dilger
20 Copyright (c) 1996, 1997 Andreas Dilger
22 libpng 1.0 beta 2 - version 0.88 - January 26, 1996
23 For conditions of distribution and use, see copyright
24 notice in png.h. Copyright (c) 1995, 1996 Guy Eric
25 Schalnat, Group 42, Inc.
27 Updated/rewritten per request in the libpng FAQ
28 Copyright (c) 1995, 1996 Frank J. T. Wojcik
29 December 18, 1995 & January 20, 1996
33 I. Introduction
34 II. Structures
35 III. Reading
36 IV. Writing
37 V. Simplified API
38 VI. Modifying/Customizing libpng
39 VII. MNG support
40 VIII. Changes to Libpng from version 0.88
41 IX. Changes to Libpng from version 1.0.x to 1.2.x
42 X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
43 XI. Changes to Libpng from version 1.4.x to 1.5.x
44 XII. Changes to Libpng from version 1.5.x to 1.6.x
45 XIII. Detecting libpng
46 XIV. Source code repository
47 XV. Coding style
48 XVI. Y2K Compliance in libpng
50 I. Introduction
52 This file describes how to use and modify the PNG reference library
53 (known as libpng) for your own use. In addition to this
54 file, example.c is a good starting point for using the library, as
55 it is heavily commented and should include everything most people
56 will need. We assume that libpng is already installed; see the
57 INSTALL file for instructions on how to configure and install libpng.
59 For examples of libpng usage, see the files "example.c", "pngtest.c",
60 and the files in the "contrib" directory, all of which are included in
61 the libpng distribution.
63 Libpng was written as a companion to the PNG specification, as a way
64 of reducing the amount of time and effort it takes to support the PNG
65 file format in application programs.
67 The PNG specification (second edition), November 2003, is available as
68 a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
69 <http://www.w3.org/TR/2003/REC-PNG-20031110/
70 The W3C and ISO documents have identical technical content.
72 The PNG-1.2 specification is available at
73 <http://www.libpng.org/pub/png/documents/>. It is technically equivalent
74 to the PNG specification (second edition) but has some additional material.
76 The PNG-1.0 specification is available
77 as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
78 W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
80 Some additional chunks are described in the special-purpose public chunks
81 documents at <http://www.libpng.org/pub/png/documents/>.
83 Other information
84 about PNG, and the latest version of libpng, can be found at the PNG home
85 page, <http://www.libpng.org/pub/png/>.
87 Most users will not have to modify the library significantly; advanced
88 users may want to modify it more. All attempts were made to make it as
89 complete as possible, while keeping the code easy to understand.
90 Currently, this library only supports C. Support for other languages
91 is being considered.
93 Libpng has been designed to handle multiple sessions at one time,
94 to be easily modifiable, to be portable to the vast majority of
95 machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
96 to use. The ultimate goal of libpng is to promote the acceptance of
97 the PNG file format in whatever way possible. While there is still
98 work to be done (see the TODO file), libpng should cover the
99 majority of the needs of its users.
101 Libpng uses zlib for its compression and decompression of PNG files.
102 Further information about zlib, and the latest version of zlib, can
103 be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
104 The zlib compression utility is a general purpose utility that is
105 useful for more than PNG files, and can be used without libpng.
106 See the documentation delivered with zlib for more details.
107 You can usually find the source files for the zlib utility wherever you
108 find the libpng source files.
110 Libpng is thread safe, provided the threads are using different
111 instances of the structures. Each thread should have its own
112 png_struct and png_info instances, and thus its own image.
113 Libpng does not protect itself against two threads using the
114 same instance of a structure.
116 II. Structures
118 There are two main structures that are important to libpng, png_struct
119 and png_info. Both are internal structures that are no longer exposed
120 in the libpng interface (as of libpng 1.5.0).
122 The png_info structure is designed to provide information about the
123 PNG file. At one time, the fields of png_info were intended to be
124 directly accessible to the user. However, this tended to cause problems
125 with applications using dynamically loaded libraries, and as a result
126 a set of interface functions for png_info (the png_get_*() and png_set_*()
127 functions) was developed, and direct access to the png_info fields was
128 deprecated..
130 The png_struct structure is the object used by the library to decode a
131 single image. As of 1.5.0 this structure is also not exposed.
133 Almost all libpng APIs require a pointer to a png_struct as the first argument.
134 Many (in particular the png_set and png_get APIs) also require a pointer
135 to png_info as the second argument. Some application visible macros
136 defined in png.h designed for basic data access (reading and writing
137 integers in the PNG format) don't take a png_info pointer, but it's almost
138 always safe to assume that a (png_struct*) has to be passed to call an API
139 function.
141 You can have more than one png_info structure associated with an image,
142 as illustrated in pngtest.c, one for information valid prior to the
143 IDAT chunks and another (called "end_info" below) for things after them.
145 The png.h header file is an invaluable reference for programming with libpng.
146 And while I'm on the topic, make sure you include the libpng header file:
148 #include <png.h>
150 and also (as of libpng-1.5.0) the zlib header file, if you need it:
152 #include <zlib.h>
154 Types
156 The png.h header file defines a number of integral types used by the
157 APIs. Most of these are fairly obvious; for example types corresponding
158 to integers of particular sizes and types for passing color values.
160 One exception is how non-integral numbers are handled. For application
161 convenience most APIs that take such numbers have C (double) arguments;
162 however, internally PNG, and libpng, use 32 bit signed integers and encode
163 the value by multiplying by 100,000. As of libpng 1.5.0 a convenience
164 macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
165 which is simply (png_int_32).
167 All APIs that take (double) arguments also have a matching API that
168 takes the corresponding fixed point integer arguments. The fixed point
169 API has the same name as the floating point one with "_fixed" appended.
170 The actual range of values permitted in the APIs is frequently less than
171 the full range of (png_fixed_point) (-21474 to +21474). When APIs require
172 a non-negative argument the type is recorded as png_uint_32 above. Consult
173 the header file and the text below for more information.
175 Special care must be take with sCAL chunk handling because the chunk itself
176 uses non-integral values encoded as strings containing decimal floating point
177 numbers. See the comments in the header file.
179 Configuration
181 The main header file function declarations are frequently protected by C
182 preprocessing directives of the form:
184 #ifdef PNG_feature_SUPPORTED
185 declare-function
186 #endif
187 ...
188 #ifdef PNG_feature_SUPPORTED
189 use-function
190 #endif
192 The library can be built without support for these APIs, although a
193 standard build will have all implemented APIs. Application programs
194 should check the feature macros before using an API for maximum
195 portability. From libpng 1.5.0 the feature macros set during the build
196 of libpng are recorded in the header file "pnglibconf.h" and this file
197 is always included by png.h.
199 If you don't need to change the library configuration from the default, skip to
200 the next section ("Reading").
202 Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
203 of the build project files in the 'projects' directory simply copy
204 scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build
205 systems do not permit easy auto-configuration of the library - they only
206 support the default configuration.
208 The easiest way to make minor changes to the libpng configuration when
209 auto-configuration is supported is to add definitions to the command line
210 using (typically) CPPFLAGS. For example:
214 will change the internal libpng math implementation for gamma correction and
215 other arithmetic calculations to fixed point, avoiding the need for fast
216 floating point support. The result can be seen in the generated pnglibconf.h -
217 make sure it contains the changed feature macro setting.
219 If you need to make more extensive configuration changes - more than one or two
220 feature macro settings - you can either add -DPNG_USER_CONFIG to the build
221 command line and put a list of feature macro settings in pngusr.h or you can set
222 DFA_XTRA (a makefile variable) to a file containing the same information in the
223 form of 'option' settings.
225 A. Changing pnglibconf.h
227 A variety of methods exist to build libpng. Not all of these support
228 reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be
229 rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
231 Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
232 pnglibconf.h and changing the lines defining the supported features, paying
233 very close attention to the 'option' information in scripts/pnglibconf.dfa
234 that describes those features and their requirements. This is easy to get
235 wrong.
237 B. Configuration using DFA_XTRA
239 Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
240 variant such as 'nawk' or 'gawk', is available. The configure build will
241 automatically find an appropriate awk and build pnglibconf.h.
242 The scripts/pnglibconf.mak file contains a set of make rules for doing the
243 same thing if configure is not used, and many of the makefiles in the scripts
244 directory use this approach.
246 When rebuilding simply write a new file containing changed options and set
247 DFA_XTRA to the name of this file. This causes the build to append the new file
248 to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines
249 of the following forms:
251 everything = off
253 This turns all optional features off. Include it at the start of pngusr.dfa to
254 make it easier to build a minimal configuration. You will need to turn at least
255 some features on afterward to enable either reading or writing code, or both.
257 option feature on
258 option feature off
260 Enable or disable a single feature. This will automatically enable other
261 features required by a feature that is turned on or disable other features that
262 require a feature which is turned off. Conflicting settings will cause an error
263 message to be emitted by awk.
265 setting feature default value
267 Changes the default value of setting 'feature' to 'value'. There are a small
268 number of settings listed at the top of pnglibconf.h, they are documented in the
269 source code. Most of these values have performance implications for the library
270 but most of them have no visible effect on the API. Some can also be overridden
271 from the API.
273 This method of building a customized pnglibconf.h is illustrated in
274 contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and
275 pngusr.dfa in these directories.
277 C. Configuration using PNG_USER_CONFIG
279 If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built,
280 the file pngusr.h will automatically be included before the options in
281 scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only
282 macro definitions turning features on or off or setting settings.
284 Apart from the global setting "everything = off" all the options listed above
285 can be set using macros in pngusr.h:
287 #define PNG_feature_SUPPORTED
289 is equivalent to:
291 option feature on
293 #define PNG_NO_feature
295 is equivalent to:
297 option feature off
299 #define PNG_feature value
301 is equivalent to:
303 setting feature default value
305 Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
306 pngusr file you supply override the contents of scripts/pnglibconf.dfa
308 If confusing or incomprehensible behavior results it is possible to
309 examine the intermediate file pnglibconf.dfn to find the full set of
310 dependency information for each setting and option. Simply locate the
311 feature in the file and read the C comments that precede it.
313 This method is also illustrated in the contrib/pngminim/* makefiles and
314 pngusr.h.
316 III. Reading
318 We'll now walk you through the possible functions to call when reading
319 in a PNG file sequentially, briefly explaining the syntax and purpose
320 of each one. See example.c and png.h for more detail. While
321 progressive reading is covered in the next section, you will still
322 need some of the functions discussed in this section to read a PNG
323 file.
325 Setup
327 You will want to do the I/O initialization(*) before you get into libpng,
328 so if it doesn't work, you don't have much to undo. Of course, you
329 will also want to insure that you are, in fact, dealing with a PNG
330 file. Libpng provides a simple check to see if a file is a PNG file.
331 To use it, pass in the first 1 to 8 bytes of the file to the function
332 png_sig_cmp(), and it will return 0 (false) if the bytes match the
333 corresponding bytes of the PNG signature, or nonzero (true) otherwise.
334 Of course, the more bytes you pass in, the greater the accuracy of the
335 prediction.
337 If you are intending to keep the file pointer open for use in libpng,
338 you must ensure you don't read more than 8 bytes from the beginning
339 of the file, and you also have to make a call to png_set_sig_bytes()
340 with the number of bytes you read from the beginning. Libpng will
341 then only check the bytes (if any) that your program didn't read.
343 (*): If you are not using the standard I/O functions, you will need
344 to replace them with custom functions. See the discussion under
345 Customizing libpng.
347 FILE *fp = fopen(file_name, "rb");
348 if (!fp)
349 {
350 return (ERROR);
351 }
353 if (fread(header, 1, number, fp) != number)
354 {
355 return (ERROR);
356 }
358 is_png = !png_sig_cmp(header, 0, number);
359 if (!is_png)
360 {
361 return (NOT_PNG);
362 }
364 Next, png_struct and png_info need to be allocated and initialized. In
365 order to ensure that the size of these structures is correct even with a
366 dynamically linked libpng, there are functions to initialize and
367 allocate the structures. We also pass the library version, optional
368 pointers to error handling functions, and a pointer to a data struct for
369 use by the error functions, if necessary (the pointer and functions can
370 be NULL if the default error handlers are to be used). See the section
371 on Changes to Libpng below regarding the old initialization functions.
372 The structure allocation functions quietly return NULL if they fail to
373 create the structure, so your application should check for that.
375 png_structp png_ptr = png_create_read_struct
376 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
377 user_error_fn, user_warning_fn);
379 if (!png_ptr)
380 return (ERROR);
382 png_infop info_ptr = png_create_info_struct(png_ptr);
384 if (!info_ptr)
385 {
386 png_destroy_read_struct(&png_ptr,
387 (png_infopp)NULL, (png_infopp)NULL);
388 return (ERROR);
389 }
391 If you want to use your own memory allocation routines,
392 use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
393 png_create_read_struct_2() instead of png_create_read_struct():
395 png_structp png_ptr = png_create_read_struct_2
396 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
397 user_error_fn, user_warning_fn, (png_voidp)
398 user_mem_ptr, user_malloc_fn, user_free_fn);
400 The error handling routines passed to png_create_read_struct()
401 and the memory alloc/free routines passed to png_create_struct_2()
402 are only necessary if you are not using the libpng supplied error
403 handling and memory alloc/free functions.
405 When libpng encounters an error, it expects to longjmp back
406 to your routine. Therefore, you will need to call setjmp and pass
407 your png_jmpbuf(png_ptr). If you read the file from different
408 routines, you will need to update the longjmp buffer every time you enter
409 a new routine that will call a png_*() function.
411 See your documentation of setjmp/longjmp for your compiler for more
412 information on setjmp/longjmp. See the discussion on libpng error
413 handling in the Customizing Libpng section below for more information
414 on the libpng error handling. If an error occurs, and libpng longjmp's
415 back to your setjmp, you will want to call png_destroy_read_struct() to
416 free any memory.
418 if (setjmp(png_jmpbuf(png_ptr)))
419 {
420 png_destroy_read_struct(&png_ptr, &info_ptr,
421 &end_info);
422 fclose(fp);
423 return (ERROR);
424 }
426 Pass (png_infopp)NULL instead of &end_info if you didn't create
427 an end_info structure.
429 If you would rather avoid the complexity of setjmp/longjmp issues,
430 you can compile libpng with PNG_NO_SETJMP, in which case
431 errors will result in a call to PNG_ABORT() which defaults to abort().
433 You can #define PNG_ABORT() to a function that does something
434 more useful than abort(), as long as your function does not
435 return.
437 Now you need to set up the input code. The default for libpng is to
438 use the C function fread(). If you use this, you will need to pass a
439 valid FILE * in the function png_init_io(). Be sure that the file is
440 opened in binary mode. If you wish to handle reading data in another
441 way, you need not call the png_init_io() function, but you must then
442 implement the libpng I/O methods discussed in the Customizing Libpng
443 section below.
445 png_init_io(png_ptr, fp);
447 If you had previously opened the file and read any of the signature from
448 the beginning in order to see if this was a PNG file, you need to let
449 libpng know that there are some bytes missing from the start of the file.
451 png_set_sig_bytes(png_ptr, number);
453 You can change the zlib compression buffer size to be used while
454 reading compressed data with
456 png_set_compression_buffer_size(png_ptr, buffer_size);
458 where the default size is 8192 bytes. Note that the buffer size
459 is changed immediately and the buffer is reallocated immediately,
460 instead of setting a flag to be acted upon later.
462 If you want CRC errors to be handled in a different manner than
463 the default, use
465 png_set_crc_action(png_ptr, crit_action, ancil_action);
467 The values for png_set_crc_action() say how libpng is to handle CRC errors in
468 ancillary and critical chunks, and whether to use the data contained
469 therein. Note that it is impossible to "discard" data in a critical
470 chunk.
472 Choices for (int) crit_action are
473 PNG_CRC_DEFAULT 0 error/quit
474 PNG_CRC_ERROR_QUIT 1 error/quit
475 PNG_CRC_WARN_USE 3 warn/use data
476 PNG_CRC_QUIET_USE 4 quiet/use data
477 PNG_CRC_NO_CHANGE 5 use the current value
479 Choices for (int) ancil_action are
480 PNG_CRC_DEFAULT 0 error/quit
481 PNG_CRC_ERROR_QUIT 1 error/quit
482 PNG_CRC_WARN_DISCARD 2 warn/discard data
483 PNG_CRC_WARN_USE 3 warn/use data
484 PNG_CRC_QUIET_USE 4 quiet/use data
485 PNG_CRC_NO_CHANGE 5 use the current value
487 Setting up callback code
489 You can set up a callback function to handle any unknown chunks in the
490 input stream. You must supply the function
492 read_chunk_callback(png_structp png_ptr,
493 png_unknown_chunkp chunk);
494 {
495 /* The unknown chunk structure contains your
496 chunk data, along with similar data for any other
497 unknown chunks: */
499 png_byte name[5];
500 png_byte *data;
501 png_size_t size;
503 /* Note that libpng has already taken care of
504 the CRC handling */
506 /* put your code here. Search for your chunk in the
507 unknown chunk structure, process it, and return one
508 of the following: */
510 return (-n); /* chunk had an error */
511 return (0); /* did not recognize */
512 return (n); /* success */
513 }
515 (You can give your function another name that you like instead of
516 "read_chunk_callback")
518 To inform libpng about your function, use
520 png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
521 read_chunk_callback);
523 This names not only the callback function, but also a user pointer that
524 you can retrieve with
526 png_get_user_chunk_ptr(png_ptr);
528 If you call the png_set_read_user_chunk_fn() function, then all unknown
529 chunks which the callback does not handle will be saved when read. You can
530 cause them to be discarded by returning '1' ("handled") instead of '0'. This
531 behavior will change in libpng 1.7 and the default handling set by the
532 png_set_keep_unknown_chunks() function, described below, will be used when the
533 callback returns 0. If you want the existing behavior you should set the global
534 default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
535 versions of libpng and with 1.7. Libpng 1.6 issues a warning if you keep the
536 default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0.
538 At this point, you can set up a callback function that will be
539 called after each row has been read, which you can use to control
540 a progress meter or the like. It's demonstrated in pngtest.c.
541 You must supply a function
543 void read_row_callback(png_structp png_ptr,
544 png_uint_32 row, int pass);
545 {
546 /* put your code here */
547 }
549 (You can give it another name that you like instead of "read_row_callback")
551 To inform libpng about your function, use
553 png_set_read_status_fn(png_ptr, read_row_callback);
555 When this function is called the row has already been completely processed and
556 the 'row' and 'pass' refer to the next row to be handled. For the
557 non-interlaced case the row that was just handled is simply one less than the
558 passed in row number, and pass will always be 0. For the interlaced case the
559 same applies unless the row value is 0, in which case the row just handled was
560 the last one from one of the preceding passes. Because interlacing may skip a
561 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
562 need to know what the last pass is record (row,pass) from the callback and use
563 the last recorded value each time.
565 As with the user transform you can find the output row using the
568 Unknown-chunk handling
570 Now you get to set the way the library processes unknown chunks in the
571 input PNG stream. Both known and unknown chunks will be read. Normal
572 behavior is that known chunks will be parsed into information in
573 various info_ptr members while unknown chunks will be discarded. This
574 behavior can be wasteful if your application will never use some known
575 chunk types. To change this, you can call:
577 png_set_keep_unknown_chunks(png_ptr, keep,
578 chunk_list, num_chunks);
580 keep - 0: default unknown chunk handling
581 1: ignore; do not keep
582 2: keep only if safe-to-copy
583 3: keep even if unsafe-to-copy
585 You can use these definitions:
591 chunk_list - list of chunks affected (a byte string,
592 five bytes per chunk, NULL or '\0' if
593 num_chunks is positive; ignored if
594 numchunks <= 0).
596 num_chunks - number of chunks affected; if 0, all
597 unknown chunks are affected. If positive,
598 only the chunks in the list are affected,
599 and if negative all unknown chunks and
600 all known chunks except for the IHDR,
601 PLTE, tRNS, IDAT, and IEND chunks are
602 affected.
604 Unknown chunks declared in this way will be saved as raw data onto a
605 list of png_unknown_chunk structures. If a chunk that is normally
606 known to libpng is named in the list, it will be handled as unknown,
607 according to the "keep" directive. If a chunk is named in successive
608 instances of png_set_keep_unknown_chunks(), the final instance will
609 take precedence. The IHDR and IEND chunks should not be named in
610 chunk_list; if they are, libpng will process them normally anyway.
611 If you know that your application will never make use of some particular
612 chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
614 Here is an example of the usage of png_set_keep_unknown_chunks(),
615 where the private "vpAg" chunk will later be processed by a user chunk
616 callback function:
618 png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'};
621 png_byte unused_chunks[]=
622 {
623 104, 73, 83, 84, (png_byte) '\0', /* hIST */
624 105, 84, 88, 116, (png_byte) '\0', /* iTXt */
625 112, 67, 65, 76, (png_byte) '\0', /* pCAL */
626 115, 67, 65, 76, (png_byte) '\0', /* sCAL */
627 115, 80, 76, 84, (png_byte) '\0', /* sPLT */
628 116, 73, 77, 69, (png_byte) '\0', /* tIME */
629 };
630 #endif
632 ...
635 /* ignore all unknown chunks
636 * (use global setting "2" for libpng16 and earlier):
637 */
638 png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
640 /* except for vpAg: */
641 png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
643 /* also ignore unused known chunks: */
644 png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
645 (int)(sizeof unused_chunks)/5);
646 #endif
648 User limits
650 The PNG specification allows the width and height of an image to be as
651 large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
652 Larger images will be rejected immediately with a png_error() call. If
653 you wish to change these limits, you can use
655 png_set_user_limits(png_ptr, width_max, height_max);
657 to set your own limits (libpng may reject some very wide images
658 anyway because of potential buffer overflow conditions).
660 You should put this statement after you create the PNG structure and
661 before calling png_read_info(), png_read_png(), or png_process_data().
663 When writing a PNG datastream, put this statement before calling
664 png_write_info() or png_write_png().
666 If you need to retrieve the limits that are being applied, use
668 width_max = png_get_user_width_max(png_ptr);
669 height_max = png_get_user_height_max(png_ptr);
671 The PNG specification sets no limit on the number of ancillary chunks
672 allowed in a PNG datastream. You can impose a limit on the total number
673 of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
675 png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
677 where 0x7fffffffL means unlimited. You can retrieve this limit with
679 chunk_cache_max = png_get_chunk_cache_max(png_ptr);
681 You can also set a limit on the amount of memory that a compressed chunk
682 other than IDAT can occupy, with
684 png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
686 and you can retrieve the limit with
688 chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
690 Any chunks that would cause either of these limits to be exceeded will
691 be ignored.
693 Information about your system
695 If you intend to display the PNG or to incorporate it in other image data you
696 need to tell libpng information about your display or drawing surface so that
697 libpng can convert the values in the image to match the display.
699 From libpng-1.5.4 this information can be set before reading the PNG file
700 header. In earlier versions png_set_gamma() existed but behaved incorrectly if
701 called before the PNG file header had been read and png_set_alpha_mode() did not
702 exist.
704 If you need to support versions prior to libpng-1.5.4 test the version number
705 as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
706 described in the appropriate manual page.
708 You give libpng the encoding expected by your system expressed as a 'gamma'
709 value. You can also specify a default encoding for the PNG file in
710 case the required information is missing from the file. By default libpng
711 assumes that the PNG data matches your system, to keep this default call:
713 png_set_gamma(png_ptr, screen_gamma, output_gamma);
715 or you can use the fixed point equivalent:
717 png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
718 PNG_FP_1*output_gamma);
720 If you don't know the gamma for your system it is probably 2.2 - a good
721 approximation to the IEC standard for display systems (sRGB). If images are
722 too contrasty or washed out you got the value wrong - check your system
723 documentation!
725 Many systems permit the system gamma to be changed via a lookup table in the
726 display driver, a few systems, including older Macs, change the response by
727 default. As of 1.5.4 three special values are available to handle common
728 situations:
730 PNG_DEFAULT_sRGB: Indicates that the system conforms to the
731 IEC 61966-2-1 standard. This matches almost
732 all systems.
733 PNG_GAMMA_MAC_18: Indicates that the system is an older
734 (pre Mac OS 10.6) Apple Macintosh system with
735 the default settings.
736 PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
737 that the system expects data with no gamma
738 encoding.
740 You would use the linear (unencoded) value if you need to process the pixel
741 values further because this avoids the need to decode and re-encode each
742 component value whenever arithmetic is performed. A lot of graphics software
743 uses linear values for this reason, often with higher precision component values
744 to preserve overall accuracy.
747 The output_gamma value expresses how to decode the output values, not how
748 they are encoded. The values used correspond to the normal numbers used to
749 describe the overall gamma of a computer display system; for example 2.2 for
750 an sRGB conformant system. The values are scaled by 100000 in the _fixed
751 version of the API (so 220000 for sRGB.)
753 The inverse of the value is always used to provide a default for the PNG file
754 encoding if it has no gAMA chunk and if png_set_gamma() has not been called
755 to override the PNG gamma information.
757 When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
758 opaque pixels however pixels with lower alpha values are not encoded,
759 regardless of the output gamma setting.
761 When the standard Porter Duff handling is requested with mode 1 the output
762 encoding is set to be linear and the output_gamma value is only relevant
763 as a default for input data that has no gamma information. The linear output
764 encoding will be overridden if png_set_gamma() is called - the results may be
765 highly unexpected!
767 The following numbers are derived from the sRGB standard and the research
768 behind it. sRGB is defined to be approximated by a PNG gAMA chunk value of
769 0.45455 (1/2.2) for PNG. The value implicitly includes any viewing
770 correction required to take account of any differences in the color
771 environment of the original scene and the intended display environment; the
772 value expresses how to *decode* the image for display, not how the original
773 data was *encoded*.
775 sRGB provides a peg for the PNG standard by defining a viewing environment.
776 sRGB itself, and earlier TV standards, actually use a more complex transform
777 (a linear portion then a gamma 2.4 power law) than PNG can express. (PNG is
778 limited to simple power laws.) By saying that an image for direct display on
779 an sRGB conformant system should be stored with a gAMA chunk value of 45455
780 ( and of the ISO PNG specification) the PNG specification
781 makes it possible to derive values for other display systems and
782 environments.
784 The Mac value is deduced from the sRGB based on an assumption that the actual
785 extra viewing correction used in early Mac display systems was implemented as
786 a power 1.45 lookup table.
788 Any system where a programmable lookup table is used or where the behavior of
789 the final display device characteristics can be changed requires system
790 specific code to obtain the current characteristic. However this can be
791 difficult and most PNG gamma correction only requires an approximate value.
793 By default, if png_set_alpha_mode() is not called, libpng assumes that all
794 values are unencoded, linear, values and that the output device also has a
795 linear characteristic. This is only very rarely correct - it is invariably
796 better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
797 default if you don't know what the right answer is!
799 The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS
800 10.6) which used a correction table to implement a somewhat lower gamma on an
801 otherwise sRGB system.
803 Both these values are reserved (not simple gamma values) in order to allow
804 more precise correction internally in the future.
806 NOTE: the values can be passed to either the fixed or floating
807 point APIs, but the floating point API will also accept floating point
808 values.
810 The second thing you may need to tell libpng about is how your system handles
811 alpha channel information. Some, but not all, PNG files contain an alpha
812 channel. To display these files correctly you need to compose the data onto a
813 suitable background, as described in the PNG specification.
815 Libpng only supports composing onto a single color (using png_set_background;
816 see below). Otherwise you must do the composition yourself and, in this case,
817 you may need to call png_set_alpha_mode:
819 #if PNG_LIBPNG_VER >= 10504
820 png_set_alpha_mode(png_ptr, mode, screen_gamma);
821 #else
822 png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
823 #endif
825 The screen_gamma value is the same as the argument to png_set_gamma; however,
826 how it affects the output depends on the mode. png_set_alpha_mode() sets the
827 file gamma default to 1/screen_gamma, so normally you don't need to call
828 png_set_gamma. If you need different defaults call png_set_gamma() before
829 png_set_alpha_mode() - if you call it after it will override the settings made
830 by png_set_alpha_mode().
832 The mode is as follows:
834 PNG_ALPHA_PNG: The data is encoded according to the PNG
835 specification. Red, green and blue, or gray, components are
836 gamma encoded color values and are not premultiplied by the
837 alpha value. The alpha value is a linear measure of the
838 contribution of the pixel to the corresponding final output pixel.
840 You should normally use this format if you intend to perform
841 color correction on the color values; most, maybe all, color
842 correction software has no handling for the alpha channel and,
843 anyway, the math to handle pre-multiplied component values is
844 unnecessarily complex.
846 Before you do any arithmetic on the component values you need
847 to remove the gamma encoding and multiply out the alpha
848 channel. See the PNG specification for more detail. It is
849 important to note that when an image with an alpha channel is
850 scaled, linear encoded, pre-multiplied component values must
851 be used!
853 The remaining modes assume you don't need to do any further color correction or
854 that if you do, your color correction software knows all about alpha (it
855 probably doesn't!). They 'associate' the alpha with the color information by
856 storing color channel values that have been scaled by the alpha. The
857 advantage is that the color channels can be resampled (the image can be
858 scaled) in this form. The disadvantage is that normal practice is to store
859 linear, not (gamma) encoded, values and this requires 16-bit channels for
860 still images rather than the 8-bit channels that are just about sufficient if
861 gamma encoding is used. In addition all non-transparent pixel values,
862 including completely opaque ones, must be gamma encoded to produce the final
863 image. These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes
864 described below (the latter being the two common names for associated alpha
865 color channels). Note that PNG files always contain non-associated color
866 channels; png_set_alpha_mode() with one of the modes causes the decoder to
867 convert the pixels to an associated form before returning them to your
868 application.
870 Since it is not necessary to perform arithmetic on opaque color values so
871 long as they are not to be resampled and are in the final color space it is
872 possible to optimize the handling of alpha by storing the opaque pixels in
873 the PNG format (adjusted for the output color space) while storing partially
874 opaque pixels in the standard, linear, format. The accuracy required for
875 standard alpha composition is relatively low, because the pixels are
876 isolated, therefore typically the accuracy loss in storing 8-bit linear
877 values is acceptable. (This is not true if the alpha channel is used to
878 simulate transparency over large areas - use 16 bits or the PNG mode in
879 this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is
880 treated as opaque only if the alpha value is equal to the maximum value.
882 PNG_ALPHA_STANDARD: The data libpng produces is encoded in the
883 standard way assumed by most correctly written graphics software.
884 The gamma encoding will be removed by libpng and the
885 linear component values will be pre-multiplied by the
886 alpha channel.
888 With this format the final image must be re-encoded to
889 match the display gamma before the image is displayed.
890 If your system doesn't do that, yet still seems to
891 perform arithmetic on the pixels without decoding them,
892 it is broken - check out the modes below.
894 With PNG_ALPHA_STANDARD libpng always produces linear
895 component values, whatever screen_gamma you supply. The
896 screen_gamma value is, however, used as a default for
897 the file gamma if the PNG file has no gamma information.
899 If you call png_set_gamma() after png_set_alpha_mode() you
900 will override the linear encoding. Instead the
901 pre-multiplied pixel values will be gamma encoded but
902 the alpha channel will still be linear. This may
903 actually match the requirements of some broken software,
904 but it is unlikely.
906 While linear 8-bit data is often used it has
907 insufficient precision for any image with a reasonable
908 dynamic range. To avoid problems, and if your software
909 supports it, use png_set_expand_16() to force all
910 components to 16 bits.
912 PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD
913 except that completely opaque pixels are gamma encoded according to
914 the screen_gamma value. Pixels with alpha less than 1.0
915 will still have linear components.
917 Use this format if you have control over your
918 compositing software and so don't do other arithmetic
919 (such as scaling) on the data you get from libpng. Your
920 compositing software can simply copy opaque pixels to
921 the output but still has linear values for the
922 non-opaque pixels.
924 In normal compositing, where the alpha channel encodes
925 partial pixel coverage (as opposed to broad area
926 translucency), the inaccuracies of the 8-bit
927 representation of non-opaque pixels are irrelevant.
929 You can also try this format if your software is broken;
930 it might look better.
932 PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component
933 values, including the alpha channel are gamma encoded. This is
934 broken because, in practice, no implementation that uses this choice
935 correctly undoes the encoding before handling alpha composition. Use this
936 choice only if other serious errors in the software or hardware you use
937 mandate it. In most cases of broken software or hardware the bug in the
938 final display manifests as a subtle halo around composited parts of the
939 image. You may not even perceive this as a halo; the composited part of
940 the image may simply appear separate from the background, as though it had
941 been cut out of paper and pasted on afterward.
943 If you don't have to deal with bugs in software or hardware, or if you can fix
944 them, there are three recommended ways of using png_set_alpha_mode():
946 png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
947 screen_gamma);
949 You can do color correction on the result (libpng does not currently
950 support color correction internally). When you handle the alpha channel
951 you need to undo the gamma encoding and multiply out the alpha.
953 png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
954 screen_gamma);
955 png_set_expand_16(png_ptr);
957 If you are using the high level interface, don't call png_set_expand_16();
958 instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
960 With this mode you can't do color correction, but you can do arithmetic,
961 including composition and scaling, on the data without further processing.
963 png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
964 screen_gamma);
966 You can avoid the expansion to 16-bit components with this mode, but you
967 lose the ability to scale the image or perform other linear arithmetic.
968 All you can do is compose the result onto a matching output. Since this
969 mode is libpng-specific you also need to write your own composition
970 software.
972 The following are examples of calls to png_set_alpha_mode to achieve the
973 required overall gamma correction and, where necessary, alpha
974 premultiplication.
976 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
978 This is the default libpng handling of the alpha channel - it is not
979 pre-multiplied into the color components. In addition the call states
980 that the output is for a sRGB system and causes all PNG files without gAMA
981 chunks to be assumed to be encoded using sRGB.
983 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
985 In this case the output is assumed to be something like an sRGB conformant
986 display preceeded by a power-law lookup table of power 1.45. This is how
987 early Mac systems behaved.
989 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
991 This is the classic Jim Blinn approach and will work in academic
992 environments where everything is done by the book. It has the shortcoming
993 of assuming that input PNG data with no gamma information is linear - this
994 is unlikely to be correct unless the PNG files where generated locally.
995 Most of the time the output precision will be so low as to show
996 significant banding in dark areas of the image.
998 png_set_expand_16(pp);
999 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
1001 This is a somewhat more realistic Jim Blinn inspired approach. PNG files
1002 are assumed to have the sRGB encoding if not marked with a gamma value and
1003 the output is always 16 bits per component. This permits accurate scaling
1004 and processing of the data. If you know that your input PNG files were
1005 generated locally you might need to replace PNG_DEFAULT_sRGB with the
1006 correct value for your system.
1008 png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
1010 If you just need to composite the PNG image onto an existing background
1011 and if you control the code that does this you can use the optimization
1012 setting. In this case you just copy completely opaque pixels to the
1013 output. For pixels that are not completely transparent (you just skip
1014 those) you do the composition math using png_composite or png_composite_16
1015 below then encode the resultant 8-bit or 16-bit values to match the output
1016 encoding.
1018 Other cases
1020 If neither the PNG nor the standard linear encoding work for you because
1021 of the software or hardware you use then you have a big problem. The PNG
1022 case will probably result in halos around the image. The linear encoding
1023 will probably result in a washed out, too bright, image (it's actually too
1024 contrasty.) Try the ALPHA_OPTIMIZED mode above - this will probably
1025 substantially reduce the halos. Alternatively try:
1027 png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
1029 This option will also reduce the halos, but there will be slight dark
1030 halos round the opaque parts of the image where the background is light.
1031 In the OPTIMIZED mode the halos will be light halos where the background
1032 is dark. Take your pick - the halos are unavoidable unless you can get
1033 your hardware/software fixed! (The OPTIMIZED approach is slightly
1034 faster.)
1036 When the default gamma of PNG files doesn't match the output gamma.
1037 If you have PNG files with no gamma information png_set_alpha_mode allows
1038 you to provide a default gamma, but it also sets the ouput gamma to the
1039 matching value. If you know your PNG files have a gamma that doesn't
1040 match the output you can take advantage of the fact that
1041 png_set_alpha_mode always sets the output gamma but only sets the PNG
1042 default if it is not already set:
1044 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
1045 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
1047 The first call sets both the default and the output gamma values, the
1048 second call overrides the output gamma without changing the default. This
1049 is easier than achieving the same effect with png_set_gamma. You must use
1050 PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
1051 fire if more than one call to png_set_alpha_mode and png_set_background is
1052 made in the same read operation, however multiple calls with PNG_ALPHA_PNG
1053 are ignored.
1055 If you don't need, or can't handle, the alpha channel you can call
1056 png_set_background() to remove it by compositing against a fixed color. Don't
1057 call png_set_strip_alpha() to do this - it will leave spurious pixel values in
1058 transparent parts of this image.
1060 png_set_background(png_ptr, &background_color,
1063 The background_color is an RGB or grayscale value according to the data format
1064 libpng will produce for you. Because you don't yet know the format of the PNG
1065 file, if you call png_set_background at this point you must arrange for the
1066 format produced by libpng to always have 8-bit or 16-bit components and then
1067 store the color as an 8-bit or 16-bit color as appropriate. The color contains
1068 separate gray and RGB component values, so you can let libpng produce gray or
1069 RGB output according to the input format, but low bit depth grayscale images
1070 must always be converted to at least 8-bit format. (Even though low bit depth
1071 grayscale images can't have an alpha channel they can have a transparent
1072 color!)
1074 You set the transforms you need later, either as flags to the high level
1075 interface or libpng API calls for the low level interface. For reference the
1076 settings and API calls required are:
1078 8-bit values:
1080 png_set_expand(png_ptr); png_set_scale_16(png_ptr);
1082 If you must get exactly the same inaccurate results
1083 produced by default in versions prior to libpng-1.5.4,
1084 use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
1085 instead.
1087 16-bit values:
1089 png_set_expand_16(png_ptr);
1091 In either case palette image data will be expanded to RGB. If you just want
1092 color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
1093 to the list.
1095 Calling png_set_background before the PNG file header is read will not work
1096 prior to libpng-1.5.4. Because the failure may result in unexpected warnings or
1097 errors it is therefore much safer to call png_set_background after the head has
1098 been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be
1099 used with the high level interface.
1101 The high-level read interface
1103 At this point there are two ways to proceed; through the high-level
1104 read interface, or through a sequence of low-level read operations.
1105 You can use the high-level interface if (a) you are willing to read
1106 the entire image into memory, and (b) the input transformations
1107 you want to do are limited to the following set:
1109 PNG_TRANSFORM_IDENTITY No transformation
1110 PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to
1111 8-bit accurately
1112 PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to
1113 8-bit less accurately
1114 PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
1115 PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
1116 samples to bytes
1117 PNG_TRANSFORM_PACKSWAP Change order of packed
1118 pixels to LSB first
1119 PNG_TRANSFORM_EXPAND Perform set_expand()
1120 PNG_TRANSFORM_INVERT_MONO Invert monochrome images
1121 PNG_TRANSFORM_SHIFT Normalize pixels to the
1122 sBIT depth
1124 to BGRA
1126 to AG
1127 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
1128 to transparency
1129 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
1130 PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples
1131 to RGB (or GA to RGBA)
1132 PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits
1134 (This excludes setting a background color, doing gamma transformation,
1135 quantizing, and setting filler.) If this is the case, simply do this:
1137 png_read_png(png_ptr, info_ptr, png_transforms, NULL)
1139 where png_transforms is an integer containing the bitwise OR of some
1140 set of transformation flags. This call is equivalent to png_read_info(),
1141 followed the set of transformations indicated by the transform mask,
1142 then png_read_image(), and finally png_read_end().
1144 (The final parameter of this call is not yet used. Someday it might point
1145 to transformation parameters required by some future input transform.)
1147 You must use png_transforms and not call any png_set_transform() functions
1148 when you use png_read_png().
1150 After you have called png_read_png(), you can retrieve the image data
1151 with
1153 row_pointers = png_get_rows(png_ptr, info_ptr);
1155 where row_pointers is an array of pointers to the pixel data for each row:
1157 png_bytep row_pointers[height];
1159 If you know your image size and pixel size ahead of time, you can allocate
1160 row_pointers prior to calling png_read_png() with
1162 if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
1163 png_error (png_ptr,
1164 "Image is too tall to process in memory");
1166 if (width > PNG_UINT_32_MAX/pixel_size)
1167 png_error (png_ptr,
1168 "Image is too wide to process in memory");
1170 row_pointers = png_malloc(png_ptr,
1171 height*(sizeof (png_bytep)));
1173 for (int i=0; i<height, i++)
1174 row_pointers[i]=NULL; /* security precaution */
1176 for (int i=0; i<height, i++)
1177 row_pointers[i]=png_malloc(png_ptr,
1178 width*pixel_size);
1180 png_set_rows(png_ptr, info_ptr, &row_pointers);
1182 Alternatively you could allocate your image in one big block and define
1183 row_pointers[i] to point into the proper places in your block.
1185 If you use png_set_rows(), the application is responsible for freeing
1186 row_pointers (and row_pointers[i], if they were separately allocated).
1188 If you don't allocate row_pointers ahead of time, png_read_png() will
1189 do it, and it'll be free'ed by libpng when you call png_destroy_*().
1191 The low-level read interface
1193 If you are going the low-level route, you are now ready to read all
1194 the file information up to the actual image data. You do this with a
1195 call to png_read_info().
1197 png_read_info(png_ptr, info_ptr);
1199 This will process all chunks up to but not including the image data.
1201 This also copies some of the data from the PNG file into the decode structure
1202 for use in later transformations. Important information copied in is:
1204 1) The PNG file gamma from the gAMA chunk. This overwrites the default value
1205 provided by an earlier call to png_set_gamma or png_set_alpha_mode.
1207 2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This
1208 damages the information provided by an earlier call to png_set_background
1209 resulting in unexpected behavior. Libpng-1.5.4 no longer does this.
1211 3) The number of significant bits in each component value. Libpng uses this to
1212 optimize gamma handling by reducing the internal lookup table sizes.
1214 4) The transparent color information from a tRNS chunk. This can be modified by
1215 a later call to png_set_tRNS.
1217 Querying the info structure
1219 Functions are used to get the information from the info_ptr once it
1220 has been read. Note that these fields may not be completely filled
1221 in until png_read_end() has read the chunk data following the image.
1223 png_get_IHDR(png_ptr, info_ptr, &width, &height,
1224 &bit_depth, &color_type, &interlace_type,
1225 &compression_type, &filter_method);
1227 width - holds the width of the image
1228 in pixels (up to 2^31).
1230 height - holds the height of the image
1231 in pixels (up to 2^31).
1233 bit_depth - holds the bit depth of one of the
1234 image channels. (valid values are
1235 1, 2, 4, 8, 16 and depend also on
1236 the color_type. See also
1237 significant bits (sBIT) below).
1239 color_type - describes which color/alpha channels
1240 are present.
1242 (bit depths 1, 2, 4, 8, 16)
1244 (bit depths 8, 16)
1246 (bit depths 1, 2, 4, 8)
1248 (bit_depths 8, 16)
1250 (bit_depths 8, 16)
1256 interlace_type - (PNG_INTERLACE_NONE or
1259 compression_type - (must be PNG_COMPRESSION_TYPE_BASE
1260 for PNG 1.0)
1262 filter_method - (must be PNG_FILTER_TYPE_BASE
1263 for PNG 1.0, and can also be
1265 the PNG datastream is embedded in
1266 a MNG-1.0 datastream)
1268 Any of width, height, color_type, bit_depth,
1269 interlace_type, compression_type, or filter_method can
1270 be NULL if you are not interested in their values.
1272 Note that png_get_IHDR() returns 32-bit data into
1273 the application's width and height variables.
1274 This is an unsafe situation if these are not png_uint_32
1275 variables. In such situations, the
1276 png_get_image_width() and png_get_image_height()
1277 functions described below are safer.
1279 width = png_get_image_width(png_ptr,
1280 info_ptr);
1282 height = png_get_image_height(png_ptr,
1283 info_ptr);
1285 bit_depth = png_get_bit_depth(png_ptr,
1286 info_ptr);
1288 color_type = png_get_color_type(png_ptr,
1289 info_ptr);
1291 interlace_type = png_get_interlace_type(png_ptr,
1292 info_ptr);
1294 compression_type = png_get_compression_type(png_ptr,
1295 info_ptr);
1297 filter_method = png_get_filter_type(png_ptr,
1298 info_ptr);
1300 channels = png_get_channels(png_ptr, info_ptr);
1302 channels - number of channels of info for the
1303 color type (valid values are 1 (GRAY,
1304 PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
1305 4 (RGB_ALPHA or RGB + filler byte))
1307 rowbytes = png_get_rowbytes(png_ptr, info_ptr);
1309 rowbytes - number of bytes needed to hold a row
1311 signature = png_get_signature(png_ptr, info_ptr);
1313 signature - holds the signature read from the
1314 file (if any). The data is kept in
1315 the same offset it would be if the
1316 whole signature were read (i.e. if an
1317 application had already read in 4
1318 bytes of signature before starting
1319 libpng, the remaining 4 bytes would
1320 be in signature[4] through signature[7]
1321 (see png_set_sig_bytes())).
1323 These are also important, but their validity depends on whether the chunk
1324 has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
1325 png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
1326 data has been read, or zero if it is missing. The parameters to the
1327 png_get_<chunk> are set directly if they are simple data types, or a
1328 pointer into the info_ptr is returned for any complex types.
1330 The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks
1331 is simply returned to give the application information about how the
1332 image was encoded. Libpng itself only does transformations using the file
1333 gamma when combining semitransparent pixels with the background color, and,
1334 since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels
1335 within the simplified API. Libpng also uses the file gamma when converting
1336 RGB to gray, beginning with libpng-1.0.5, if the application calls
1337 png_set_rgb_to_gray()).
1339 png_get_PLTE(png_ptr, info_ptr, &palette,
1340 &num_palette);
1342 palette - the palette for the file
1343 (array of png_color)
1345 num_palette - number of entries in the palette
1347 png_get_gAMA(png_ptr, info_ptr, &file_gamma);
1348 png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
1350 file_gamma - the gamma at which the file is
1351 written (PNG_INFO_gAMA)
1353 int_file_gamma - 100,000 times the gamma at which the
1354 file is written
1356 png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x,
1357 &red_y, &green_x, &green_y, &blue_x, &blue_y)
1358 png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
1359 &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
1360 &blue_Z)
1361 png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
1362 &int_white_y, &int_red_x, &int_red_y,
1363 &int_green_x, &int_green_y, &int_blue_x,
1364 &int_blue_y)
1365 png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
1366 &int_red_Z, &int_green_X, &int_green_Y,
1367 &int_green_Z, &int_blue_X, &int_blue_Y,
1368 &int_blue_Z)
1370 {white,red,green,blue}_{x,y}
1371 A color space encoding specified using the
1372 chromaticities of the end points and the
1373 white point. (PNG_INFO_cHRM)
1375 {red,green,blue}_{X,Y,Z}
1376 A color space encoding specified using the
1377 encoding end points - the CIE tristimulus
1378 specification of the intended color of the red,
1379 green and blue channels in the PNG RGB data.
1380 The white point is simply the sum of the three
1381 end points. (PNG_INFO_cHRM)
1383 png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
1385 srgb_intent - the rendering intent (PNG_INFO_sRGB)
1386 The presence of the sRGB chunk
1387 means that the pixel data is in the
1388 sRGB color space. This chunk also
1389 implies specific values of gAMA and
1390 cHRM.
1392 png_get_iCCP(png_ptr, info_ptr, &name,
1393 &compression_type, &profile, &proflen);
1395 name - The profile name.
1397 compression_type - The compression type; always
1399 You may give NULL to this argument to
1400 ignore it.
1402 profile - International Color Consortium color
1403 profile data. May contain NULs.
1405 proflen - length of profile data in bytes.
1407 png_get_sBIT(png_ptr, info_ptr, &sig_bit);
1409 sig_bit - the number of significant bits for
1410 (PNG_INFO_sBIT) each of the gray,
1411 red, green, and blue channels,
1412 whichever are appropriate for the
1413 given color type (png_color_16)
1415 png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
1416 &num_trans, &trans_color);
1418 trans_alpha - array of alpha (transparency)
1419 entries for palette (PNG_INFO_tRNS)
1421 num_trans - number of transparent entries
1422 (PNG_INFO_tRNS)
1424 trans_color - graylevel or color sample values of
1425 the single transparent color for
1426 non-paletted images (PNG_INFO_tRNS)
1428 png_get_hIST(png_ptr, info_ptr, &hist);
1429 (PNG_INFO_hIST)
1431 hist - histogram of palette (array of
1432 png_uint_16)
1434 png_get_tIME(png_ptr, info_ptr, &mod_time);
1436 mod_time - time image was last modified
1439 png_get_bKGD(png_ptr, info_ptr, &background);
1441 background - background color (of type
1442 png_color_16p) (PNG_VALID_bKGD)
1443 valid 16-bit red, green and blue
1444 values, regardless of color_type
1446 num_comments = png_get_text(png_ptr, info_ptr,
1447 &text_ptr, &num_text);
1449 num_comments - number of comments
1451 text_ptr - array of png_text holding image
1454 text_ptr[i].compression - type of compression used
1460 text_ptr[i].key - keyword for comment. Must contain
1461 1-79 characters.
1463 text_ptr[i].text - text comments for current
1464 keyword. Can be empty.
1466 text_ptr[i].text_length - length of text string,
1467 after decompression, 0 for iTXt
1469 text_ptr[i].itxt_length - length of itxt string,
1470 after decompression, 0 for tEXt/zTXt
1472 text_ptr[i].lang - language of comment (empty
1473 string for unknown).
1475 text_ptr[i].lang_key - keyword in UTF-8
1476 (empty string for unknown).
1478 Note that the itxt_length, lang, and lang_key
1479 members of the text_ptr structure only exist when the
1480 library is built with iTXt chunk support. Prior to
1481 libpng-1.4.0 the library was built by default without
1482 iTXt support. Also note that when iTXt is supported,
1483 they contain NULL pointers when the "compression"
1484 field contains PNG_TEXT_COMPRESSION_NONE or
1487 num_text - number of comments (same as
1488 num_comments; you can put NULL here
1489 to avoid the duplication)
1491 Note while png_set_text() will accept text, language,
1492 and translated keywords that can be NULL pointers, the
1493 structure returned by png_get_text will always contain
1494 regular zero-terminated C strings. They might be
1495 empty strings but they will never be NULL pointers.
1497 num_spalettes = png_get_sPLT(png_ptr, info_ptr,
1498 &palette_ptr);
1500 num_spalettes - number of sPLT chunks read.
1502 palette_ptr - array of palette structures holding
1503 contents of one or more sPLT chunks
1504 read.
1506 png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
1507 &unit_type);
1509 offset_x - positive offset from the left edge
1510 of the screen (can be negative)
1512 offset_y - positive offset from the top edge
1513 of the screen (can be negative)
1517 png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
1518 &unit_type);
1520 res_x - pixels/unit physical resolution in
1521 x direction
1523 res_y - pixels/unit physical resolution in
1524 x direction
1526 unit_type - PNG_RESOLUTION_UNKNOWN,
1529 png_get_sCAL(png_ptr, info_ptr, &unit, &width,
1530 &height)
1532 unit - physical scale units (an integer)
1534 width - width of a pixel in physical scale units
1536 height - height of a pixel in physical scale units
1537 (width and height are doubles)
1539 png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
1540 &height)
1542 unit - physical scale units (an integer)
1544 width - width of a pixel in physical scale units
1545 (expressed as a string)
1547 height - height of a pixel in physical scale units
1548 (width and height are strings like "2.54")
1550 num_unknown_chunks = png_get_unknown_chunks(png_ptr,
1551 info_ptr, &unknowns)
1553 unknowns - array of png_unknown_chunk
1554 structures holding unknown chunks
1556 unknowns[i].name - name of unknown chunk
1558 unknowns[i].data - data of unknown chunk
1560 unknowns[i].size - size of unknown chunk's data
1562 unknowns[i].location - position of chunk in file
1564 The value of "i" corresponds to the order in which the
1565 chunks were read from the PNG file or inserted with the
1566 png_set_unknown_chunks() function.
1568 The value of "location" is a bitwise "or" of
1570 PNG_HAVE_IHDR (0x01)
1571 PNG_HAVE_PLTE (0x02)
1572 PNG_AFTER_IDAT (0x08)
1574 The data from the pHYs chunk can be retrieved in several convenient
1575 forms:
1577 res_x = png_get_x_pixels_per_meter(png_ptr,
1578 info_ptr)
1580 res_y = png_get_y_pixels_per_meter(png_ptr,
1581 info_ptr)
1583 res_x_and_y = png_get_pixels_per_meter(png_ptr,
1584 info_ptr)
1586 res_x = png_get_x_pixels_per_inch(png_ptr,
1587 info_ptr)
1589 res_y = png_get_y_pixels_per_inch(png_ptr,
1590 info_ptr)
1592 res_x_and_y = png_get_pixels_per_inch(png_ptr,
1593 info_ptr)
1595 aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
1596 info_ptr)
1598 Each of these returns 0 [signifying "unknown"] if
1599 the data is not present or if res_x is 0;
1600 res_x_and_y is 0 if res_x != res_y
1602 Note that because of the way the resolutions are
1603 stored internally, the inch conversions won't
1604 come out to exactly even number. For example,
1605 72 dpi is stored as 0.28346 pixels/meter, and
1606 when this is retrieved it is 71.9988 dpi, so
1607 be sure to round the returned value appropriately
1608 if you want to display a reasonable-looking result.
1610 The data from the oFFs chunk can be retrieved in several convenient
1611 forms:
1613 x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
1615 y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
1617 x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
1619 y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
1621 Each of these returns 0 [signifying "unknown" if both
1622 x and y are 0] if the data is not present or if the
1623 chunk is present but the unit is the pixel. The
1624 remark about inexact inch conversions applies here
1625 as well, because a value in inches can't always be
1626 converted to microns and back without some loss
1627 of precision.
1629 For more information, see the
1630 PNG specification for chunk contents. Be careful with trusting
1631 rowbytes, as some of the transformations could increase the space
1632 needed to hold a row (expand, filler, gray_to_rgb, etc.).
1633 See png_read_update_info(), below.
1635 A quick word about text_ptr and num_text. PNG stores comments in
1636 keyword/text pairs, one pair per chunk, with no limit on the number
1637 of text chunks, and a 2^31 byte limit on their size. While there are
1638 suggested keywords, there is no requirement to restrict the use to these
1639 strings. It is strongly suggested that keywords and text be sensible
1640 to humans (that's the point), so don't use abbreviations. Non-printing
1641 symbols are not allowed. See the PNG specification for more details.
1642 There is also no requirement to have text after the keyword.
1644 Keywords should be limited to 79 Latin-1 characters without leading or
1645 trailing spaces, but non-consecutive spaces are allowed within the
1646 keyword. It is possible to have the same keyword any number of times.
1647 The text_ptr is an array of png_text structures, each holding a
1648 pointer to a language string, a pointer to a keyword and a pointer to
1649 a text string. The text string, language code, and translated
1650 keyword may be empty or NULL pointers. The keyword/text
1651 pairs are put into the array in the order that they are received.
1652 However, some or all of the text chunks may be after the image, so, to
1653 make sure you have read all the text chunks, don't mess with these
1654 until after you read the stuff after the image. This will be
1655 mentioned again below in the discussion that goes with png_read_end().
1657 Input transformations
1659 After you've read the header information, you can set up the library
1660 to handle any special transformations of the image data. The various
1661 ways to transform the data will be described in the order that they
1662 should occur. This is important, as some of these change the color
1663 type and/or bit depth of the data, and some others only work on
1664 certain color types and bit depths.
1666 Transformations you request are ignored if they don't have any meaning for a
1667 particular input data format. However some transformations can have an effect
1668 as a result of a previous transformation. If you specify a contradictory set of
1669 transformations, for example both adding and removing the alpha channel, you
1670 cannot predict the final result.
1672 The color used for the transparency values should be supplied in the same
1673 format/depth as the current image data. It is stored in the same format/depth
1674 as the image data in a tRNS chunk, so this is what libpng expects for this data.
1676 The color used for the background value depends on the need_expand argument as
1677 described below.
1679 Data will be decoded into the supplied row buffers packed into bytes
1680 unless the library has been told to transform it into another format.
1681 For example, 4 bit/pixel paletted or grayscale data will be returned
1682 2 pixels/byte with the leftmost pixel in the high-order bits of the
1683 byte, unless png_set_packing() is called. 8-bit RGB data will be stored
1684 in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
1685 is called to insert filler bytes, either before or after each RGB triplet.
1686 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
1687 byte of the color value first, unless png_set_scale_16() is called to
1688 transform it to regular RGB RGB triplets, or png_set_filler() or
1689 png_set_add alpha() is called to insert filler bytes, either before or
1690 after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
1691 be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
1692 or png_set_scale_16().
1694 The following code transforms grayscale images of less than 8 to 8 bits,
1695 changes paletted images to RGB, and adds a full alpha channel if there is
1696 transparency information in a tRNS chunk. This is most useful on
1697 grayscale images with bit depths of 2 or 4 or if there is a multiple-image
1698 viewing application that wishes to treat all images in the same way.
1700 if (color_type == PNG_COLOR_TYPE_PALETTE)
1701 png_set_palette_to_rgb(png_ptr);
1703 if (png_get_valid(png_ptr, info_ptr,
1704 PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
1706 if (color_type == PNG_COLOR_TYPE_GRAY &&
1707 bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
1709 The first two functions are actually aliases for png_set_expand(), added
1710 in libpng version 1.0.4, with the function names expanded to improve code
1711 readability. In some future version they may actually do different
1712 things.
1714 As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
1715 added. It expands the sample depth without changing tRNS to alpha.
1717 As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as
1718 png_set_expand(); however, the resultant channels have 16 bits rather than 8.
1719 Use this when the output color or gray channels are made linear to avoid fairly
1720 severe accuracy loss.
1722 if (bit_depth < 16)
1723 png_set_expand_16(png_ptr);
1725 PNG can have files with 16 bits per channel. If you only can handle
1726 8 bits per channel, this will strip the pixels down to 8-bit.
1728 if (bit_depth == 16)
1729 #if PNG_LIBPNG_VER >= 10504
1730 png_set_scale_16(png_ptr);
1731 #else
1732 png_set_strip_16(png_ptr);
1733 #endif
1735 (The more accurate "png_set_scale_16()" API became available in libpng version
1736 1.5.4).
1738 If you need to process the alpha channel on the image separately from the image
1739 data (for example if you convert it to a bitmap mask) it is possible to have
1740 libpng strip the channel leaving just RGB or gray data:
1742 if (color_type & PNG_COLOR_MASK_ALPHA)
1743 png_set_strip_alpha(png_ptr);
1745 If you strip the alpha channel you need to find some other way of dealing with
1746 the information. If, instead, you want to convert the image to an opaque
1747 version with no alpha channel use png_set_background; see below.
1749 As of libpng version 1.5.2, almost all useful expansions are supported, the
1750 major ommissions are conversion of grayscale to indexed images (which can be
1751 done trivially in the application) and conversion of indexed to grayscale (which
1752 can be done by a trivial manipulation of the palette.)
1754 In the following table, the 01 means grayscale with depth<8, 31 means
1755 indexed with depth<8, other numerals represent the color type, "T" means
1756 the tRNS chunk is present, A means an alpha channel is present, and O
1757 means tRNS or alpha is present but all pixels in the image are opaque.
1759 FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O
1760 TO
1761 01 - [G] - - - - - - - - - - - - -
1762 31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q
1763 0 1 G + . . G G G G G G B B GB GB
1764 0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt
1765 0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt
1766 2 C P C C C + . . C - - CB CB B B
1767 2T Ct - Ct C C t + t - - - CBt CBt Bt Bt
1768 2O Ct - Ct C C t t + - - - CBt CBt Bt Bt
1769 3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q
1770 3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt
1771 3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt
1774 6A CA PA CA C C A T tT PA P P C CBA + BA
1775 6O CA PBA CA C C A tT T PA P P CBA C BA +
1777 Within the matrix,
1778 "+" identifies entries where 'from' and 'to' are the same.
1779 "-" means the transformation is not supported.
1780 "." means nothing is necessary (a tRNS chunk can just be ignored).
1781 "t" means the transformation is obtained by png_set_tRNS.
1782 "A" means the transformation is obtained by png_set_add_alpha().
1783 "X" means the transformation is obtained by png_set_expand().
1784 "1" means the transformation is obtained by
1785 png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
1786 if there is no transparency in the original or the final
1787 format).
1788 "C" means the transformation is obtained by png_set_gray_to_rgb().
1789 "G" means the transformation is obtained by png_set_rgb_to_gray().
1790 "P" means the transformation is obtained by
1791 png_set_expand_palette_to_rgb().
1792 "p" means the transformation is obtained by png_set_packing().
1793 "Q" means the transformation is obtained by png_set_quantize().
1794 "T" means the transformation is obtained by
1795 png_set_tRNS_to_alpha().
1796 "B" means the transformation is obtained by
1797 png_set_background(), or png_strip_alpha().
1799 When an entry has multiple transforms listed all are required to cause the
1800 right overall transformation. When two transforms are separated by a comma
1801 either will do the job. When transforms are enclosed in [] the transform should
1802 do the job but this is currently unimplemented - a different format will result
1803 if the suggested transformations are used.
1805 In PNG files, the alpha channel in an image
1806 is the level of opacity. If you need the alpha channel in an image to
1807 be the level of transparency instead of opacity, you can invert the
1808 alpha channel (or the tRNS chunk data) after it's read, so that 0 is
1809 fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
1810 images) is fully transparent, with
1812 png_set_invert_alpha(png_ptr);
1814 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
1815 they can, resulting in, for example, 8 pixels per byte for 1 bit
1816 files. This code expands to 1 pixel per byte without changing the
1817 values of the pixels:
1819 if (bit_depth < 8)
1820 png_set_packing(png_ptr);
1822 PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
1823 stored in a PNG image have been "scaled" or "shifted" up to the next
1824 higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
1825 to 8 bits/sample in the range [0, 255]). However, it is also possible
1826 to convert the PNG pixel data back to the original bit depth of the
1827 image. This call reduces the pixels back down to the original bit depth:
1829 png_color_8p sig_bit;
1831 if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
1832 png_set_shift(png_ptr, sig_bit);
1834 PNG files store 3-color pixels in red, green, blue order. This code
1835 changes the storage of the pixels to blue, green, red:
1837 if (color_type == PNG_COLOR_TYPE_RGB ||
1838 color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1839 png_set_bgr(png_ptr);
1841 PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
1842 into 4 or 8 bytes for windowing systems that need them in this format:
1844 if (color_type == PNG_COLOR_TYPE_RGB)
1845 png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
1847 where "filler" is the 8 or 16-bit number to fill with, and the location is
1848 either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
1849 you want the filler before the RGB or after. This transformation
1850 does not affect images that already have full alpha channels. To add an
1851 opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
1852 will generate RGBA pixels.
1854 Note that png_set_filler() does not change the color type. If you want
1855 to do that, you can add a true alpha channel with
1857 if (color_type == PNG_COLOR_TYPE_RGB ||
1858 color_type == PNG_COLOR_TYPE_GRAY)
1859 png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
1861 where "filler" contains the alpha value to assign to each pixel.
1862 This function was added in libpng-1.2.7.
1864 If you are reading an image with an alpha channel, and you need the
1865 data as ARGB instead of the normal PNG format RGBA:
1867 if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1868 png_set_swap_alpha(png_ptr);
1870 For some uses, you may want a grayscale image to be represented as
1871 RGB. This code will do that conversion:
1873 if (color_type == PNG_COLOR_TYPE_GRAY ||
1874 color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
1875 png_set_gray_to_rgb(png_ptr);
1877 Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
1878 with alpha.
1880 if (color_type == PNG_COLOR_TYPE_RGB ||
1881 color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1882 png_set_rgb_to_gray(png_ptr, error_action,
1883 double red_weight, double green_weight);
1885 error_action = 1: silently do the conversion
1887 error_action = 2: issue a warning if the original
1888 image has any pixel where
1889 red != green or red != blue
1891 error_action = 3: issue an error and abort the
1892 conversion if the original
1893 image has any pixel where
1894 red != green or red != blue
1896 red_weight: weight of red component
1898 green_weight: weight of green component
1899 If either weight is negative, default
1900 weights are used.
1902 In the corresponding fixed point API the red_weight and green_weight values are
1903 simply scaled by 100,000:
1905 png_set_rgb_to_gray(png_ptr, error_action,
1906 png_fixed_point red_weight,
1907 png_fixed_point green_weight);
1909 If you have set error_action = 1 or 2, you can
1910 later check whether the image really was gray, after processing
1911 the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
1912 It will return a png_byte that is zero if the image was gray or
1913 1 if there were any non-gray pixels. Background and sBIT data
1914 will be silently converted to grayscale, using the green channel
1915 data for sBIT, regardless of the error_action setting.
1917 The default values come from the PNG file cHRM chunk if present; otherwise, the
1918 defaults correspond to the ITU-R recommendation 709, and also the sRGB color
1919 space, as recommended in the Charles Poynton's Colour FAQ,
1920 <http://www.poynton.com/>, in section 9:
1922 <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
1924 Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
1926 Previous versions of this document, 1998 through 2002, recommended a slightly
1927 different formula:
1929 Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
1931 Libpng uses an integer approximation:
1933 Y = (6968 * R + 23434 * G + 2366 * B)/32768
1935 The calculation is done in a linear colorspace, if the image gamma
1936 can be determined.
1938 The png_set_background() function has been described already; it tells libpng to
1939 composite images with alpha or simple transparency against the supplied
1940 background color. For compatibility with versions of libpng earlier than
1941 libpng-1.5.4 it is recommended that you call the function after reading the file
1942 header, even if you don't want to use the color in a bKGD chunk, if one exists.
1944 If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
1945 you may use this color, or supply another color more suitable for
1946 the current display (e.g., the background color from a web page). You
1947 need to tell libpng how the color is represented, both the format of the
1948 component values in the color (the number of bits) and the gamma encoding of the
1949 color. The function takes two arguments, background_gamma_mode and need_expand
1950 to convey this information; however, only two combinations are likely to be
1951 useful:
1953 png_color_16 my_background;
1954 png_color_16p image_background;
1956 if (png_get_bKGD(png_ptr, info_ptr, &image_background))
1957 png_set_background(png_ptr, image_background,
1958 PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
1959 else
1960 png_set_background(png_ptr, &my_background,
1961 PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);
1963 The second call was described above - my_background is in the format of the
1964 final, display, output produced by libpng. Because you now know the format of
1965 the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
1966 output and to retain palette images (the palette colors will be modified
1967 appropriately and the tRNS chunk removed.) However, if you are doing this,
1968 take great care not to ask for transformations without checking first that
1969 they apply!
1971 In the first call the background color has the original bit depth and color type
1972 of the PNG file. So, for palette images the color is supplied as a palette
1973 index and for low bit greyscale images the color is a reduced bit value in
1974 image_background->gray.
1976 If you didn't call png_set_gamma() before reading the file header, for example
1977 if you need your code to remain compatible with older versions of libpng prior
1978 to libpng-1.5.4, this is the place to call it.
1980 Do not call it if you called png_set_alpha_mode(); doing so will damage the
1981 settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is
1982 supported then you can certainly do png_set_gamma() before reading the PNG
1983 header.)
1985 This API unconditionally sets the screen and file gamma values, so it will
1986 override the value in the PNG file unless it is called before the PNG file
1987 reading starts. For this reason you must always call it with the PNG file
1988 value when you call it in this position:
1990 if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
1991 png_set_gamma(png_ptr, screen_gamma, file_gamma);
1993 else
1994 png_set_gamma(png_ptr, screen_gamma, 0.45455);
1996 If you need to reduce an RGB file to a paletted file, or if a paletted
1997 file has more entries than will fit on your screen, png_set_quantize()
1998 will do that. Note that this is a simple match quantization that merely
1999 finds the closest color available. This should work fairly well with
2000 optimized palettes, but fairly badly with linear color cubes. If you
2001 pass a palette that is larger than maximum_colors, the file will
2002 reduce the number of colors in the palette so it will fit into
2003 maximum_colors. If there is a histogram, libpng will use it to make
2004 more intelligent choices when reducing the palette. If there is no
2005 histogram, it may not do as good a job.
2007 if (color_type & PNG_COLOR_MASK_COLOR)
2008 {
2009 if (png_get_valid(png_ptr, info_ptr,
2011 {
2012 png_uint_16p histogram = NULL;
2014 png_get_hIST(png_ptr, info_ptr,
2015 &histogram);
2016 png_set_quantize(png_ptr, palette, num_palette,
2017 max_screen_colors, histogram, 1);
2018 }
2020 else
2021 {
2022 png_color std_color_cube[MAX_SCREEN_COLORS] =
2023 { ... colors ... };
2025 png_set_quantize(png_ptr, std_color_cube,
2027 NULL,0);
2028 }
2029 }
2031 PNG files describe monochrome as black being zero and white being one.
2032 The following code will reverse this (make black be one and white be
2033 zero):
2035 if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
2036 png_set_invert_mono(png_ptr);
2038 This function can also be used to invert grayscale and gray-alpha images:
2040 if (color_type == PNG_COLOR_TYPE_GRAY ||
2041 color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
2042 png_set_invert_mono(png_ptr);
2044 PNG files store 16-bit pixels in network byte order (big-endian,
2045 ie. most significant bits first). This code changes the storage to the
2046 other way (little-endian, i.e. least significant bits first, the
2047 way PCs store them):
2049 if (bit_depth == 16)
2050 png_set_swap(png_ptr);
2052 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
2053 need to change the order the pixels are packed into bytes, you can use:
2055 if (bit_depth < 8)
2056 png_set_packswap(png_ptr);
2058 Finally, you can write your own transformation function if none of
2059 the existing ones meets your needs. This is done by setting a callback
2060 with
2062 png_set_read_user_transform_fn(png_ptr,
2063 read_transform_fn);
2065 You must supply the function
2067 void read_transform_fn(png_structp png_ptr, png_row_infop
2068 row_info, png_bytep data)
2070 See pngtest.c for a working example. Your function will be called
2071 after all of the other transformations have been processed. Take care with
2072 interlaced images if you do the interlace yourself - the width of the row is the
2073 width in 'row_info', not the overall image width.
2075 If supported, libpng provides two information routines that you can use to find
2076 where you are in processing the image:
2078 png_get_current_pass_number(png_structp png_ptr);
2079 png_get_current_row_number(png_structp png_ptr);
2081 Don't try using these outside a transform callback - firstly they are only
2082 supported if user transforms are supported, secondly they may well return
2083 unexpected results unless the row is actually being processed at the moment they
2084 are called.
2086 With interlaced
2087 images the value returned is the row in the input sub-image image. Use
2088 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
2089 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
2091 The discussion of interlace handling above contains more information on how to
2092 use these values.
2094 You can also set up a pointer to a user structure for use by your
2095 callback function, and you can inform libpng that your transform
2096 function will change the number of channels or bit depth with the
2097 function
2099 png_set_user_transform_info(png_ptr, user_ptr,
2100 user_depth, user_channels);
2102 The user's application, not libpng, is responsible for allocating and
2103 freeing any memory required for the user structure.
2105 You can retrieve the pointer via the function
2106 png_get_user_transform_ptr(). For example:
2108 voidp read_user_transform_ptr =
2109 png_get_user_transform_ptr(png_ptr);
2111 The last thing to handle is interlacing; this is covered in detail below,
2112 but you must call the function here if you want libpng to handle expansion
2113 of the interlaced image.
2115 number_of_passes = png_set_interlace_handling(png_ptr);
2117 After setting the transformations, libpng can update your png_info
2118 structure to reflect any transformations you've requested with this
2119 call.
2121 png_read_update_info(png_ptr, info_ptr);
2123 This is most useful to update the info structure's rowbytes
2124 field so you can use it to allocate your image memory. This function
2125 will also update your palette with the correct screen_gamma and
2126 background if these have been given with the calls above. You may
2127 only call png_read_update_info() once with a particular info_ptr.
2129 After you call png_read_update_info(), you can allocate any
2130 memory you need to hold the image. The row data is simply
2131 raw byte data for all forms of images. As the actual allocation
2132 varies among applications, no example will be given. If you
2133 are allocating one large chunk, you will need to build an
2134 array of pointers to each row, as it will be needed for some
2135 of the functions below.
2137 Remember: Before you call png_read_update_info(), the png_get_*()
2138 functions return the values corresponding to the original PNG image.
2139 After you call png_read_update_info the values refer to the image
2140 that libpng will output. Consequently you must call all the png_set_
2141 functions before you call png_read_update_info(). This is particularly
2142 important for png_set_interlace_handling() - if you are going to call
2143 png_read_update_info() you must call png_set_interlace_handling() before
2144 it unless you want to receive interlaced output.
2146 Reading image data
2148 After you've allocated memory, you can read the image data.
2149 The simplest way to do this is in one function call. If you are
2150 allocating enough memory to hold the whole image, you can just
2151 call png_read_image() and libpng will read in all the image data
2152 and put it in the memory area supplied. You will need to pass in
2153 an array of pointers to each row.
2155 This function automatically handles interlacing, so you don't
2156 need to call png_set_interlace_handling() (unless you call
2157 png_read_update_info()) or call this function multiple times, or any
2158 of that other stuff necessary with png_read_rows().
2160 png_read_image(png_ptr, row_pointers);
2162 where row_pointers is:
2164 png_bytep row_pointers[height];
2166 You can point to void or char or whatever you use for pixels.
2168 If you don't want to read in the whole image at once, you can
2169 use png_read_rows() instead. If there is no interlacing (check
2170 interlace_type == PNG_INTERLACE_NONE), this is simple:
2172 png_read_rows(png_ptr, row_pointers, NULL,
2173 number_of_rows);
2175 where row_pointers is the same as in the png_read_image() call.
2177 If you are doing this just one row at a time, you can do this with
2178 a single row_pointer instead of an array of row_pointers:
2180 png_bytep row_pointer = row;
2181 png_read_row(png_ptr, row_pointer, NULL);
2183 If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
2184 get somewhat harder. The only current (PNG Specification version 1.2)
2185 interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
2186 a somewhat complicated 2D interlace scheme, known as Adam7, that
2187 breaks down an image into seven smaller images of varying size, based
2188 on an 8x8 grid. This number is defined (from libpng 1.5) as
2191 libpng can fill out those images or it can give them to you "as is".
2192 It is almost always better to have libpng handle the interlacing for you.
2193 If you want the images filled out, there are two ways to do that. The one
2194 mentioned in the PNG specification is to expand each pixel to cover
2195 those pixels that have not been read yet (the "rectangle" method).
2196 This results in a blocky image for the first pass, which gradually
2197 smooths out as more pixels are read. The other method is the "sparkle"
2198 method, where pixels are drawn only in their final locations, with the
2199 rest of the image remaining whatever colors they were initialized to
2200 before the start of the read. The first method usually looks better,
2201 but tends to be slower, as there are more pixels to put in the rows.
2203 If, as is likely, you want libpng to expand the images, call this before
2204 calling png_start_read_image() or png_read_update_info():
2206 if (interlace_type == PNG_INTERLACE_ADAM7)
2207 number_of_passes
2208 = png_set_interlace_handling(png_ptr);
2210 This will return the number of passes needed. Currently, this is seven,
2211 but may change if another interlace type is added. This function can be
2212 called even if the file is not interlaced, where it will return one pass.
2213 You then need to read the whole image 'number_of_passes' times. Each time
2214 will distribute the pixels from the current pass to the correct place in
2215 the output image, so you need to supply the same rows to png_read_rows in
2216 each pass.
2218 If you are not going to display the image after each pass, but are
2219 going to wait until the entire image is read in, use the sparkle
2220 effect. This effect is faster and the end result of either method
2221 is exactly the same. If you are planning on displaying the image
2222 after each pass, the "rectangle" effect is generally considered the
2223 better looking one.
2225 If you only want the "sparkle" effect, just call png_read_rows() as
2226 normal, with the third parameter NULL. Make sure you make pass over
2227 the image number_of_passes times, and you don't change the data in the
2228 rows between calls. You can change the locations of the data, just
2229 not the data. Each pass only writes the pixels appropriate for that
2230 pass, and assumes the data from previous passes is still valid.
2232 png_read_rows(png_ptr, row_pointers, NULL,
2233 number_of_rows);
2235 If you only want the first effect (the rectangles), do the same as
2236 before except pass the row buffer in the third parameter, and leave
2237 the second parameter NULL.
2239 png_read_rows(png_ptr, NULL, row_pointers,
2240 number_of_rows);
2242 If you don't want libpng to handle the interlacing details, just call
2243 png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
2244 Each of the images is a valid image by itself; however, you will almost
2245 certainly need to distribute the pixels from each sub-image to the
2246 correct place. This is where everything gets very tricky.
2248 If you want to retrieve the separate images you must pass the correct
2249 number of rows to each successive call of png_read_rows(). The calculation
2250 gets pretty complicated for small images, where some sub-images may
2251 not even exist because either their width or height ends up zero.
2252 libpng provides two macros to help you in 1.5 and later versions:
2254 png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
2255 png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);
2257 Respectively these tell you the width and height of the sub-image
2258 corresponding to the numbered pass. 'pass' is in in the range 0 to 6 -
2259 this can be confusing because the specification refers to the same passes
2260 as 1 to 7! Be careful, you must check both the width and height before
2261 calling png_read_rows() and not call it for that pass if either is zero.
2263 You can, of course, read each sub-image row by row. If you want to
2264 produce optimal code to make a pixel-by-pixel transformation of an
2265 interlaced image this is the best approach; read each row of each pass,
2266 transform it, and write it out to a new interlaced image.
2268 If you want to de-interlace the image yourself libpng provides further
2269 macros to help that tell you where to place the pixels in the output image.
2270 Because the interlacing scheme is rectangular - sub-image pixels are always
2271 arranged on a rectangular grid - all you need to know for each pass is the
2272 starting column and row in the output image of the first pixel plus the
2273 spacing between each pixel. As of libpng 1.5 there are four macros to
2274 retrieve this information:
2276 png_uint_32 x = PNG_PASS_START_COL(pass);
2277 png_uint_32 y = PNG_PASS_START_ROW(pass);
2278 png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
2279 png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);
2281 These allow you to write the obvious loop:
2283 png_uint_32 input_y = 0;
2284 png_uint_32 output_y = PNG_PASS_START_ROW(pass);
2286 while (output_y < output_image_height)
2287 {
2288 png_uint_32 input_x = 0;
2289 png_uint_32 output_x = PNG_PASS_START_COL(pass);
2291 while (output_x < output_image_width)
2292 {
2293 image[output_y][output_x] =
2294 subimage[pass][input_y][input_x++];
2296 output_x += xStep;
2297 }
2299 ++input_y;
2300 output_y += yStep;
2301 }
2303 Notice that the steps between successive output rows and columns are
2304 returned as shifts. This is possible because the pixels in the subimages
2305 are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
2306 image. In practice you may need to directly calculate the output coordinate
2307 given an input coordinate. libpng provides two further macros for this
2308 purpose:
2310 png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
2311 png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);
2313 Finally a pair of macros are provided to tell you if a particular image
2314 row or column appears in a given pass:
2316 int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
2317 int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);
2319 Bear in mind that you will probably also need to check the width and height
2320 of the pass in addition to the above to be sure the pass even exists!
2322 With any luck you are convinced by now that you don't want to do your own
2323 interlace handling. In reality normally the only good reason for doing this
2324 is if you are processing PNG files on a pixel-by-pixel basis and don't want
2325 to load the whole file into memory when it is interlaced.
2327 libpng includes a test program, pngvalid, that illustrates reading and
2328 writing of interlaced images. If you can't get interlacing to work in your
2329 code and don't want to leave it to libpng (the recommended approach), see
2330 how pngvalid.c does it.
2332 Finishing a sequential read
2334 After you are finished reading the image through the
2335 low-level interface, you can finish reading the file.
2337 If you want to use a different crc action for handling CRC errors in
2338 chunks after the image data, you can call png_set_crc_action()
2339 again at this point.
2341 If you are interested in comments or time, which may be stored either
2342 before or after the image data, you should pass the separate png_info
2343 struct if you want to keep the comments from before and after the image
2344 separate.
2346 png_infop end_info = png_create_info_struct(png_ptr);
2348 if (!end_info)
2349 {
2350 png_destroy_read_struct(&png_ptr, &info_ptr,
2351 (png_infopp)NULL);
2352 return (ERROR);
2353 }
2355 png_read_end(png_ptr, end_info);
2357 If you are not interested, you should still call png_read_end()
2358 but you can pass NULL, avoiding the need to create an end_info structure.
2359 If you do this, libpng will not process any chunks after IDAT other than
2360 skipping over them and perhaps (depending on whether you have called
2361 png_set_crc_action) checking their CRCs while looking for the IEND chunk.
2363 png_read_end(png_ptr, (png_infop)NULL);
2365 If you don't call png_read_end(), then your file pointer will be
2366 left pointing to the first chunk after the last IDAT, which is probably
2367 not what you want if you expect to read something beyond the end of
2368 the PNG datastream.
2370 When you are done, you can free all memory allocated by libpng like this:
2372 png_destroy_read_struct(&png_ptr, &info_ptr,
2373 &end_info);
2375 or, if you didn't create an end_info structure,
2377 png_destroy_read_struct(&png_ptr, &info_ptr,
2378 (png_infopp)NULL);
2380 It is also possible to individually free the info_ptr members that
2381 point to libpng-allocated storage with the following function:
2383 png_free_data(png_ptr, info_ptr, mask, seq)
2385 mask - identifies data to be freed, a mask
2386 containing the bitwise OR of one or
2387 more of
2393 or simply PNG_FREE_ALL
2395 seq - sequence number of item to be freed
2396 (-1 for all items)
2398 This function may be safely called when the relevant storage has
2399 already been freed, or has not yet been allocated, or was allocated
2400 by the user and not by libpng, and will in those cases do nothing.
2401 The "seq" parameter is ignored if only one item of the selected data
2402 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
2403 are allowed for the data type identified in the mask, such as text or
2404 sPLT, only the n'th item in the structure is freed, where n is "seq".
2406 The default behavior is only to free data that was allocated internally
2407 by libpng. This can be changed, so that libpng will not free the data,
2408 or so that it will free data that was allocated by the user with png_malloc()
2409 or png_calloc() and passed in via a png_set_*() function, with
2411 png_data_freer(png_ptr, info_ptr, freer, mask)
2413 freer - one of
2418 mask - which data elements are affected
2419 same choices as in png_free_data()
2421 This function only affects data that has already been allocated.
2422 You can call this function after reading the PNG data but before calling
2423 any png_set_*() functions, to control whether the user or the png_set_*()
2424 function is responsible for freeing any existing data that might be present,
2425 and again after the png_set_*() functions to control whether the user
2426 or png_destroy_*() is supposed to free the data. When the user assumes
2427 responsibility for libpng-allocated data, the application must use
2428 png_free() to free it, and when the user transfers responsibility to libpng
2429 for data that the user has allocated, the user must have used png_malloc()
2430 or png_calloc() to allocate it.
2432 If you allocated your row_pointers in a single block, as suggested above in
2433 the description of the high level read interface, you must not transfer
2434 responsibility for freeing it to the png_set_rows or png_read_destroy function,
2435 because they would also try to free the individual row_pointers[i].
2437 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
2438 separately, do not transfer responsibility for freeing text_ptr to libpng,
2439 because when libpng fills a png_text structure it combines these members with
2440 the key member, and png_free_data() will free only text_ptr.key. Similarly,
2441 if you transfer responsibility for free'ing text_ptr from libpng to your
2442 application, your application must not separately free those members.
2444 The png_free_data() function will turn off the "valid" flag for anything
2445 it frees. If you need to turn the flag off for a chunk that was freed by
2446 your application instead of by libpng, you can use
2448 png_set_invalid(png_ptr, info_ptr, mask);
2450 mask - identifies the chunks to be made invalid,
2451 containing the bitwise OR of one or
2452 more of
2462 For a more compact example of reading a PNG image, see the file example.c.
2464 Reading PNG files progressively
2466 The progressive reader is slightly different from the non-progressive
2467 reader. Instead of calling png_read_info(), png_read_rows(), and
2468 png_read_end(), you make one call to png_process_data(), which calls
2469 callbacks when it has the info, a row, or the end of the image. You
2470 set up these callbacks with png_set_progressive_read_fn(). You don't
2471 have to worry about the input/output functions of libpng, as you are
2472 giving the library the data directly in png_process_data(). I will
2473 assume that you have read the section on reading PNG files above,
2474 so I will only highlight the differences (although I will show
2475 all of the code).
2477 png_structp png_ptr;
2478 png_infop info_ptr;
2480 /* An example code fragment of how you would
2481 initialize the progressive reader in your
2482 application. */
2483 int
2484 initialize_png_reader()
2485 {
2486 png_ptr = png_create_read_struct
2487 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
2488 user_error_fn, user_warning_fn);
2490 if (!png_ptr)
2491 return (ERROR);
2493 info_ptr = png_create_info_struct(png_ptr);
2495 if (!info_ptr)
2496 {
2497 png_destroy_read_struct(&png_ptr,
2498 (png_infopp)NULL, (png_infopp)NULL);
2499 return (ERROR);
2500 }
2502 if (setjmp(png_jmpbuf(png_ptr)))
2503 {
2504 png_destroy_read_struct(&png_ptr, &info_ptr,
2505 (png_infopp)NULL);
2506 return (ERROR);
2507 }
2509 /* This one's new. You can provide functions
2510 to be called when the header info is valid,
2511 when each row is completed, and when the image
2512 is finished. If you aren't using all functions,
2513 you can specify NULL parameters. Even when all
2514 three functions are NULL, you need to call
2515 png_set_progressive_read_fn(). You can use
2516 any struct as the user_ptr (cast to a void pointer
2517 for the function call), and retrieve the pointer
2518 from inside the callbacks using the function
2520 png_get_progressive_ptr(png_ptr);
2522 which will return a void pointer, which you have
2523 to cast appropriately.
2524 */
2525 png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
2526 info_callback, row_callback, end_callback);
2528 return 0;
2529 }
2531 /* A code fragment that you call as you receive blocks
2532 of data */
2533 int
2534 process_data(png_bytep buffer, png_uint_32 length)
2535 {
2536 if (setjmp(png_jmpbuf(png_ptr)))
2537 {
2538 png_destroy_read_struct(&png_ptr, &info_ptr,
2539 (png_infopp)NULL);
2540 return (ERROR);
2541 }
2543 /* This one's new also. Simply give it a chunk
2544 of data from the file stream (in order, of
2545 course). On machines with segmented memory
2546 models machines, don't give it any more than
2547 64K. The library seems to run fine with sizes
2548 of 4K. Although you can give it much less if
2549 necessary (I assume you can give it chunks of
2550 1 byte, I haven't tried less than 256 bytes
2551 yet). When this function returns, you may
2552 want to display any rows that were generated
2553 in the row callback if you don't already do
2554 so there.
2555 */
2556 png_process_data(png_ptr, info_ptr, buffer, length);
2558 /* At this point you can call png_process_data_skip if
2559 you want to handle data the library will skip yourself;
2560 it simply returns the number of bytes to skip (and stops
2561 libpng skipping that number of bytes on the next
2562 png_process_data call).
2563 return 0;
2564 }
2566 /* This function is called (as set by
2567 png_set_progressive_read_fn() above) when enough data
2568 has been supplied so all of the header has been
2569 read.
2570 */
2571 void
2572 info_callback(png_structp png_ptr, png_infop info)
2573 {
2574 /* Do any setup here, including setting any of
2575 the transformations mentioned in the Reading
2576 PNG files section. For now, you _must_ call
2577 either png_start_read_image() or
2578 png_read_update_info() after all the
2579 transformations are set (even if you don't set
2580 any). You may start getting rows before
2581 png_process_data() returns, so this is your
2582 last chance to prepare for that.
2584 This is where you turn on interlace handling,
2585 assuming you don't want to do it yourself.
2587 If you need to you can stop the processing of
2588 your original input data at this point by calling
2589 png_process_data_pause. This returns the number
2590 of unprocessed bytes from the last png_process_data
2591 call - it is up to you to ensure that the next call
2592 sees these bytes again. If you don't want to bother
2593 with this you can get libpng to cache the unread
2594 bytes by setting the 'save' parameter (see png.h) but
2595 then libpng will have to copy the data internally.
2596 */
2597 }
2599 /* This function is called when each row of image
2600 data is complete */
2601 void
2602 row_callback(png_structp png_ptr, png_bytep new_row,
2603 png_uint_32 row_num, int pass)
2604 {
2605 /* If the image is interlaced, and you turned
2606 on the interlace handler, this function will
2607 be called for every row in every pass. Some
2608 of these rows will not be changed from the
2609 previous pass. When the row is not changed,
2610 the new_row variable will be NULL. The rows
2611 and passes are called in order, so you don't
2612 really need the row_num and pass, but I'm
2613 supplying them because it may make your life
2614 easier.
2616 If you did not turn on interlace handling then
2617 the callback is called for each row of each
2618 sub-image when the image is interlaced. In this
2619 case 'row_num' is the row in the sub-image, not
2620 the row in the output image as it is in all other
2621 cases.
2623 For the non-NULL rows of interlaced images when
2624 you have switched on libpng interlace handling,
2625 you must call png_progressive_combine_row()
2626 passing in the row and the old row. You can
2627 call this function for NULL rows (it will just
2628 return) and for non-interlaced images (it just
2629 does the memcpy for you) if it will make the
2630 code easier. Thus, you can just do this for
2631 all cases if you switch on interlace handling;
2632 */
2634 png_progressive_combine_row(png_ptr, old_row,
2635 new_row);
2637 /* where old_row is what was displayed
2638 previously for the row. Note that the first
2639 pass (pass == 0, really) will completely cover
2640 the old row, so the rows do not have to be
2641 initialized. After the first pass (and only
2642 for interlaced images), you will have to pass
2643 the current row, and the function will combine
2644 the old row and the new row.
2646 You can also call png_process_data_pause in this
2647 callback - see above.
2648 */
2649 }
2651 void
2652 end_callback(png_structp png_ptr, png_infop info)
2653 {
2654 /* This function is called after the whole image
2655 has been read, including any chunks after the
2656 image (up to and including the IEND). You
2657 will usually have the same info chunk as you
2658 had in the header, although some data may have
2659 been added to the comments and time fields.
2661 Most people won't do much here, perhaps setting
2662 a flag that marks the image as finished.
2663 */
2664 }
2668 IV. Writing
2670 Much of this is very similar to reading. However, everything of
2671 importance is repeated here, so you won't have to constantly look
2672 back up in the reading section to understand writing.
2674 Setup
2676 You will want to do the I/O initialization before you get into libpng,
2677 so if it doesn't work, you don't have anything to undo. If you are not
2678 using the standard I/O functions, you will need to replace them with
2679 custom writing functions. See the discussion under Customizing libpng.
2681 FILE *fp = fopen(file_name, "wb");
2683 if (!fp)
2684 return (ERROR);
2686 Next, png_struct and png_info need to be allocated and initialized.
2687 As these can be both relatively large, you may not want to store these
2688 on the stack, unless you have stack space to spare. Of course, you
2689 will want to check if they return NULL. If you are also reading,
2690 you won't want to name your read structure and your write structure
2691 both "png_ptr"; you can call them anything you like, such as
2692 "read_ptr" and "write_ptr". Look at pngtest.c, for example.
2694 png_structp png_ptr = png_create_write_struct
2695 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
2696 user_error_fn, user_warning_fn);
2698 if (!png_ptr)
2699 return (ERROR);
2701 png_infop info_ptr = png_create_info_struct(png_ptr);
2702 if (!info_ptr)
2703 {
2704 png_destroy_write_struct(&png_ptr,
2705 (png_infopp)NULL);
2706 return (ERROR);
2707 }
2709 If you want to use your own memory allocation routines,
2710 define PNG_USER_MEM_SUPPORTED and use
2711 png_create_write_struct_2() instead of png_create_write_struct():
2713 png_structp png_ptr = png_create_write_struct_2
2714 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
2715 user_error_fn, user_warning_fn, (png_voidp)
2716 user_mem_ptr, user_malloc_fn, user_free_fn);
2718 After you have these structures, you will need to set up the
2719 error handling. When libpng encounters an error, it expects to
2720 longjmp() back to your routine. Therefore, you will need to call
2721 setjmp() and pass the png_jmpbuf(png_ptr). If you
2722 write the file from different routines, you will need to update
2723 the png_jmpbuf(png_ptr) every time you enter a new routine that will
2724 call a png_*() function. See your documentation of setjmp/longjmp
2725 for your compiler for more information on setjmp/longjmp. See
2726 the discussion on libpng error handling in the Customizing Libpng
2727 section below for more information on the libpng error handling.
2729 if (setjmp(png_jmpbuf(png_ptr)))
2730 {
2731 png_destroy_write_struct(&png_ptr, &info_ptr);
2732 fclose(fp);
2733 return (ERROR);
2734 }
2735 ...
2736 return;
2738 If you would rather avoid the complexity of setjmp/longjmp issues,
2739 you can compile libpng with PNG_NO_SETJMP, in which case
2740 errors will result in a call to PNG_ABORT() which defaults to abort().
2742 You can #define PNG_ABORT() to a function that does something
2743 more useful than abort(), as long as your function does not
2744 return.
2746 Checking for invalid palette index on write was added at libpng
2747 1.5.10. If a pixel contains an invalid (out-of-range) index libpng issues
2748 a benign error. This is enabled by default because this condition is an
2749 error according to the PNG specification, Clause 11.3.2, but the error can
2750 be ignored in each png_ptr with
2752 png_set_check_for_invalid_index(png_ptr, 0);
2754 If the error is ignored, or if png_benign_error() treats it as a warning,
2755 any invalid pixels are written as-is by the encoder, resulting in an
2756 invalid PNG datastream as output. In this case the application is
2757 responsible for ensuring that the pixel indexes are in range when it writes
2758 a PLTE chunk with fewer entries than the bit depth would allow.
2760 Now you need to set up the output code. The default for libpng is to
2761 use the C function fwrite(). If you use this, you will need to pass a
2762 valid FILE * in the function png_init_io(). Be sure that the file is
2763 opened in binary mode. Again, if you wish to handle writing data in
2764 another way, see the discussion on libpng I/O handling in the Customizing
2765 Libpng section below.
2767 png_init_io(png_ptr, fp);
2769 If you are embedding your PNG into a datastream such as MNG, and don't
2770 want libpng to write the 8-byte signature, or if you have already
2771 written the signature in your application, use
2773 png_set_sig_bytes(png_ptr, 8);
2775 to inform libpng that it should not write a signature.
2777 Write callbacks
2779 At this point, you can set up a callback function that will be
2780 called after each row has been written, which you can use to control
2781 a progress meter or the like. It's demonstrated in pngtest.c.
2782 You must supply a function
2784 void write_row_callback(png_structp png_ptr, png_uint_32 row,
2785 int pass);
2786 {
2787 /* put your code here */
2788 }
2790 (You can give it another name that you like instead of "write_row_callback")
2792 To inform libpng about your function, use
2794 png_set_write_status_fn(png_ptr, write_row_callback);
2796 When this function is called the row has already been completely processed and
2797 it has also been written out. The 'row' and 'pass' refer to the next row to be
2798 handled. For the
2799 non-interlaced case the row that was just handled is simply one less than the
2800 passed in row number, and pass will always be 0. For the interlaced case the
2801 same applies unless the row value is 0, in which case the row just handled was
2802 the last one from one of the preceding passes. Because interlacing may skip a
2803 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
2804 need to know what the last pass is record (row,pass) from the callback and use
2805 the last recorded value each time.
2807 As with the user transform you can find the output row using the
2810 You now have the option of modifying how the compression library will
2811 run. The following functions are mainly for testing, but may be useful
2812 in some cases, like if you need to write PNG files extremely fast and
2813 are willing to give up some compression, or if you want to get the
2814 maximum possible compression at the expense of slower writing. If you
2815 have no special needs in this area, let the library do what it wants by
2816 not calling this function at all, as it has been tuned to deliver a good
2817 speed/compression ratio. The second parameter to png_set_filter() is
2818 the filter method, for which the only valid values are 0 (as of the
2819 July 1999 PNG specification, version 1.2) or 64 (if you are writing
2820 a PNG datastream that is to be embedded in a MNG datastream). The third
2821 parameter is a flag that indicates which filter type(s) are to be tested
2822 for each scanline. See the PNG specification for details on the specific
2823 filter types.
2826 /* turn on or off filtering, and/or choose
2827 specific filters. You can use either a single
2828 PNG_FILTER_VALUE_NAME or the bitwise OR of one
2829 or more PNG_FILTER_NAME masks.
2830 */
2831 png_set_filter(png_ptr, 0,
2839 If an application wants to start and stop using particular filters during
2840 compression, it should start out with all of the filters (to ensure that
2841 the previous row of pixels will be stored in case it's needed later),
2842 and then add and remove them after the start of compression.
2844 If you are writing a PNG datastream that is to be embedded in a MNG
2845 datastream, the second parameter can be either 0 or 64.
2847 The png_set_compression_*() functions interface to the zlib compression
2848 library, and should mostly be ignored unless you really know what you are
2849 doing. The only generally useful call is png_set_compression_level()
2850 which changes how much time zlib spends on trying to compress the image
2851 data. See the Compression Library (zlib.h and algorithm.txt, distributed
2852 with zlib) for details on the compression levels.
2854 #include zlib.h
2856 /* Set the zlib compression level */
2857 png_set_compression_level(png_ptr,
2860 /* Set other zlib parameters for compressing IDAT */
2861 png_set_compression_mem_level(png_ptr, 8);
2862 png_set_compression_strategy(png_ptr,
2864 png_set_compression_window_bits(png_ptr, 15);
2865 png_set_compression_method(png_ptr, 8);
2866 png_set_compression_buffer_size(png_ptr, 8192)
2868 /* Set zlib parameters for text compression
2869 * If you don't call these, the parameters
2870 * fall back on those defined for IDAT chunks
2871 */
2872 png_set_text_compression_mem_level(png_ptr, 8);
2873 png_set_text_compression_strategy(png_ptr,
2875 png_set_text_compression_window_bits(png_ptr, 15);
2876 png_set_text_compression_method(png_ptr, 8);
2878 Setting the contents of info for output
2880 You now need to fill in the png_info structure with all the data you
2881 wish to write before the actual image. Note that the only thing you
2882 are allowed to write after the image is the text chunks and the time
2883 chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
2884 the latest PNG specification for more information on that. If you
2885 wish to write them before the image, fill them in now, and flag that
2886 data as being valid. If you want to wait until after the data, don't
2887 fill them until png_write_end(). For all the fields in png_info and
2888 their data types, see png.h. For explanations of what the fields
2889 contain, see the PNG specification.
2891 Some of the more important parts of the png_info are:
2893 png_set_IHDR(png_ptr, info_ptr, width, height,
2894 bit_depth, color_type, interlace_type,
2895 compression_type, filter_method)
2897 width - holds the width of the image
2898 in pixels (up to 2^31).
2900 height - holds the height of the image
2901 in pixels (up to 2^31).
2903 bit_depth - holds the bit depth of one of the
2904 image channels.
2905 (valid values are 1, 2, 4, 8, 16
2906 and depend also on the
2907 color_type. See also significant
2908 bits (sBIT) below).
2910 color_type - describes which color/alpha
2911 channels are present.
2913 (bit depths 1, 2, 4, 8, 16)
2915 (bit depths 8, 16)
2917 (bit depths 1, 2, 4, 8)
2919 (bit_depths 8, 16)
2921 (bit_depths 8, 16)
2927 interlace_type - PNG_INTERLACE_NONE or
2930 compression_type - (must be
2933 filter_method - (must be PNG_FILTER_TYPE_DEFAULT
2934 or, if you are writing a PNG to
2935 be embedded in a MNG datastream,
2936 can also be
2939 If you call png_set_IHDR(), the call must appear before any of the
2940 other png_set_*() functions, because they might require access to some of
2941 the IHDR settings. The remaining png_set_*() functions can be called
2942 in any order.
2944 If you wish, you can reset the compression_type, interlace_type, or
2945 filter_method later by calling png_set_IHDR() again; if you do this, the
2946 width, height, bit_depth, and color_type must be the same in each call.
2948 png_set_PLTE(png_ptr, info_ptr, palette,
2949 num_palette);
2951 palette - the palette for the file
2952 (array of png_color)
2953 num_palette - number of entries in the palette
2955 png_set_gAMA(png_ptr, info_ptr, file_gamma);
2956 png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
2958 file_gamma - the gamma at which the image was
2959 created (PNG_INFO_gAMA)
2961 int_file_gamma - 100,000 times the gamma at which
2962 the image was created
2964 png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y,
2965 green_x, green_y, blue_x, blue_y)
2966 png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
2967 green_Y, green_Z, blue_X, blue_Y, blue_Z)
2968 png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
2969 int_red_x, int_red_y, int_green_x, int_green_y,
2970 int_blue_x, int_blue_y)
2971 png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
2972 int_red_Z, int_green_X, int_green_Y, int_green_Z,
2973 int_blue_X, int_blue_Y, int_blue_Z)
2975 {white,red,green,blue}_{x,y}
2976 A color space encoding specified using the chromaticities
2977 of the end points and the white point.
2979 {red,green,blue}_{X,Y,Z}
2980 A color space encoding specified using the encoding end
2981 points - the CIE tristimulus specification of the intended
2982 color of the red, green and blue channels in the PNG RGB
2983 data. The white point is simply the sum of the three end
2984 points.
2986 png_set_sRGB(png_ptr, info_ptr, srgb_intent);
2988 srgb_intent - the rendering intent
2989 (PNG_INFO_sRGB) The presence of
2990 the sRGB chunk means that the pixel
2991 data is in the sRGB color space.
2992 This chunk also implies specific
2993 values of gAMA and cHRM. Rendering
2994 intent is the CSS-1 property that
2995 has been defined by the International
2996 Color Consortium
2997 (http://www.color.org).
2998 It can be one of
3005 png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
3006 srgb_intent);
3008 srgb_intent - the rendering intent
3009 (PNG_INFO_sRGB) The presence of the
3010 sRGB chunk means that the pixel
3011 data is in the sRGB color space.
3012 This function also causes gAMA and
3013 cHRM chunks with the specific values
3014 that are consistent with sRGB to be
3015 written.
3017 png_set_iCCP(png_ptr, info_ptr, name, compression_type,
3018 profile, proflen);
3020 name - The profile name.
3022 compression_type - The compression type; always
3024 You may give NULL to this argument to
3025 ignore it.
3027 profile - International Color Consortium color
3028 profile data. May contain NULs.
3030 proflen - length of profile data in bytes.
3032 png_set_sBIT(png_ptr, info_ptr, sig_bit);
3034 sig_bit - the number of significant bits for
3035 (PNG_INFO_sBIT) each of the gray, red,
3036 green, and blue channels, whichever are
3037 appropriate for the given color type
3038 (png_color_16)
3040 png_set_tRNS(png_ptr, info_ptr, trans_alpha,
3041 num_trans, trans_color);
3043 trans_alpha - array of alpha (transparency)
3044 entries for palette (PNG_INFO_tRNS)
3046 num_trans - number of transparent entries
3047 (PNG_INFO_tRNS)
3049 trans_color - graylevel or color sample values
3050 (in order red, green, blue) of the
3051 single transparent color for
3052 non-paletted images (PNG_INFO_tRNS)
3054 png_set_hIST(png_ptr, info_ptr, hist);
3056 hist - histogram of palette (array of
3057 png_uint_16) (PNG_INFO_hIST)
3059 png_set_tIME(png_ptr, info_ptr, mod_time);
3061 mod_time - time image was last modified
3064 png_set_bKGD(png_ptr, info_ptr, background);
3066 background - background color (of type
3067 png_color_16p) (PNG_VALID_bKGD)
3069 png_set_text(png_ptr, info_ptr, text_ptr, num_text);
3071 text_ptr - array of png_text holding image
3074 text_ptr[i].compression - type of compression used
3079 text_ptr[i].key - keyword for comment. Must contain
3080 1-79 characters.
3081 text_ptr[i].text - text comments for current
3082 keyword. Can be NULL or empty.
3083 text_ptr[i].text_length - length of text string,
3084 after decompression, 0 for iTXt
3085 text_ptr[i].itxt_length - length of itxt string,
3086 after decompression, 0 for tEXt/zTXt
3087 text_ptr[i].lang - language of comment (NULL or
3088 empty for unknown).
3089 text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
3090 or empty for unknown).
3092 Note that the itxt_length, lang, and lang_key
3093 members of the text_ptr structure only exist when the
3094 library is built with iTXt chunk support. Prior to
3095 libpng-1.4.0 the library was built by default without
3096 iTXt support. Also note that when iTXt is supported,
3097 they contain NULL pointers when the "compression"
3098 field contains PNG_TEXT_COMPRESSION_NONE or
3101 num_text - number of comments
3103 png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
3104 num_spalettes);
3106 palette_ptr - array of png_sPLT_struct structures
3107 to be added to the list of palettes
3108 in the info structure.
3109 num_spalettes - number of palette structures to be
3110 added.
3112 png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
3113 unit_type);
3115 offset_x - positive offset from the left
3116 edge of the screen
3118 offset_y - positive offset from the top
3119 edge of the screen
3123 png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
3124 unit_type);
3126 res_x - pixels/unit physical resolution
3127 in x direction
3129 res_y - pixels/unit physical resolution
3130 in y direction
3132 unit_type - PNG_RESOLUTION_UNKNOWN,
3135 png_set_sCAL(png_ptr, info_ptr, unit, width, height)
3137 unit - physical scale units (an integer)
3139 width - width of a pixel in physical scale units
3141 height - height of a pixel in physical scale units
3142 (width and height are doubles)
3144 png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
3146 unit - physical scale units (an integer)
3148 width - width of a pixel in physical scale units
3149 expressed as a string
3151 height - height of a pixel in physical scale units
3152 (width and height are strings like "2.54")
3154 png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
3155 num_unknowns)
3157 unknowns - array of png_unknown_chunk
3158 structures holding unknown chunks
3159 unknowns[i].name - name of unknown chunk
3160 unknowns[i].data - data of unknown chunk
3161 unknowns[i].size - size of unknown chunk's data
3162 unknowns[i].location - position to write chunk in file
3163 0: do not write chunk
3164 PNG_HAVE_IHDR: before PLTE
3165 PNG_HAVE_PLTE: before IDAT
3168 The "location" member is set automatically according to
3169 what part of the output file has already been written.
3170 You can change its value after calling png_set_unknown_chunks()
3171 as demonstrated in pngtest.c. Within each of the "locations",
3172 the chunks are sequenced according to their position in the
3173 structure (that is, the value of "i", which is the order in which
3174 the chunk was either read from the input file or defined with
3175 png_set_unknown_chunks).
3177 A quick word about text and num_text. text is an array of png_text
3178 structures. num_text is the number of valid structures in the array.
3179 Each png_text structure holds a language code, a keyword, a text value,
3180 and a compression type.
3182 The compression types have the same valid numbers as the compression
3183 types of the image data. Currently, the only valid number is zero.
3184 However, you can store text either compressed or uncompressed, unlike
3185 images, which always have to be compressed. So if you don't want the
3186 text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
3187 Because tEXt and zTXt chunks don't have a language field, if you
3189 any language code or translated keyword will not be written out.
3191 Until text gets around a few hundred bytes, it is not worth compressing it.
3192 After the text has been written out to the file, the compression type
3194 so that it isn't written out again at the end (in case you are calling
3195 png_write_end() with the same struct).
3197 The keywords that are given in the PNG Specification are:
3199 Title Short (one line) title or
3200 caption for image
3202 Author Name of image's creator
3204 Description Description of image (possibly long)
3206 Copyright Copyright notice
3208 Creation Time Time of original image creation
3209 (usually RFC 1123 format, see below)
3211 Software Software used to create the image
3213 Disclaimer Legal disclaimer
3215 Warning Warning of nature of content
3217 Source Device used to create the image
3219 Comment Miscellaneous comment; conversion
3220 from other image format
3222 The keyword-text pairs work like this. Keywords should be short
3223 simple descriptions of what the comment is about. Some typical
3224 keywords are found in the PNG specification, as is some recommendations
3225 on keywords. You can repeat keywords in a file. You can even write
3226 some text before the image and some after. For example, you may want
3227 to put a description of the image before the image, but leave the
3228 disclaimer until after, so viewers working over modem connections
3229 don't have to wait for the disclaimer to go over the modem before
3230 they start seeing the image. Finally, keywords should be full
3231 words, not abbreviations. Keywords and text are in the ISO 8859-1
3232 (Latin-1) character set (a superset of regular ASCII) and can not
3233 contain NUL characters, and should not contain control or other
3234 unprintable characters. To make the comments widely readable, stick
3235 with basic ASCII, and avoid machine specific character set extensions
3236 like the IBM-PC character set. The keyword must be present, but
3237 you can leave off the text string on non-compressed pairs.
3238 Compressed pairs must have a text string, as only the text string
3239 is compressed anyway, so the compression would be meaningless.
3241 PNG supports modification time via the png_time structure. Two
3242 conversion routines are provided, png_convert_from_time_t() for
3243 time_t and png_convert_from_struct_tm() for struct tm. The
3244 time_t routine uses gmtime(). You don't have to use either of
3245 these, but if you wish to fill in the png_time structure directly,
3246 you should provide the time in universal time (GMT) if possible
3247 instead of your local time. Note that the year number is the full
3248 year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
3249 that months start with 1.
3251 If you want to store the time of the original image creation, you should
3252 use a plain tEXt chunk with the "Creation Time" keyword. This is
3253 necessary because the "creation time" of a PNG image is somewhat vague,
3254 depending on whether you mean the PNG file, the time the image was
3255 created in a non-PNG format, a still photo from which the image was
3256 scanned, or possibly the subject matter itself. In order to facilitate
3257 machine-readable dates, it is recommended that the "Creation Time"
3258 tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
3259 although this isn't a requirement. Unlike the tIME chunk, the
3260 "Creation Time" tEXt chunk is not expected to be automatically changed
3261 by the software. To facilitate the use of RFC 1123 dates, a function
3262 png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to
3263 convert from PNG time to an RFC 1123 format string. The caller must provide
3264 a writeable buffer of at least 29 bytes.
3266 Writing unknown chunks
3268 You can use the png_set_unknown_chunks function to queue up private chunks
3269 for writing. You give it a chunk name, location, raw data, and a size. You
3270 also must use png_set_keep_unknown_chunks() to ensure that libpng will
3271 handle them. That's all there is to it. The chunks will be written by the
3272 next following png_write_info_before_PLTE, png_write_info, or png_write_end
3273 function, depending upon the specified location. Any chunks previously
3274 read into the info structure's unknown-chunk list will also be written out
3275 in a sequence that satisfies the PNG specification's ordering rules.
3277 Here is an example of writing two private chunks, prVt and miNE:
3280 /* Set unknown chunk data */
3281 png_unknown_chunk unk_chunk[2];
3282 strcpy((char *) unk_chunk[0].name, "prVt";
3283 unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
3284 unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
3285 unk_chunk[0].location = PNG_HAVE_IHDR;
3286 strcpy((char *) unk_chunk[1].name, "miNE";
3287 unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
3288 unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
3289 unk_chunk[1].location = PNG_AFTER_IDAT;
3290 png_set_unknown_chunks(write_ptr, write_info_ptr,
3291 unk_chunk, 2);
3292 /* Needed because miNE is not safe-to-copy */
3293 png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
3294 (png_bytep) "miNE", 1);
3295 # if PNG_LIBPNG_VER < 10600
3296 /* Deal with unknown chunk location bug in 1.5.x and earlier */
3297 png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
3298 png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
3299 # endif
3300 # if PNG_LIBPNG_VER < 10500
3301 /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0,
3302 * one before IDAT and another after IDAT, so don't use it; only use
3303 * PNG_HAVE_IHDR location. This call resets the location previously
3304 * set by assignment and png_set_unknown_chunk_location() for chunk 1.
3305 */
3306 png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
3307 # endif
3308 #endif
3310 The high-level write interface
3312 At this point there are two ways to proceed; through the high-level
3313 write interface, or through a sequence of low-level write operations.
3314 You can use the high-level interface if your image data is present
3315 in the info structure. All defined output
3316 transformations are permitted, enabled by the following masks.
3318 PNG_TRANSFORM_IDENTITY No transformation
3319 PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
3320 PNG_TRANSFORM_PACKSWAP Change order of packed
3321 pixels to LSB first
3322 PNG_TRANSFORM_INVERT_MONO Invert monochrome images
3323 PNG_TRANSFORM_SHIFT Normalize pixels to the
3324 sBIT depth
3326 to BGRA
3328 to AG
3329 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
3330 to transparency
3331 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
3332 PNG_TRANSFORM_STRIP_FILLER Strip out filler
3333 bytes (deprecated).
3335 filler bytes
3337 filler bytes
3339 If you have valid image data in the info structure (you can use
3340 png_set_rows() to put image data in the info structure), simply do this:
3342 png_write_png(png_ptr, info_ptr, png_transforms, NULL)
3344 where png_transforms is an integer containing the bitwise OR of some set of
3345 transformation flags. This call is equivalent to png_write_info(),
3346 followed the set of transformations indicated by the transform mask,
3347 then png_write_image(), and finally png_write_end().
3349 (The final parameter of this call is not yet used. Someday it might point
3350 to transformation parameters required by some future output transform.)
3352 You must use png_transforms and not call any png_set_transform() functions
3353 when you use png_write_png().
3355 The low-level write interface
3357 If you are going the low-level route instead, you are now ready to
3358 write all the file information up to the actual image data. You do
3359 this with a call to png_write_info().
3361 png_write_info(png_ptr, info_ptr);
3363 Note that there is one transformation you may need to do before
3364 png_write_info(). In PNG files, the alpha channel in an image is the
3365 level of opacity. If your data is supplied as a level of transparency,
3366 you can invert the alpha channel before you write it, so that 0 is
3367 fully transparent and 255 (in 8-bit or paletted images) or 65535
3368 (in 16-bit images) is fully opaque, with
3370 png_set_invert_alpha(png_ptr);
3372 This must appear before png_write_info() instead of later with the
3373 other transformations because in the case of paletted images the tRNS
3374 chunk data has to be inverted before the tRNS chunk is written. If
3375 your image is not a paletted image, the tRNS data (which in such cases
3376 represents a single color to be rendered as transparent) won't need to
3377 be changed, and you can safely do this transformation after your
3378 png_write_info() call.
3380 If you need to write a private chunk that you want to appear before
3381 the PLTE chunk when PLTE is present, you can write the PNG info in
3382 two steps, and insert code to write your own chunk between them:
3384 png_write_info_before_PLTE(png_ptr, info_ptr);
3385 png_set_unknown_chunks(png_ptr, info_ptr, ...);
3386 png_write_info(png_ptr, info_ptr);
3388 After you've written the file information, you can set up the library
3389 to handle any special transformations of the image data. The various
3390 ways to transform the data will be described in the order that they
3391 should occur. This is important, as some of these change the color
3392 type and/or bit depth of the data, and some others only work on
3393 certain color types and bit depths. Even though each transformation
3394 checks to see if it has data that it can do something with, you should
3395 make sure to only enable a transformation if it will be valid for the
3396 data. For example, don't swap red and blue on grayscale data.
3398 PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
3399 the library to strip input data that has 4 or 8 bytes per pixel down
3400 to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
3401 bytes per pixel).
3403 png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
3405 where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
3406 PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
3407 is stored XRGB or RGBX.
3409 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
3410 they can, resulting in, for example, 8 pixels per byte for 1 bit files.
3411 If the data is supplied at 1 pixel per byte, use this code, which will
3412 correctly pack the pixels into a single byte:
3414 png_set_packing(png_ptr);
3416 PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
3417 data is of another bit depth, you can write an sBIT chunk into the
3418 file so that decoders can recover the original data if desired.
3420 /* Set the true bit depth of the image data */
3421 if (color_type & PNG_COLOR_MASK_COLOR)
3422 {
3423 sig_bit.red = true_bit_depth;
3424 sig_bit.green = true_bit_depth;
3425 sig_bit.blue = true_bit_depth;
3426 }
3428 else
3429 {
3430 sig_bit.gray = true_bit_depth;
3431 }
3433 if (color_type & PNG_COLOR_MASK_ALPHA)
3434 {
3435 sig_bit.alpha = true_bit_depth;
3436 }
3438 png_set_sBIT(png_ptr, info_ptr, &sig_bit);
3440 If the data is stored in the row buffer in a bit depth other than
3441 one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
3442 this will scale the values to appear to be the correct bit depth as
3443 is required by PNG.
3445 png_set_shift(png_ptr, &sig_bit);
3447 PNG files store 16-bit pixels in network byte order (big-endian,
3448 ie. most significant bits first). This code would be used if they are
3449 supplied the other way (little-endian, i.e. least significant bits
3450 first, the way PCs store them):
3452 if (bit_depth > 8)
3453 png_set_swap(png_ptr);
3455 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
3456 need to change the order the pixels are packed into bytes, you can use:
3458 if (bit_depth < 8)
3459 png_set_packswap(png_ptr);
3461 PNG files store 3 color pixels in red, green, blue order. This code
3462 would be used if they are supplied as blue, green, red:
3464 png_set_bgr(png_ptr);
3466 PNG files describe monochrome as black being zero and white being
3467 one. This code would be used if the pixels are supplied with this reversed
3468 (black being one and white being zero):
3470 png_set_invert_mono(png_ptr);
3472 Finally, you can write your own transformation function if none of
3473 the existing ones meets your needs. This is done by setting a callback
3474 with
3476 png_set_write_user_transform_fn(png_ptr,
3477 write_transform_fn);
3479 You must supply the function
3481 void write_transform_fn(png_structp png_ptr, png_row_infop
3482 row_info, png_bytep data)
3484 See pngtest.c for a working example. Your function will be called
3485 before any of the other transformations are processed. If supported
3486 libpng also supplies an information routine that may be called from
3487 your callback:
3489 png_get_current_row_number(png_ptr);
3490 png_get_current_pass_number(png_ptr);
3492 This returns the current row passed to the transform. With interlaced
3493 images the value returned is the row in the input sub-image image. Use
3494 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
3495 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
3497 The discussion of interlace handling above contains more information on how to
3498 use these values.
3500 You can also set up a pointer to a user structure for use by your
3501 callback function.
3503 png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
3505 The user_channels and user_depth parameters of this function are ignored
3506 when writing; you can set them to zero as shown.
3508 You can retrieve the pointer via the function png_get_user_transform_ptr().
3509 For example:
3511 voidp write_user_transform_ptr =
3512 png_get_user_transform_ptr(png_ptr);
3514 It is possible to have libpng flush any pending output, either manually,
3515 or automatically after a certain number of lines have been written. To
3516 flush the output stream a single time call:
3518 png_write_flush(png_ptr);
3520 and to have libpng flush the output stream periodically after a certain
3521 number of scanlines have been written, call:
3523 png_set_flush(png_ptr, nrows);
3525 Note that the distance between rows is from the last time png_write_flush()
3526 was called, or the first row of the image if it has never been called.
3527 So if you write 50 lines, and then png_set_flush 25, it will flush the
3528 output on the next scanline, and every 25 lines thereafter, unless
3529 png_write_flush() is called before 25 more lines have been written.
3530 If nrows is too small (less than about 10 lines for a 640 pixel wide
3531 RGB image) the image compression may decrease noticeably (although this
3532 may be acceptable for real-time applications). Infrequent flushing will
3533 only degrade the compression performance by a few percent over images
3534 that do not use flushing.
3536 Writing the image data
3538 That's it for the transformations. Now you can write the image data.
3539 The simplest way to do this is in one function call. If you have the
3540 whole image in memory, you can just call png_write_image() and libpng
3541 will write the image. You will need to pass in an array of pointers to
3542 each row. This function automatically handles interlacing, so you don't
3543 need to call png_set_interlace_handling() or call this function multiple
3544 times, or any of that other stuff necessary with png_write_rows().
3546 png_write_image(png_ptr, row_pointers);
3548 where row_pointers is:
3550 png_byte *row_pointers[height];
3552 You can point to void or char or whatever you use for pixels.
3554 If you don't want to write the whole image at once, you can
3555 use png_write_rows() instead. If the file is not interlaced,
3556 this is simple:
3558 png_write_rows(png_ptr, row_pointers,
3559 number_of_rows);
3561 row_pointers is the same as in the png_write_image() call.
3563 If you are just writing one row at a time, you can do this with
3564 a single row_pointer instead of an array of row_pointers:
3566 png_bytep row_pointer = row;
3568 png_write_row(png_ptr, row_pointer);
3570 When the file is interlaced, things can get a good deal more complicated.
3571 The only currently (as of the PNG Specification version 1.2, dated July
3572 1999) defined interlacing scheme for PNG files is the "Adam7" interlace
3573 scheme, that breaks down an image into seven smaller images of varying
3574 size. libpng will build these images for you, or you can do them
3575 yourself. If you want to build them yourself, see the PNG specification
3576 for details of which pixels to write when.
3578 If you don't want libpng to handle the interlacing details, just
3579 use png_set_interlace_handling() and call png_write_rows() the
3580 correct number of times to write all the sub-images
3581 (png_set_interlace_handling() returns the number of sub-images.)
3583 If you want libpng to build the sub-images, call this before you start
3584 writing any rows:
3586 number_of_passes = png_set_interlace_handling(png_ptr);
3588 This will return the number of passes needed. Currently, this is seven,
3589 but may change if another interlace type is added.
3591 Then write the complete image number_of_passes times.
3593 png_write_rows(png_ptr, row_pointers, number_of_rows);
3595 Think carefully before you write an interlaced image. Typically code that
3596 reads such images reads all the image data into memory, uncompressed, before
3597 doing any processing. Only code that can display an image on the fly can
3598 take advantage of the interlacing and even then the image has to be exactly
3599 the correct size for the output device, because scaling an image requires
3600 adjacent pixels and these are not available until all the passes have been
3601 read.
3603 If you do write an interlaced image you will hardly ever need to handle
3604 the interlacing yourself. Call png_set_interlace_handling() and use the
3605 approach described above.
3607 The only time it is conceivable that you will really need to write an
3608 interlaced image pass-by-pass is when you have read one pass by pass and
3609 made some pixel-by-pixel transformation to it, as described in the read
3610 code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros
3611 to determine the size of each sub-image in turn and simply write the rows
3612 you obtained from the read code.
3614 Finishing a sequential write
3616 After you are finished writing the image, you should finish writing
3617 the file. If you are interested in writing comments or time, you should
3618 pass an appropriately filled png_info pointer. If you are not interested,
3619 you can pass NULL.
3621 png_write_end(png_ptr, info_ptr);
3623 When you are done, you can free all memory used by libpng like this:
3625 png_destroy_write_struct(&png_ptr, &info_ptr);
3627 It is also possible to individually free the info_ptr members that
3628 point to libpng-allocated storage with the following function:
3630 png_free_data(png_ptr, info_ptr, mask, seq)
3632 mask - identifies data to be freed, a mask
3633 containing the bitwise OR of one or
3634 more of
3640 or simply PNG_FREE_ALL
3642 seq - sequence number of item to be freed
3643 (-1 for all items)
3645 This function may be safely called when the relevant storage has
3646 already been freed, or has not yet been allocated, or was allocated
3647 by the user and not by libpng, and will in those cases do nothing.
3648 The "seq" parameter is ignored if only one item of the selected data
3649 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
3650 are allowed for the data type identified in the mask, such as text or
3651 sPLT, only the n'th item in the structure is freed, where n is "seq".
3653 If you allocated data such as a palette that you passed in to libpng
3654 with png_set_*, you must not free it until just before the call to
3655 png_destroy_write_struct().
3657 The default behavior is only to free data that was allocated internally
3658 by libpng. This can be changed, so that libpng will not free the data,
3659 or so that it will free data that was allocated by the user with png_malloc()
3660 or png_calloc() and passed in via a png_set_*() function, with
3662 png_data_freer(png_ptr, info_ptr, freer, mask)
3664 freer - one of
3669 mask - which data elements are affected
3670 same choices as in png_free_data()
3672 For example, to transfer responsibility for some data from a read structure
3673 to a write structure, you could use
3675 png_data_freer(read_ptr, read_info_ptr,
3679 png_data_freer(write_ptr, write_info_ptr,
3683 thereby briefly reassigning responsibility for freeing to the user but
3684 immediately afterwards reassigning it once more to the write_destroy
3685 function. Having done this, it would then be safe to destroy the read
3686 structure and continue to use the PLTE, tRNS, and hIST data in the write
3687 structure.
3689 This function only affects data that has already been allocated.
3690 You can call this function before calling after the png_set_*() functions
3691 to control whether the user or png_destroy_*() is supposed to free the data.
3692 When the user assumes responsibility for libpng-allocated data, the
3693 application must use
3694 png_free() to free it, and when the user transfers responsibility to libpng
3695 for data that the user has allocated, the user must have used png_malloc()
3696 or png_calloc() to allocate it.
3698 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
3699 separately, do not transfer responsibility for freeing text_ptr to libpng,
3700 because when libpng fills a png_text structure it combines these members with
3701 the key member, and png_free_data() will free only text_ptr.key. Similarly,
3702 if you transfer responsibility for free'ing text_ptr from libpng to your
3703 application, your application must not separately free those members.
3704 For a more compact example of writing a PNG image, see the file example.c.
3706 V. Simplified API
3708 The simplified API, which became available in libpng-1.6.0, hides the details
3709 of both libpng and the PNG file format itself.
3710 It allows PNG files to be read into a very limited number of
3711 in-memory bitmap formats or to be written from the same formats. If these
3712 formats do not accommodate your needs then you can, and should, use the more
3713 sophisticated APIs above - these support a wide variety of in-memory formats
3714 and a wide variety of sophisticated transformations to those formats as well
3715 as a wide variety of APIs to manipulate ancilliary information.
3717 To read a PNG file using the simplified API:
3719 1) Declare a 'png_image' structure (see below) on the
3720 stack and memset() it to all zero.
3722 2) Call the appropriate png_image_begin_read... function.
3724 3) Set the png_image 'format' member to the required
3725 format and allocate a buffer for the image.
3727 4) Call png_image_finish_read to read the image into
3728 your buffer.
3730 There are no restrictions on the format of the PNG input itself; all valid
3731 color types, bit depths, and interlace methods are acceptable, and the
3732 input image is transformed as necessary to the requested in-memory format
3733 during the png_image_finish_read() step.
3735 To write a PNG file using the simplified API:
3737 1) Declare a 'png_image' structure on the stack and memset()
3738 it to all zero.
3740 2) Initialize the members of the structure that describe the
3741 image, setting the 'format' member to the format of the
3742 image in memory.
3744 3) Call the appropriate png_image_write... function with a
3745 pointer to the image to write the PNG data.
3747 png_image is a structure that describes the in-memory format of an image
3748 when it is being read or define the in-memory format of an image that you
3749 need to write. The "png_image" structure contains the following members:
3751 png_uint_32 version Set to PNG_IMAGE_VERSION
3752 png_uint_32 width Image width in pixels (columns)
3753 png_uint_32 height Image height in pixels (rows)
3754 png_uint_32 format Image format as defined below
3755 png_uint_32 flags A bit mask containing informational flags
3756 png_controlp opaque Initialize to NULL, free with png_image_free
3757 png_uint_32 colormap_entries; Number of entries in the color-map
3758 png_uint_32 warning_or_error;
3759 char message[64];
3761 In the event of an error or warning the following field warning_or_error
3762 field will be set to a non-zero value and the 'message' field will contain
3763 a '\0' terminated string with the libpng error or warning message. If both
3764 warnings and an error were encountered, only the error is recorded. If there
3765 are multiple warnings, only the first one is recorded.
3767 The upper 30 bits of this value are reserved; the low two bits contain
3768 a two bit code such that a value more than 1 indicates a failure in the API
3769 just called:
3771 0 - no warning or error
3772 1 - warning
3773 2 - error
3774 3 - error preceded by warning
3776 The pixels (samples) of the image have one to four channels whose components
3777 have original values in the range 0 to 1.0:
3779 1: A single gray or luminance channel (G).
3780 2: A gray/luminance channel and an alpha channel (GA).
3781 3: Three red, green, blue color channels (RGB).
3782 4: Three color channels and an alpha channel (RGBA).
3784 The channels are encoded in one of two ways:
3786 a) As a small integer, value 0..255, contained in a single byte. For the
3787 alpha channel the original value is simply value/255. For the color or
3788 luminance channels the value is encoded according to the sRGB specification
3789 and matches the 8-bit format expected by typical display devices.
3791 The color/gray channels are not scaled (pre-multiplied) by the alpha
3792 channel and are suitable for passing to color management software.
3794 b) As a value in the range 0..65535, contained in a 2-byte integer, in
3795 the native byte order of the platform on which the application is running.
3796 All channels can be converted to the original value by dividing by 65535; all
3797 channels are linear. Color channels use the RGB encoding (RGB end-points) of
3798 the sRGB specification. This encoding is identified by the
3799 PNG_FORMAT_FLAG_LINEAR flag below.
3801 When an alpha channel is present it is expected to denote pixel coverage
3802 of the color or luminance channels and is returned as an associated alpha
3803 channel: the color/gray channels are scaled (pre-multiplied) by the alpha
3804 value.
3806 When a color-mapped image is used as a result of calling
3807 png_image_read_colormap or png_image_write_colormap the channels are encoded
3808 in the color-map and the descriptions above apply to the color-map entries.
3809 The image data is encoded as small integers, value 0..255, that index the
3810 entries in the color-map. One integer (one byte) is stored for each pixel.
3814 The #defines to be used in png_image::format. Each #define identifies a
3815 particular layout of channel data and, if present, alpha values. There are
3816 separate defines for each of the two channel encodings.
3818 A format is built up using single bit flag values. Not all combinations are
3819 valid: use the bit flag values below for testing a format returned by the
3820 read APIs, but set formats from the derived values.
3822 When reading or writing color-mapped images the format should be set to the
3823 format of the entries in the color-map then png_image_{read,write}_colormap
3824 called to read or write the color-map and set the format correctly for the
3825 image data. Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly!
3827 NOTE: libpng can be built with particular features disabled, if you see
3828 compiler errors because the definition of one of the following flags has been
3829 compiled out it is because libpng does not have the required support. It is
3830 possible, however, for the libpng configuration to enable the format on just
3831 read or just write; in that case you may see an error at run time. You can
3832 guard against this by checking for the definition of:
3836 PNG_FORMAT_FLAG_ALPHA 0x01 format with an alpha channel
3837 PNG_FORMAT_FLAG_COLOR 0x02 color format: otherwise grayscale
3838 PNG_FORMAT_FLAG_LINEAR 0x04 png_uint_16 channels else png_byte
3839 PNG_FORMAT_FLAG_COLORMAP 0x08 libpng use only
3840 PNG_FORMAT_FLAG_BGR 0x10 BGR colors, else order is RGB
3841 PNG_FORMAT_FLAG_AFIRST 0x20 alpha channel comes first
3843 Supported formats are as follows. Future versions of libpng may support more
3844 formats; for compatibility with older versions simply check if the format
3845 macro is defined using #ifdef. These defines describe the in-memory layout
3846 of the components of the pixels of the image.
3848 First the single byte formats:
3860 Then the linear 2-byte formats. When naming these "Y" is used to
3861 indicate a luminance (gray) channel. The component order within the pixel
3862 is always the same - there is no provision for swapping the order of the
3863 components in the linear format. The components are 16-bit integers in
3864 the native byte order for your platform, and there is no provision for
3865 swapping the bytes to a different endian condition.
3876 Color-mapped formats are obtained by calling png_image_{read,write}_colormap,
3877 as appropriate after setting png_image::format to the format of the color-map
3878 to be read or written. Applications may check the value of
3879 PNG_FORMAT_FLAG_COLORMAP to see if they have called the colormap API. The
3880 format of the color-map may be extracted using the following macro.
3884 PNG_IMAGE macros
3886 These are convenience macros to derive information from a png_image
3887 structure. The PNG_IMAGE_SAMPLE_ macros return values appropriate to the
3888 actual image sample values - either the entries in the color-map or the
3889 pixels in the image. The PNG_IMAGE_PIXEL_ macros return corresponding values
3890 for the pixels and will always return 1 after a call to
3891 png_image_{read,write}_colormap. The remaining macros return information
3892 about the rows in the image and the complete image.
3894 NOTE: All the macros that take a png_image::format parameter are compile time
3895 constants if the format parameter is, itself, a constant. Therefore these
3896 macros can be used in array declarations and case labels where required.
3897 Similarly the macros are also pre-processor constants (sizeof is not used) so
3898 they can be used in #if tests.
3900 First the information about the samples.
3903 Returns the total number of channels in a given format: 1..4
3906 Returns the size in bytes of a single component of a pixel or color-map
3907 entry (as appropriate) in the image.
3910 This is the size of the sample data for one sample. If the image is
3911 color-mapped it is the size of one color-map entry (and image pixels are
3912 one byte in size), otherwise it is the size of one image pixel.
3915 The size of the color-map required by the format; this is the size of the
3916 color-map buffer passed to the png_image_{read,write}_colormap APIs, it is
3917 a fixed number determined by the format so can easily be allocated on the
3918 stack if necessary.
3922 /* The maximum size of the color-map required by the format expressed in a
3923 * count of components. This can be used to compile-time allocate a
3924 * color-map:
3925 *
3926 * png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];
3927 *
3928 * png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
3929 *
3930 * Alternatively, use the PNG_IMAGE_COLORMAP_SIZE macro below to use the
3931 * information from one of the png_image_begin_read_ APIs and dynamically
3932 * allocate the required memory.
3933 */
3936 Corresponding information about the pixels
3938 PNG_IMAGE_PIXEL_(test,fmt)
3941 The number of separate channels (components) in a pixel; 1 for a
3942 color-mapped image.
3945 The size, in bytes, of each component in a pixel; 1 for a color-mapped
3946 image.
3949 The size, in bytes, of a complete pixel; 1 for a color-mapped image.
3951 Information about the whole row, or whole image
3954 Returns the total number of components in a single row of the image; this
3955 is the minimum 'row stride', the minimum count of components between each
3956 row. For a color-mapped image this is the minimum number of bytes in a
3957 row.
3959 If you need the stride measured in bytes, row_stride_bytes is
3961 plus any padding bytes that your application might need, for example
3962 to start the next row on a 4-byte boundary.
3964 PNG_IMAGE_BUFFER_SIZE(image, row_stride)
3965 Returns the size, in bytes, of an image buffer given a png_image and a row
3966 stride - the number of components to leave space for in each row. This
3967 macro takes care of multiplying row_stride by PNG_IMAGE_PIXEL_COMONENT_SIZE
3968 when the image has 2-byte components.
3971 This indicates the the RGB values of the in-memory bitmap do not
3972 correspond to the red, green and blue end-points defined by sRGB.
3975 The PNG is color-mapped. If this flag is set png_image_read_colormap
3976 can be used without further loss of image information. If it is not set
3977 png_image_read_colormap will cause significant loss if the image has any
3979 READ APIs
3981 The png_image passed to the read APIs must have been initialized by setting
3982 the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.)
3984 int png_image_begin_read_from_file( png_imagep image,
3985 const char *file_name)
3987 The named file is opened for read and the image header
3988 is filled in from the PNG header in the file.
3990 int png_image_begin_read_from_stdio (png_imagep image,
3991 FILE* file)
3993 The PNG header is read from the stdio FILE object.
3995 int png_image_begin_read_from_memory(png_imagep image,
3996 png_const_voidp memory, png_size_t size)
3998 The PNG header is read from the given memory buffer.
4000 int png_image_finish_read(png_imagep image,
4001 png_colorp background, void *buffer,
4002 png_int_32 row_stride, void *colormap));
4004 Finish reading the image into the supplied buffer and
4005 clean up the png_image structure.
4007 row_stride is the step, in png_byte or png_uint_16 units
4008 as appropriate, between adjacent rows. A positive stride
4009 indicates that the top-most row is first in the buffer -
4010 the normal top-down arrangement. A negative stride
4011 indicates that the bottom-most row is first in the buffer.
4013 background need only be supplied if an alpha channel must
4014 be removed from a png_byte format and the removal is to be
4015 done by compositing on a solid color; otherwise it may be
4016 NULL and any composition will be done directly onto the
4017 buffer. The value is an sRGB color to use for the
4018 background, for grayscale output the green channel is used.
4020 For linear output removing the alpha channel is always done
4021 by compositing on black.
4023 void png_image_free(png_imagep image)
4025 Free any data allocated by libpng in image->opaque,
4026 setting the pointer to NULL. May be called at any time
4027 after the structure is initialized.
4029 When the simplified API needs to convert between sRGB and linear colorspaces,
4030 the actual sRGB transfer curve defined in the sRGB specification (see the
4031 article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
4032 approximation used elsewhere in libpng.
4036 For write you must initialize a png_image structure to describe the image to
4037 be written:
4039 version: must be set to PNG_IMAGE_VERSION
4040 opaque: must be initialized to NULL
4041 width: image width in pixels
4042 height: image height in rows
4043 format: the format of the data you wish to write
4044 flags: set to 0 unless one of the defined flags applies; set
4045 PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
4046 where the RGB values do not correspond to the colors in sRGB.
4047 colormap_entries: set to the number of entries in the color-map (0 to 256)
4049 int png_image_write_to_file, (png_imagep image,
4050 const char *file, int convert_to_8bit, const void *buffer,
4051 png_int_32 row_stride, const void *colormap));
4053 Write the image to the named file.
4055 int png_image_write_to_stdio(png_imagep image, FILE *file,
4056 int convert_to_8_bit, const void *buffer,
4057 png_int_32 row_stride, const void *colormap)
4059 Write the image to the given (FILE*).
4061 With all write APIs if image is in one of the linear formats with
4062 (png_uint_16) data then setting convert_to_8_bit will cause the output to be
4063 a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise
4064 a 16-bit linear encoded PNG file is written.
4066 With all APIs row_stride is handled as in the read APIs - it is the spacing
4067 from one row to the next in component sized units (float) and if negative
4068 indicates a bottom-up row layout in the buffer. If you pass zero, libpng will
4069 calculate the row_stride for you from the width and number of channels.
4071 Note that the write API does not support interlacing, sub-8-bit pixels,
4072 indexed (paletted) images, or most ancillary chunks.
4074 VI. Modifying/Customizing libpng
4076 There are two issues here. The first is changing how libpng does
4077 standard things like memory allocation, input/output, and error handling.
4078 The second deals with more complicated things like adding new chunks,
4079 adding new transformations, and generally changing how libpng works.
4080 Both of those are compile-time issues; that is, they are generally
4081 determined at the time the code is written, and there is rarely a need
4082 to provide the user with a means of changing them.
4084 Memory allocation, input/output, and error handling
4086 All of the memory allocation, input/output, and error handling in libpng
4087 goes through callbacks that are user-settable. The default routines are
4088 in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change
4089 these functions, call the appropriate png_set_*_fn() function.
4091 Memory allocation is done through the functions png_malloc(), png_calloc(),
4092 and png_free(). The png_malloc() and png_free() functions currently just
4093 call the standard C functions and png_calloc() calls png_malloc() and then
4094 clears the newly allocated memory to zero; note that png_calloc(png_ptr, size)
4095 is not the same as the calloc(number, size) function provided by stdlib.h.
4096 There is limited support for certain systems with segmented memory
4097 architectures and the types of pointers declared by png.h match this; you
4098 will have to use appropriate pointers in your application. If you prefer
4099 to use a different method of allocating and freeing data, you can use
4100 png_create_read_struct_2() or png_create_write_struct_2() to register your
4101 own functions as described above. These functions also provide a void
4102 pointer that can be retrieved via
4104 mem_ptr=png_get_mem_ptr(png_ptr);
4106 Your replacement memory functions must have prototypes as follows:
4108 png_voidp malloc_fn(png_structp png_ptr,
4109 png_alloc_size_t size);
4111 void free_fn(png_structp png_ptr, png_voidp ptr);
4113 Your malloc_fn() must return NULL in case of failure. The png_malloc()
4114 function will normally call png_error() if it receives a NULL from the
4115 system memory allocator or from your replacement malloc_fn().
4117 Your free_fn() will never be called with a NULL ptr, since libpng's
4118 png_free() checks for NULL before calling free_fn().
4120 Input/Output in libpng is done through png_read() and png_write(),
4121 which currently just call fread() and fwrite(). The FILE * is stored in
4122 png_struct and is initialized via png_init_io(). If you wish to change
4123 the method of I/O, the library supplies callbacks that you can set
4124 through the function png_set_read_fn() and png_set_write_fn() at run
4125 time, instead of calling the png_init_io() function. These functions
4126 also provide a void pointer that can be retrieved via the function
4127 png_get_io_ptr(). For example:
4129 png_set_read_fn(png_structp read_ptr,
4130 voidp read_io_ptr, png_rw_ptr read_data_fn)
4132 png_set_write_fn(png_structp write_ptr,
4133 voidp write_io_ptr, png_rw_ptr write_data_fn,
4134 png_flush_ptr output_flush_fn);
4136 voidp read_io_ptr = png_get_io_ptr(read_ptr);
4137 voidp write_io_ptr = png_get_io_ptr(write_ptr);
4139 The replacement I/O functions must have prototypes as follows:
4141 void user_read_data(png_structp png_ptr,
4142 png_bytep data, png_size_t length);
4144 void user_write_data(png_structp png_ptr,
4145 png_bytep data, png_size_t length);
4147 void user_flush_data(png_structp png_ptr);
4149 The user_read_data() function is responsible for detecting and
4150 handling end-of-data errors.
4152 Supplying NULL for the read, write, or flush functions sets them back
4153 to using the default C stream functions, which expect the io_ptr to
4154 point to a standard *FILE structure. It is probably a mistake
4155 to use NULL for one of write_data_fn and output_flush_fn but not both
4156 of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
4157 It is an error to read from a write stream, and vice versa.
4159 Error handling in libpng is done through png_error() and png_warning().
4160 Errors handled through png_error() are fatal, meaning that png_error()
4161 should never return to its caller. Currently, this is handled via
4162 setjmp() and longjmp() (unless you have compiled libpng with
4163 PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
4164 but you could change this to do things like exit() if you should wish,
4165 as long as your function does not return.
4167 On non-fatal errors, png_warning() is called
4168 to print a warning message, and then control returns to the calling code.
4169 By default png_error() and png_warning() print a message on stderr via
4170 fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
4171 (because you don't want the messages) or PNG_NO_STDIO defined (because
4172 fprintf() isn't available). If you wish to change the behavior of the error
4173 functions, you will need to set up your own message callbacks. These
4174 functions are normally supplied at the time that the png_struct is created.
4175 It is also possible to redirect errors and warnings to your own replacement
4176 functions after png_create_*_struct() has been called by calling:
4178 png_set_error_fn(png_structp png_ptr,
4179 png_voidp error_ptr, png_error_ptr error_fn,
4180 png_error_ptr warning_fn);
4182 png_voidp error_ptr = png_get_error_ptr(png_ptr);
4184 If NULL is supplied for either error_fn or warning_fn, then the libpng
4185 default function will be used, calling fprintf() and/or longjmp() if a
4186 problem is encountered. The replacement error functions should have
4187 parameters as follows:
4189 void user_error_fn(png_structp png_ptr,
4190 png_const_charp error_msg);
4192 void user_warning_fn(png_structp png_ptr,
4193 png_const_charp warning_msg);
4195 The motivation behind using setjmp() and longjmp() is the C++ throw and
4196 catch exception handling methods. This makes the code much easier to write,
4197 as there is no need to check every return code of every function call.
4198 However, there are some uncertainties about the status of local variables
4199 after a longjmp, so the user may want to be careful about doing anything
4200 after setjmp returns non-zero besides returning itself. Consult your
4201 compiler documentation for more details. For an alternative approach, you
4202 may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net),
4203 which is illustrated in pngvalid.c and in contrib/visupng.
4205 Beginning in libpng-1.4.0, the png_set_benign_errors() API became available.
4206 You can use this to handle certain errors (normally handled as errors)
4207 as warnings.
4209 png_set_benign_errors (png_ptr, int allowed);
4211 allowed: 0: treat png_benign_error() as an error.
4212 1: treat png_benign_error() as a warning.
4214 As of libpng-1.6.0, the default condition is to treat benign errors as
4215 warnings while reading and as errors while writing.
4217 Custom chunks
4219 If you need to read or write custom chunks, you may need to get deeper
4220 into the libpng code. The library now has mechanisms for storing
4221 and writing chunks of unknown type; you can even declare callbacks
4222 for custom chunks. However, this may not be good enough if the
4223 library code itself needs to know about interactions between your
4224 chunk and existing `intrinsic' chunks.
4226 If you need to write a new intrinsic chunk, first read the PNG
4227 specification. Acquire a first level of understanding of how it works.
4228 Pay particular attention to the sections that describe chunk names,
4229 and look at how other chunks were designed, so you can do things
4230 similarly. Second, check out the sections of libpng that read and
4231 write chunks. Try to find a chunk that is similar to yours and use
4232 it as a template. More details can be found in the comments inside
4233 the code. It is best to handle private or unknown chunks in a generic method,
4234 via callback functions, instead of by modifying libpng functions. This
4235 is illustrated in pngtest.c, which uses a callback function to handle a
4236 private "vpAg" chunk and the new "sTER" chunk, which are both unknown to
4237 libpng.
4239 If you wish to write your own transformation for the data, look through
4240 the part of the code that does the transformations, and check out some of
4241 the simpler ones to get an idea of how they work. Try to find a similar
4242 transformation to the one you want to add and copy off of it. More details
4243 can be found in the comments inside the code itself.
4245 Configuring for gui/windowing platforms:
4247 You will need to write new error and warning functions that use the GUI
4248 interface, as described previously, and set them to be the error and
4249 warning functions at the time that png_create_*_struct() is called,
4250 in order to have them available during the structure initialization.
4251 They can be changed later via png_set_error_fn(). On some compilers,
4252 you may also have to change the memory allocators (png_malloc, etc.).
4254 Configuring zlib:
4256 There are special functions to configure the compression. Perhaps the
4257 most useful one changes the compression level, which currently uses
4258 input compression values in the range 0 - 9. The library normally
4259 uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
4260 have shown that for a large majority of images, compression values in
4261 the range 3-6 compress nearly as well as higher levels, and do so much
4262 faster. For online applications it may be desirable to have maximum speed
4263 (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
4264 specify no compression (Z_NO_COMPRESSION = 0), but this would create
4265 files larger than just storing the raw bitmap. You can specify the
4266 compression level by calling:
4268 #include zlib.h
4269 png_set_compression_level(png_ptr, level);
4271 Another useful one is to reduce the memory level used by the library.
4272 The memory level defaults to 8, but it can be lowered if you are
4273 short on memory (running DOS, for example, where you only have 640K).
4274 Note that the memory level does have an effect on compression; among
4275 other things, lower levels will result in sections of incompressible
4276 data being emitted in smaller stored blocks, with a correspondingly
4277 larger relative overhead of up to 15% in the worst case.
4279 #include zlib.h
4280 png_set_compression_mem_level(png_ptr, level);
4282 The other functions are for configuring zlib. They are not recommended
4283 for normal use and may result in writing an invalid PNG file. See
4284 zlib.h for more information on what these mean.
4286 #include zlib.h
4287 png_set_compression_strategy(png_ptr,
4288 strategy);
4290 png_set_compression_window_bits(png_ptr,
4291 window_bits);
4293 png_set_compression_method(png_ptr, method);
4295 This controls the size of the IDAT chunks (default 8192):
4297 png_set_compression_buffer_size(png_ptr, size);
4299 As of libpng version 1.5.4, additional APIs became
4300 available to set these separately for non-IDAT
4301 compressed chunks such as zTXt, iTXt, and iCCP:
4303 #include zlib.h
4304 #if PNG_LIBPNG_VER >= 10504
4305 png_set_text_compression_level(png_ptr, level);
4307 png_set_text_compression_mem_level(png_ptr, level);
4309 png_set_text_compression_strategy(png_ptr,
4310 strategy);
4312 png_set_text_compression_window_bits(png_ptr,
4313 window_bits);
4315 png_set_text_compression_method(png_ptr, method);
4316 #endif
4318 Controlling row filtering
4320 If you want to control whether libpng uses filtering or not, which
4321 filters are used, and how it goes about picking row filters, you
4322 can call one of these functions. The selection and configuration
4323 of row filters can have a significant impact on the size and
4324 encoding speed and a somewhat lesser impact on the decoding speed
4325 of an image. Filtering is enabled by default for RGB and grayscale
4326 images (with and without alpha), but not for paletted images nor
4327 for any images with bit depths less than 8 bits/pixel.
4329 The 'method' parameter sets the main filtering method, which is
4330 currently only '0' in the PNG 1.2 specification. The 'filters'
4331 parameter sets which filter(s), if any, should be used for each
4332 scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
4333 to turn filtering on and off, respectively.
4335 Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
4336 PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
4337 ORed together with '|' to specify one or more filters to use.
4338 These filters are described in more detail in the PNG specification.
4339 If you intend to change the filter type during the course of writing
4340 the image, you should start with flags set for all of the filters
4341 you intend to use so that libpng can initialize its internal
4342 structures appropriately for all of the filter types. (Note that this
4343 means the first row must always be adaptively filtered, because libpng
4344 currently does not allocate the filter buffers until png_write_row()
4345 is called for the first time.)
4351 png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
4352 filters);
4353 The second parameter can also be
4355 writing a PNG to be embedded in a MNG
4356 datastream. This parameter must be the
4357 same as the value of filter_method used
4358 in png_set_IHDR().
4360 Requesting debug printout
4362 The macro definition PNG_DEBUG can be used to request debugging
4363 printout. Set it to an integer value in the range 0 to 3. Higher
4364 numbers result in increasing amounts of debugging information. The
4365 information is printed to the "stderr" file, unless another file
4366 name is specified in the PNG_DEBUG_FILE macro definition.
4368 When PNG_DEBUG > 0, the following functions (macros) become available:
4370 png_debug(level, message)
4371 png_debug1(level, message, p1)
4372 png_debug2(level, message, p1, p2)
4374 in which "level" is compared to PNG_DEBUG to decide whether to print
4375 the message, "message" is the formatted string to be printed,
4376 and p1 and p2 are parameters that are to be embedded in the string
4377 according to printf-style formatting directives. For example,
4379 png_debug1(2, "foo=%d", foo);
4381 is expanded to
4383 if (PNG_DEBUG > 2)
4384 fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
4386 When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
4387 can still use PNG_DEBUG to control your own debugging:
4389 #ifdef PNG_DEBUG
4390 fprintf(stderr, ...
4391 #endif
4393 When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
4394 having level = 0 will be printed. There aren't any such statements in
4395 this version of libpng, but if you insert some they will be printed.
4397 VII. MNG support
4399 The MNG specification (available at http://www.libpng.org/pub/mng) allows
4400 certain extensions to PNG for PNG images that are embedded in MNG datastreams.
4401 Libpng can support some of these extensions. To enable them, use the
4402 png_permit_mng_features() function:
4404 feature_set = png_permit_mng_features(png_ptr, mask)
4406 mask is a png_uint_32 containing the bitwise OR of the
4407 features you want to enable. These include
4412 feature_set is a png_uint_32 that is the bitwise AND of
4413 your mask with the set of MNG features that is
4414 supported by the version of libpng that you are using.
4416 It is an error to use this function when reading or writing a standalone
4417 PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped
4418 in a MNG datastream. As a minimum, it must have the MNG 8-byte signature
4419 and the MHDR and MEND chunks. Libpng does not provide support for these
4420 or any other MNG chunks; your application must provide its own support for
4421 them. You may wish to consider using libmng (available at
4422 http://www.libmng.com) instead.
4424 VIII. Changes to Libpng from version 0.88
4426 It should be noted that versions of libpng later than 0.96 are not
4427 distributed by the original libpng author, Guy Schalnat, nor by
4428 Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
4429 distributed versions 0.89 through 0.96, but rather by another member
4430 of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
4431 still alive and well, but they have moved on to other things.
4433 The old libpng functions png_read_init(), png_write_init(),
4434 png_info_init(), png_read_destroy(), and png_write_destroy() have been
4435 moved to PNG_INTERNAL in version 0.95 to discourage their use. These
4436 functions will be removed from libpng version 1.4.0.
4438 The preferred method of creating and initializing the libpng structures is
4439 via the png_create_read_struct(), png_create_write_struct(), and