[LIBPNG] Update to v1.6.21. CORE-11128
[reactos.git] / reactos / dll / 3rdparty / libpng / docs / libpng-manual.txt
1 libpng-manual.txt - A description on how to use and modify libpng
2
3 libpng version 1.6.21 - January 15, 2016
4 Updated and distributed by Glenn Randers-Pehrson
5 <glennrp at users.sourceforge.net>
6 Copyright (c) 1998-2016 Glenn Randers-Pehrson
7
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
11
12 Based on:
13
14 libpng versions 0.97, January 1998, through 1.6.21 - January 15, 2016
15 Updated and distributed by Glenn Randers-Pehrson
16 Copyright (c) 1998-2016 Glenn Randers-Pehrson
17
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
21
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.
26
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
30
31 TABLE OF CONTENTS
32
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
49
50 I. Introduction
51
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.
58
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.
62
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.
66
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.
71
72 The PNG-1.2 specification is available at
73 <http://png-mng.sourceforge.net/pub/png/spec/1.2/>.
74 It is technically equivalent
75 to the PNG specification (second edition) but has some additional material.
76
77 The PNG-1.0 specification is available as RFC 2083
78 <http://png-mng.sourceforge.net/pub/png/spec/1.0/> and as a
79 W3C Recommendation <http://www.w3.org/TR/REC-png-961001>.
80
81 Some additional chunks are described in the special-purpose public chunks
82 documents at <http://www.libpng.org/pub/png/spec/register/>
83
84 Other information
85 about PNG, and the latest version of libpng, can be found at the PNG home
86 page, <http://www.libpng.org/pub/png/>.
87
88 Most users will not have to modify the library significantly; advanced
89 users may want to modify it more. All attempts were made to make it as
90 complete as possible, while keeping the code easy to understand.
91 Currently, this library only supports C. Support for other languages
92 is being considered.
93
94 Libpng has been designed to handle multiple sessions at one time,
95 to be easily modifiable, to be portable to the vast majority of
96 machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
97 to use. The ultimate goal of libpng is to promote the acceptance of
98 the PNG file format in whatever way possible. While there is still
99 work to be done (see the TODO file), libpng should cover the
100 majority of the needs of its users.
101
102 Libpng uses zlib for its compression and decompression of PNG files.
103 Further information about zlib, and the latest version of zlib, can
104 be found at the zlib home page, <http://zlib.net/>.
105 The zlib compression utility is a general purpose utility that is
106 useful for more than PNG files, and can be used without libpng.
107 See the documentation delivered with zlib for more details.
108 You can usually find the source files for the zlib utility wherever you
109 find the libpng source files.
110
111 Libpng is thread safe, provided the threads are using different
112 instances of the structures. Each thread should have its own
113 png_struct and png_info instances, and thus its own image.
114 Libpng does not protect itself against two threads using the
115 same instance of a structure.
116
117 II. Structures
118
119 There are two main structures that are important to libpng, png_struct
120 and png_info. Both are internal structures that are no longer exposed
121 in the libpng interface (as of libpng 1.5.0).
122
123 The png_info structure is designed to provide information about the
124 PNG file. At one time, the fields of png_info were intended to be
125 directly accessible to the user. However, this tended to cause problems
126 with applications using dynamically loaded libraries, and as a result
127 a set of interface functions for png_info (the png_get_*() and png_set_*()
128 functions) was developed, and direct access to the png_info fields was
129 deprecated..
130
131 The png_struct structure is the object used by the library to decode a
132 single image. As of 1.5.0 this structure is also not exposed.
133
134 Almost all libpng APIs require a pointer to a png_struct as the first argument.
135 Many (in particular the png_set and png_get APIs) also require a pointer
136 to png_info as the second argument. Some application visible macros
137 defined in png.h designed for basic data access (reading and writing
138 integers in the PNG format) don't take a png_info pointer, but it's almost
139 always safe to assume that a (png_struct*) has to be passed to call an API
140 function.
141
142 You can have more than one png_info structure associated with an image,
143 as illustrated in pngtest.c, one for information valid prior to the
144 IDAT chunks and another (called "end_info" below) for things after them.
145
146 The png.h header file is an invaluable reference for programming with libpng.
147 And while I'm on the topic, make sure you include the libpng header file:
148
149 #include <png.h>
150
151 and also (as of libpng-1.5.0) the zlib header file, if you need it:
152
153 #include <zlib.h>
154
155 Types
156
157 The png.h header file defines a number of integral types used by the
158 APIs. Most of these are fairly obvious; for example types corresponding
159 to integers of particular sizes and types for passing color values.
160
161 One exception is how non-integral numbers are handled. For application
162 convenience most APIs that take such numbers have C (double) arguments;
163 however, internally PNG, and libpng, use 32 bit signed integers and encode
164 the value by multiplying by 100,000. As of libpng 1.5.0 a convenience
165 macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
166 which is simply (png_int_32).
167
168 All APIs that take (double) arguments also have a matching API that
169 takes the corresponding fixed point integer arguments. The fixed point
170 API has the same name as the floating point one with "_fixed" appended.
171 The actual range of values permitted in the APIs is frequently less than
172 the full range of (png_fixed_point) (-21474 to +21474). When APIs require
173 a non-negative argument the type is recorded as png_uint_32 above. Consult
174 the header file and the text below for more information.
175
176 Special care must be take with sCAL chunk handling because the chunk itself
177 uses non-integral values encoded as strings containing decimal floating point
178 numbers. See the comments in the header file.
179
180 Configuration
181
182 The main header file function declarations are frequently protected by C
183 preprocessing directives of the form:
184
185 #ifdef PNG_feature_SUPPORTED
186 declare-function
187 #endif
188 ...
189 #ifdef PNG_feature_SUPPORTED
190 use-function
191 #endif
192
193 The library can be built without support for these APIs, although a
194 standard build will have all implemented APIs. Application programs
195 should check the feature macros before using an API for maximum
196 portability. From libpng 1.5.0 the feature macros set during the build
197 of libpng are recorded in the header file "pnglibconf.h" and this file
198 is always included by png.h.
199
200 If you don't need to change the library configuration from the default, skip to
201 the next section ("Reading").
202
203 Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
204 of the build project files in the 'projects' directory simply copy
205 scripts/pnglibconf.h.prebuilt to pnglibconf.h. This means that these build
206 systems do not permit easy auto-configuration of the library - they only
207 support the default configuration.
208
209 The easiest way to make minor changes to the libpng configuration when
210 auto-configuration is supported is to add definitions to the command line
211 using (typically) CPPFLAGS. For example:
212
213 CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC
214
215 will change the internal libpng math implementation for gamma correction and
216 other arithmetic calculations to fixed point, avoiding the need for fast
217 floating point support. The result can be seen in the generated pnglibconf.h -
218 make sure it contains the changed feature macro setting.
219
220 If you need to make more extensive configuration changes - more than one or two
221 feature macro settings - you can either add -DPNG_USER_CONFIG to the build
222 command line and put a list of feature macro settings in pngusr.h or you can set
223 DFA_XTRA (a makefile variable) to a file containing the same information in the
224 form of 'option' settings.
225
226 A. Changing pnglibconf.h
227
228 A variety of methods exist to build libpng. Not all of these support
229 reconfiguration of pnglibconf.h. To reconfigure pnglibconf.h it must either be
230 rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
231
232 Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
233 pnglibconf.h and changing the lines defining the supported features, paying
234 very close attention to the 'option' information in scripts/pnglibconf.dfa
235 that describes those features and their requirements. This is easy to get
236 wrong.
237
238 B. Configuration using DFA_XTRA
239
240 Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
241 variant such as 'nawk' or 'gawk', is available. The configure build will
242 automatically find an appropriate awk and build pnglibconf.h.
243 The scripts/pnglibconf.mak file contains a set of make rules for doing the
244 same thing if configure is not used, and many of the makefiles in the scripts
245 directory use this approach.
246
247 When rebuilding simply write a new file containing changed options and set
248 DFA_XTRA to the name of this file. This causes the build to append the new file
249 to the end of scripts/pnglibconf.dfa. The pngusr.dfa file should contain lines
250 of the following forms:
251
252 everything = off
253
254 This turns all optional features off. Include it at the start of pngusr.dfa to
255 make it easier to build a minimal configuration. You will need to turn at least
256 some features on afterward to enable either reading or writing code, or both.
257
258 option feature on
259 option feature off
260
261 Enable or disable a single feature. This will automatically enable other
262 features required by a feature that is turned on or disable other features that
263 require a feature which is turned off. Conflicting settings will cause an error
264 message to be emitted by awk.
265
266 setting feature default value
267
268 Changes the default value of setting 'feature' to 'value'. There are a small
269 number of settings listed at the top of pnglibconf.h, they are documented in the
270 source code. Most of these values have performance implications for the library
271 but most of them have no visible effect on the API. Some can also be overridden
272 from the API.
273
274 This method of building a customized pnglibconf.h is illustrated in
275 contrib/pngminim/*. See the "$(PNGCONF):" target in the makefile and
276 pngusr.dfa in these directories.
277
278 C. Configuration using PNG_USER_CONFIG
279
280 If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is built,
281 the file pngusr.h will automatically be included before the options in
282 scripts/pnglibconf.dfa are processed. Your pngusr.h file should contain only
283 macro definitions turning features on or off or setting settings.
284
285 Apart from the global setting "everything = off" all the options listed above
286 can be set using macros in pngusr.h:
287
288 #define PNG_feature_SUPPORTED
289
290 is equivalent to:
291
292 option feature on
293
294 #define PNG_NO_feature
295
296 is equivalent to:
297
298 option feature off
299
300 #define PNG_feature value
301
302 is equivalent to:
303
304 setting feature default value
305
306 Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
307 pngusr file you supply override the contents of scripts/pnglibconf.dfa
308
309 If confusing or incomprehensible behavior results it is possible to
310 examine the intermediate file pnglibconf.dfn to find the full set of
311 dependency information for each setting and option. Simply locate the
312 feature in the file and read the C comments that precede it.
313
314 This method is also illustrated in the contrib/pngminim/* makefiles and
315 pngusr.h.
316
317 III. Reading
318
319 We'll now walk you through the possible functions to call when reading
320 in a PNG file sequentially, briefly explaining the syntax and purpose
321 of each one. See example.c and png.h for more detail. While
322 progressive reading is covered in the next section, you will still
323 need some of the functions discussed in this section to read a PNG
324 file.
325
326 Setup
327
328 You will want to do the I/O initialization(*) before you get into libpng,
329 so if it doesn't work, you don't have much to undo. Of course, you
330 will also want to insure that you are, in fact, dealing with a PNG
331 file. Libpng provides a simple check to see if a file is a PNG file.
332 To use it, pass in the first 1 to 8 bytes of the file to the function
333 png_sig_cmp(), and it will return 0 (false) if the bytes match the
334 corresponding bytes of the PNG signature, or nonzero (true) otherwise.
335 Of course, the more bytes you pass in, the greater the accuracy of the
336 prediction.
337
338 If you are intending to keep the file pointer open for use in libpng,
339 you must ensure you don't read more than 8 bytes from the beginning
340 of the file, and you also have to make a call to png_set_sig_bytes()
341 with the number of bytes you read from the beginning. Libpng will
342 then only check the bytes (if any) that your program didn't read.
343
344 (*): If you are not using the standard I/O functions, you will need
345 to replace them with custom functions. See the discussion under
346 Customizing libpng.
347
348 FILE *fp = fopen(file_name, "rb");
349 if (!fp)
350 {
351 return (ERROR);
352 }
353
354 if (fread(header, 1, number, fp) != number)
355 {
356 return (ERROR);
357 }
358
359 is_png = !png_sig_cmp(header, 0, number);
360 if (!is_png)
361 {
362 return (NOT_PNG);
363 }
364
365 Next, png_struct and png_info need to be allocated and initialized. In
366 order to ensure that the size of these structures is correct even with a
367 dynamically linked libpng, there are functions to initialize and
368 allocate the structures. We also pass the library version, optional
369 pointers to error handling functions, and a pointer to a data struct for
370 use by the error functions, if necessary (the pointer and functions can
371 be NULL if the default error handlers are to be used). See the section
372 on Changes to Libpng below regarding the old initialization functions.
373 The structure allocation functions quietly return NULL if they fail to
374 create the structure, so your application should check for that.
375
376 png_structp png_ptr = png_create_read_struct
377 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
378 user_error_fn, user_warning_fn);
379
380 if (!png_ptr)
381 return (ERROR);
382
383 png_infop info_ptr = png_create_info_struct(png_ptr);
384
385 if (!info_ptr)
386 {
387 png_destroy_read_struct(&png_ptr,
388 (png_infopp)NULL, (png_infopp)NULL);
389 return (ERROR);
390 }
391
392 If you want to use your own memory allocation routines,
393 use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
394 png_create_read_struct_2() instead of png_create_read_struct():
395
396 png_structp png_ptr = png_create_read_struct_2
397 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
398 user_error_fn, user_warning_fn, (png_voidp)
399 user_mem_ptr, user_malloc_fn, user_free_fn);
400
401 The error handling routines passed to png_create_read_struct()
402 and the memory alloc/free routines passed to png_create_struct_2()
403 are only necessary if you are not using the libpng supplied error
404 handling and memory alloc/free functions.
405
406 When libpng encounters an error, it expects to longjmp back
407 to your routine. Therefore, you will need to call setjmp and pass
408 your png_jmpbuf(png_ptr). If you read the file from different
409 routines, you will need to update the longjmp buffer every time you enter
410 a new routine that will call a png_*() function.
411
412 See your documentation of setjmp/longjmp for your compiler for more
413 information on setjmp/longjmp. See the discussion on libpng error
414 handling in the Customizing Libpng section below for more information
415 on the libpng error handling. If an error occurs, and libpng longjmp's
416 back to your setjmp, you will want to call png_destroy_read_struct() to
417 free any memory.
418
419 if (setjmp(png_jmpbuf(png_ptr)))
420 {
421 png_destroy_read_struct(&png_ptr, &info_ptr,
422 &end_info);
423 fclose(fp);
424 return (ERROR);
425 }
426
427 Pass (png_infopp)NULL instead of &end_info if you didn't create
428 an end_info structure.
429
430 If you would rather avoid the complexity of setjmp/longjmp issues,
431 you can compile libpng with PNG_NO_SETJMP, in which case
432 errors will result in a call to PNG_ABORT() which defaults to abort().
433
434 You can #define PNG_ABORT() to a function that does something
435 more useful than abort(), as long as your function does not
436 return.
437
438 Now you need to set up the input code. The default for libpng is to
439 use the C function fread(). If you use this, you will need to pass a
440 valid FILE * in the function png_init_io(). Be sure that the file is
441 opened in binary mode. If you wish to handle reading data in another
442 way, you need not call the png_init_io() function, but you must then
443 implement the libpng I/O methods discussed in the Customizing Libpng
444 section below.
445
446 png_init_io(png_ptr, fp);
447
448 If you had previously opened the file and read any of the signature from
449 the beginning in order to see if this was a PNG file, you need to let
450 libpng know that there are some bytes missing from the start of the file.
451
452 png_set_sig_bytes(png_ptr, number);
453
454 You can change the zlib compression buffer size to be used while
455 reading compressed data with
456
457 png_set_compression_buffer_size(png_ptr, buffer_size);
458
459 where the default size is 8192 bytes. Note that the buffer size
460 is changed immediately and the buffer is reallocated immediately,
461 instead of setting a flag to be acted upon later.
462
463 If you want CRC errors to be handled in a different manner than
464 the default, use
465
466 png_set_crc_action(png_ptr, crit_action, ancil_action);
467
468 The values for png_set_crc_action() say how libpng is to handle CRC errors in
469 ancillary and critical chunks, and whether to use the data contained
470 therein. Note that it is impossible to "discard" data in a critical
471 chunk.
472
473 Choices for (int) crit_action are
474 PNG_CRC_DEFAULT 0 error/quit
475 PNG_CRC_ERROR_QUIT 1 error/quit
476 PNG_CRC_WARN_USE 3 warn/use data
477 PNG_CRC_QUIET_USE 4 quiet/use data
478 PNG_CRC_NO_CHANGE 5 use the current value
479
480 Choices for (int) ancil_action are
481 PNG_CRC_DEFAULT 0 error/quit
482 PNG_CRC_ERROR_QUIT 1 error/quit
483 PNG_CRC_WARN_DISCARD 2 warn/discard data
484 PNG_CRC_WARN_USE 3 warn/use data
485 PNG_CRC_QUIET_USE 4 quiet/use data
486 PNG_CRC_NO_CHANGE 5 use the current value
487
488 Setting up callback code
489
490 You can set up a callback function to handle any unknown chunks in the
491 input stream. You must supply the function
492
493 read_chunk_callback(png_structp png_ptr,
494 png_unknown_chunkp chunk);
495 {
496 /* The unknown chunk structure contains your
497 chunk data, along with similar data for any other
498 unknown chunks: */
499
500 png_byte name[5];
501 png_byte *data;
502 png_size_t size;
503
504 /* Note that libpng has already taken care of
505 the CRC handling */
506
507 /* put your code here. Search for your chunk in the
508 unknown chunk structure, process it, and return one
509 of the following: */
510
511 return (-n); /* chunk had an error */
512 return (0); /* did not recognize */
513 return (n); /* success */
514 }
515
516 (You can give your function another name that you like instead of
517 "read_chunk_callback")
518
519 To inform libpng about your function, use
520
521 png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
522 read_chunk_callback);
523
524 This names not only the callback function, but also a user pointer that
525 you can retrieve with
526
527 png_get_user_chunk_ptr(png_ptr);
528
529 If you call the png_set_read_user_chunk_fn() function, then all unknown
530 chunks which the callback does not handle will be saved when read. You can
531 cause them to be discarded by returning '1' ("handled") instead of '0'. This
532 behavior will change in libpng 1.7 and the default handling set by the
533 png_set_keep_unknown_chunks() function, described below, will be used when the
534 callback returns 0. If you want the existing behavior you should set the global
535 default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
536 versions of libpng and with 1.7. Libpng 1.6 issues a warning if you keep the
537 default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0.
538
539 At this point, you can set up a callback function that will be
540 called after each row has been read, which you can use to control
541 a progress meter or the like. It's demonstrated in pngtest.c.
542 You must supply a function
543
544 void read_row_callback(png_structp png_ptr,
545 png_uint_32 row, int pass);
546 {
547 /* put your code here */
548 }
549
550 (You can give it another name that you like instead of "read_row_callback")
551
552 To inform libpng about your function, use
553
554 png_set_read_status_fn(png_ptr, read_row_callback);
555
556 When this function is called the row has already been completely processed and
557 the 'row' and 'pass' refer to the next row to be handled. For the
558 non-interlaced case the row that was just handled is simply one less than the
559 passed in row number, and pass will always be 0. For the interlaced case the
560 same applies unless the row value is 0, in which case the row just handled was
561 the last one from one of the preceding passes. Because interlacing may skip a
562 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
563 need to know what the last pass is record (row,pass) from the callback and use
564 the last recorded value each time.
565
566 As with the user transform you can find the output row using the
567 PNG_ROW_FROM_PASS_ROW macro.
568
569 Unknown-chunk handling
570
571 Now you get to set the way the library processes unknown chunks in the
572 input PNG stream. Both known and unknown chunks will be read. Normal
573 behavior is that known chunks will be parsed into information in
574 various info_ptr members while unknown chunks will be discarded. This
575 behavior can be wasteful if your application will never use some known
576 chunk types. To change this, you can call:
577
578 png_set_keep_unknown_chunks(png_ptr, keep,
579 chunk_list, num_chunks);
580
581 keep - 0: default unknown chunk handling
582 1: ignore; do not keep
583 2: keep only if safe-to-copy
584 3: keep even if unsafe-to-copy
585
586 You can use these definitions:
587 PNG_HANDLE_CHUNK_AS_DEFAULT 0
588 PNG_HANDLE_CHUNK_NEVER 1
589 PNG_HANDLE_CHUNK_IF_SAFE 2
590 PNG_HANDLE_CHUNK_ALWAYS 3
591
592 chunk_list - list of chunks affected (a byte string,
593 five bytes per chunk, NULL or '\0' if
594 num_chunks is positive; ignored if
595 numchunks <= 0).
596
597 num_chunks - number of chunks affected; if 0, all
598 unknown chunks are affected. If positive,
599 only the chunks in the list are affected,
600 and if negative all unknown chunks and
601 all known chunks except for the IHDR,
602 PLTE, tRNS, IDAT, and IEND chunks are
603 affected.
604
605 Unknown chunks declared in this way will be saved as raw data onto a
606 list of png_unknown_chunk structures. If a chunk that is normally
607 known to libpng is named in the list, it will be handled as unknown,
608 according to the "keep" directive. If a chunk is named in successive
609 instances of png_set_keep_unknown_chunks(), the final instance will
610 take precedence. The IHDR and IEND chunks should not be named in
611 chunk_list; if they are, libpng will process them normally anyway.
612 If you know that your application will never make use of some particular
613 chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
614
615 Here is an example of the usage of png_set_keep_unknown_chunks(),
616 where the private "vpAg" chunk will later be processed by a user chunk
617 callback function:
618
619 png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'};
620
621 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
622 png_byte unused_chunks[]=
623 {
624 104, 73, 83, 84, (png_byte) '\0', /* hIST */
625 105, 84, 88, 116, (png_byte) '\0', /* iTXt */
626 112, 67, 65, 76, (png_byte) '\0', /* pCAL */
627 115, 67, 65, 76, (png_byte) '\0', /* sCAL */
628 115, 80, 76, 84, (png_byte) '\0', /* sPLT */
629 116, 73, 77, 69, (png_byte) '\0', /* tIME */
630 };
631 #endif
632
633 ...
634
635 #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
636 /* ignore all unknown chunks
637 * (use global setting "2" for libpng16 and earlier):
638 */
639 png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
640
641 /* except for vpAg: */
642 png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
643
644 /* also ignore unused known chunks: */
645 png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
646 (int)(sizeof unused_chunks)/5);
647 #endif
648
649 User limits
650
651 The PNG specification allows the width and height of an image to be as
652 large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
653 For safety, libpng imposes a default limit of 1 million rows and columns.
654 Larger images will be rejected immediately with a png_error() call. If
655 you wish to change these limits, you can use
656
657 png_set_user_limits(png_ptr, width_max, height_max);
658
659 to set your own limits (libpng may reject some very wide images
660 anyway because of potential buffer overflow conditions).
661
662 You should put this statement after you create the PNG structure and
663 before calling png_read_info(), png_read_png(), or png_process_data().
664
665 When writing a PNG datastream, put this statement before calling
666 png_write_info() or png_write_png().
667
668 If you need to retrieve the limits that are being applied, use
669
670 width_max = png_get_user_width_max(png_ptr);
671 height_max = png_get_user_height_max(png_ptr);
672
673 The PNG specification sets no limit on the number of ancillary chunks
674 allowed in a PNG datastream. By default, libpng imposes a limit of
675 a total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored.
676 If you have set up both info_ptr and end_info_ptr, the limit applies
677 separately to each. You can change the limit on the total number of such
678 chunks that will be stored, with
679
680 png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
681
682 where 0x7fffffffL means unlimited. You can retrieve this limit with
683
684 chunk_cache_max = png_get_chunk_cache_max(png_ptr);
685
686 Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount of
687 memory that a compressed chunk other than IDAT can occupy, when decompressed.
688 You can change this limit with
689
690 png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
691
692 and you can retrieve the limit with
693
694 chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
695
696 Any chunks that would cause either of these limits to be exceeded will
697 be ignored.
698
699 Information about your system
700
701 If you intend to display the PNG or to incorporate it in other image data you
702 need to tell libpng information about your display or drawing surface so that
703 libpng can convert the values in the image to match the display.
704
705 From libpng-1.5.4 this information can be set before reading the PNG file
706 header. In earlier versions png_set_gamma() existed but behaved incorrectly if
707 called before the PNG file header had been read and png_set_alpha_mode() did not
708 exist.
709
710 If you need to support versions prior to libpng-1.5.4 test the version number
711 as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
712 described in the appropriate manual page.
713
714 You give libpng the encoding expected by your system expressed as a 'gamma'
715 value. You can also specify a default encoding for the PNG file in
716 case the required information is missing from the file. By default libpng
717 assumes that the PNG data matches your system, to keep this default call:
718
719 png_set_gamma(png_ptr, screen_gamma, output_gamma);
720
721 or you can use the fixed point equivalent:
722
723 png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
724 PNG_FP_1*output_gamma);
725
726 If you don't know the gamma for your system it is probably 2.2 - a good
727 approximation to the IEC standard for display systems (sRGB). If images are
728 too contrasty or washed out you got the value wrong - check your system
729 documentation!
730
731 Many systems permit the system gamma to be changed via a lookup table in the
732 display driver, a few systems, including older Macs, change the response by
733 default. As of 1.5.4 three special values are available to handle common
734 situations:
735
736 PNG_DEFAULT_sRGB: Indicates that the system conforms to the
737 IEC 61966-2-1 standard. This matches almost
738 all systems.
739 PNG_GAMMA_MAC_18: Indicates that the system is an older
740 (pre Mac OS 10.6) Apple Macintosh system with
741 the default settings.
742 PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
743 that the system expects data with no gamma
744 encoding.
745
746 You would use the linear (unencoded) value if you need to process the pixel
747 values further because this avoids the need to decode and re-encode each
748 component value whenever arithmetic is performed. A lot of graphics software
749 uses linear values for this reason, often with higher precision component values
750 to preserve overall accuracy.
751
752
753 The output_gamma value expresses how to decode the output values, not how
754 they are encoded. The values used correspond to the normal numbers used to
755 describe the overall gamma of a computer display system; for example 2.2 for
756 an sRGB conformant system. The values are scaled by 100000 in the _fixed
757 version of the API (so 220000 for sRGB.)
758
759 The inverse of the value is always used to provide a default for the PNG file
760 encoding if it has no gAMA chunk and if png_set_gamma() has not been called
761 to override the PNG gamma information.
762
763 When the ALPHA_OPTIMIZED mode is selected the output gamma is used to encode
764 opaque pixels however pixels with lower alpha values are not encoded,
765 regardless of the output gamma setting.
766
767 When the standard Porter Duff handling is requested with mode 1 the output
768 encoding is set to be linear and the output_gamma value is only relevant
769 as a default for input data that has no gamma information. The linear output
770 encoding will be overridden if png_set_gamma() is called - the results may be
771 highly unexpected!
772
773 The following numbers are derived from the sRGB standard and the research
774 behind it. sRGB is defined to be approximated by a PNG gAMA chunk value of
775 0.45455 (1/2.2) for PNG. The value implicitly includes any viewing
776 correction required to take account of any differences in the color
777 environment of the original scene and the intended display environment; the
778 value expresses how to *decode* the image for display, not how the original
779 data was *encoded*.
780
781 sRGB provides a peg for the PNG standard by defining a viewing environment.
782 sRGB itself, and earlier TV standards, actually use a more complex transform
783 (a linear portion then a gamma 2.4 power law) than PNG can express. (PNG is
784 limited to simple power laws.) By saying that an image for direct display on
785 an sRGB conformant system should be stored with a gAMA chunk value of 45455
786 (11.3.3.2 and 11.3.3.5 of the ISO PNG specification) the PNG specification
787 makes it possible to derive values for other display systems and
788 environments.
789
790 The Mac value is deduced from the sRGB based on an assumption that the actual
791 extra viewing correction used in early Mac display systems was implemented as
792 a power 1.45 lookup table.
793
794 Any system where a programmable lookup table is used or where the behavior of
795 the final display device characteristics can be changed requires system
796 specific code to obtain the current characteristic. However this can be
797 difficult and most PNG gamma correction only requires an approximate value.
798
799 By default, if png_set_alpha_mode() is not called, libpng assumes that all
800 values are unencoded, linear, values and that the output device also has a
801 linear characteristic. This is only very rarely correct - it is invariably
802 better to call png_set_alpha_mode() with PNG_DEFAULT_sRGB than rely on the
803 default if you don't know what the right answer is!
804
805 The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre Mac OS
806 10.6) which used a correction table to implement a somewhat lower gamma on an
807 otherwise sRGB system.
808
809 Both these values are reserved (not simple gamma values) in order to allow
810 more precise correction internally in the future.
811
812 NOTE: the values can be passed to either the fixed or floating
813 point APIs, but the floating point API will also accept floating point
814 values.
815
816 The second thing you may need to tell libpng about is how your system handles
817 alpha channel information. Some, but not all, PNG files contain an alpha
818 channel. To display these files correctly you need to compose the data onto a
819 suitable background, as described in the PNG specification.
820
821 Libpng only supports composing onto a single color (using png_set_background;
822 see below). Otherwise you must do the composition yourself and, in this case,
823 you may need to call png_set_alpha_mode:
824
825 #if PNG_LIBPNG_VER >= 10504
826 png_set_alpha_mode(png_ptr, mode, screen_gamma);
827 #else
828 png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
829 #endif
830
831 The screen_gamma value is the same as the argument to png_set_gamma; however,
832 how it affects the output depends on the mode. png_set_alpha_mode() sets the
833 file gamma default to 1/screen_gamma, so normally you don't need to call
834 png_set_gamma. If you need different defaults call png_set_gamma() before
835 png_set_alpha_mode() - if you call it after it will override the settings made
836 by png_set_alpha_mode().
837
838 The mode is as follows:
839
840 PNG_ALPHA_PNG: The data is encoded according to the PNG
841 specification. Red, green and blue, or gray, components are
842 gamma encoded color values and are not premultiplied by the
843 alpha value. The alpha value is a linear measure of the
844 contribution of the pixel to the corresponding final output pixel.
845
846 You should normally use this format if you intend to perform
847 color correction on the color values; most, maybe all, color
848 correction software has no handling for the alpha channel and,
849 anyway, the math to handle pre-multiplied component values is
850 unnecessarily complex.
851
852 Before you do any arithmetic on the component values you need
853 to remove the gamma encoding and multiply out the alpha
854 channel. See the PNG specification for more detail. It is
855 important to note that when an image with an alpha channel is
856 scaled, linear encoded, pre-multiplied component values must
857 be used!
858
859 The remaining modes assume you don't need to do any further color correction or
860 that if you do, your color correction software knows all about alpha (it
861 probably doesn't!). They 'associate' the alpha with the color information by
862 storing color channel values that have been scaled by the alpha. The
863 advantage is that the color channels can be resampled (the image can be
864 scaled) in this form. The disadvantage is that normal practice is to store
865 linear, not (gamma) encoded, values and this requires 16-bit channels for
866 still images rather than the 8-bit channels that are just about sufficient if
867 gamma encoding is used. In addition all non-transparent pixel values,
868 including completely opaque ones, must be gamma encoded to produce the final
869 image. These are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes
870 described below (the latter being the two common names for associated alpha
871 color channels). Note that PNG files always contain non-associated color
872 channels; png_set_alpha_mode() with one of the modes causes the decoder to
873 convert the pixels to an associated form before returning them to your
874 application.
875
876 Since it is not necessary to perform arithmetic on opaque color values so
877 long as they are not to be resampled and are in the final color space it is
878 possible to optimize the handling of alpha by storing the opaque pixels in
879 the PNG format (adjusted for the output color space) while storing partially
880 opaque pixels in the standard, linear, format. The accuracy required for
881 standard alpha composition is relatively low, because the pixels are
882 isolated, therefore typically the accuracy loss in storing 8-bit linear
883 values is acceptable. (This is not true if the alpha channel is used to
884 simulate transparency over large areas - use 16 bits or the PNG mode in
885 this case!) This is the 'OPTIMIZED' mode. For this mode a pixel is
886 treated as opaque only if the alpha value is equal to the maximum value.
887
888 PNG_ALPHA_STANDARD: The data libpng produces is encoded in the
889 standard way assumed by most correctly written graphics software.
890 The gamma encoding will be removed by libpng and the
891 linear component values will be pre-multiplied by the
892 alpha channel.
893
894 With this format the final image must be re-encoded to
895 match the display gamma before the image is displayed.
896 If your system doesn't do that, yet still seems to
897 perform arithmetic on the pixels without decoding them,
898 it is broken - check out the modes below.
899
900 With PNG_ALPHA_STANDARD libpng always produces linear
901 component values, whatever screen_gamma you supply. The
902 screen_gamma value is, however, used as a default for
903 the file gamma if the PNG file has no gamma information.
904
905 If you call png_set_gamma() after png_set_alpha_mode() you
906 will override the linear encoding. Instead the
907 pre-multiplied pixel values will be gamma encoded but
908 the alpha channel will still be linear. This may
909 actually match the requirements of some broken software,
910 but it is unlikely.
911
912 While linear 8-bit data is often used it has
913 insufficient precision for any image with a reasonable
914 dynamic range. To avoid problems, and if your software
915 supports it, use png_set_expand_16() to force all
916 components to 16 bits.
917
918 PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD
919 except that completely opaque pixels are gamma encoded according to
920 the screen_gamma value. Pixels with alpha less than 1.0
921 will still have linear components.
922
923 Use this format if you have control over your
924 compositing software and so don't do other arithmetic
925 (such as scaling) on the data you get from libpng. Your
926 compositing software can simply copy opaque pixels to
927 the output but still has linear values for the
928 non-opaque pixels.
929
930 In normal compositing, where the alpha channel encodes
931 partial pixel coverage (as opposed to broad area
932 translucency), the inaccuracies of the 8-bit
933 representation of non-opaque pixels are irrelevant.
934
935 You can also try this format if your software is broken;
936 it might look better.
937
938 PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all component
939 values, including the alpha channel are gamma encoded. This is
940 broken because, in practice, no implementation that uses this choice
941 correctly undoes the encoding before handling alpha composition. Use this
942 choice only if other serious errors in the software or hardware you use
943 mandate it. In most cases of broken software or hardware the bug in the
944 final display manifests as a subtle halo around composited parts of the
945 image. You may not even perceive this as a halo; the composited part of
946 the image may simply appear separate from the background, as though it had
947 been cut out of paper and pasted on afterward.
948
949 If you don't have to deal with bugs in software or hardware, or if you can fix
950 them, there are three recommended ways of using png_set_alpha_mode():
951
952 png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
953 screen_gamma);
954
955 You can do color correction on the result (libpng does not currently
956 support color correction internally). When you handle the alpha channel
957 you need to undo the gamma encoding and multiply out the alpha.
958
959 png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
960 screen_gamma);
961 png_set_expand_16(png_ptr);
962
963 If you are using the high level interface, don't call png_set_expand_16();
964 instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
965
966 With this mode you can't do color correction, but you can do arithmetic,
967 including composition and scaling, on the data without further processing.
968
969 png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
970 screen_gamma);
971
972 You can avoid the expansion to 16-bit components with this mode, but you
973 lose the ability to scale the image or perform other linear arithmetic.
974 All you can do is compose the result onto a matching output. Since this
975 mode is libpng-specific you also need to write your own composition
976 software.
977
978 The following are examples of calls to png_set_alpha_mode to achieve the
979 required overall gamma correction and, where necessary, alpha
980 premultiplication.
981
982 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
983
984 This is the default libpng handling of the alpha channel - it is not
985 pre-multiplied into the color components. In addition the call states
986 that the output is for a sRGB system and causes all PNG files without gAMA
987 chunks to be assumed to be encoded using sRGB.
988
989 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
990
991 In this case the output is assumed to be something like an sRGB conformant
992 display preceeded by a power-law lookup table of power 1.45. This is how
993 early Mac systems behaved.
994
995 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
996
997 This is the classic Jim Blinn approach and will work in academic
998 environments where everything is done by the book. It has the shortcoming
999 of assuming that input PNG data with no gamma information is linear - this
1000 is unlikely to be correct unless the PNG files where generated locally.
1001 Most of the time the output precision will be so low as to show
1002 significant banding in dark areas of the image.
1003
1004 png_set_expand_16(pp);
1005 png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
1006
1007 This is a somewhat more realistic Jim Blinn inspired approach. PNG files
1008 are assumed to have the sRGB encoding if not marked with a gamma value and
1009 the output is always 16 bits per component. This permits accurate scaling
1010 and processing of the data. If you know that your input PNG files were
1011 generated locally you might need to replace PNG_DEFAULT_sRGB with the
1012 correct value for your system.
1013
1014 png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
1015
1016 If you just need to composite the PNG image onto an existing background
1017 and if you control the code that does this you can use the optimization
1018 setting. In this case you just copy completely opaque pixels to the
1019 output. For pixels that are not completely transparent (you just skip
1020 those) you do the composition math using png_composite or png_composite_16
1021 below then encode the resultant 8-bit or 16-bit values to match the output
1022 encoding.
1023
1024 Other cases
1025
1026 If neither the PNG nor the standard linear encoding work for you because
1027 of the software or hardware you use then you have a big problem. The PNG
1028 case will probably result in halos around the image. The linear encoding
1029 will probably result in a washed out, too bright, image (it's actually too
1030 contrasty.) Try the ALPHA_OPTIMIZED mode above - this will probably
1031 substantially reduce the halos. Alternatively try:
1032
1033 png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
1034
1035 This option will also reduce the halos, but there will be slight dark
1036 halos round the opaque parts of the image where the background is light.
1037 In the OPTIMIZED mode the halos will be light halos where the background
1038 is dark. Take your pick - the halos are unavoidable unless you can get
1039 your hardware/software fixed! (The OPTIMIZED approach is slightly
1040 faster.)
1041
1042 When the default gamma of PNG files doesn't match the output gamma.
1043 If you have PNG files with no gamma information png_set_alpha_mode allows
1044 you to provide a default gamma, but it also sets the ouput gamma to the
1045 matching value. If you know your PNG files have a gamma that doesn't
1046 match the output you can take advantage of the fact that
1047 png_set_alpha_mode always sets the output gamma but only sets the PNG
1048 default if it is not already set:
1049
1050 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
1051 png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
1052
1053 The first call sets both the default and the output gamma values, the
1054 second call overrides the output gamma without changing the default. This
1055 is easier than achieving the same effect with png_set_gamma. You must use
1056 PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
1057 fire if more than one call to png_set_alpha_mode and png_set_background is
1058 made in the same read operation, however multiple calls with PNG_ALPHA_PNG
1059 are ignored.
1060
1061 If you don't need, or can't handle, the alpha channel you can call
1062 png_set_background() to remove it by compositing against a fixed color. Don't
1063 call png_set_strip_alpha() to do this - it will leave spurious pixel values in
1064 transparent parts of this image.
1065
1066 png_set_background(png_ptr, &background_color,
1067 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);
1068
1069 The background_color is an RGB or grayscale value according to the data format
1070 libpng will produce for you. Because you don't yet know the format of the PNG
1071 file, if you call png_set_background at this point you must arrange for the
1072 format produced by libpng to always have 8-bit or 16-bit components and then
1073 store the color as an 8-bit or 16-bit color as appropriate. The color contains
1074 separate gray and RGB component values, so you can let libpng produce gray or
1075 RGB output according to the input format, but low bit depth grayscale images
1076 must always be converted to at least 8-bit format. (Even though low bit depth
1077 grayscale images can't have an alpha channel they can have a transparent
1078 color!)
1079
1080 You set the transforms you need later, either as flags to the high level
1081 interface or libpng API calls for the low level interface. For reference the
1082 settings and API calls required are:
1083
1084 8-bit values:
1085 PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
1086 png_set_expand(png_ptr); png_set_scale_16(png_ptr);
1087
1088 If you must get exactly the same inaccurate results
1089 produced by default in versions prior to libpng-1.5.4,
1090 use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
1091 instead.
1092
1093 16-bit values:
1094 PNG_TRANSFORM_EXPAND_16
1095 png_set_expand_16(png_ptr);
1096
1097 In either case palette image data will be expanded to RGB. If you just want
1098 color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
1099 to the list.
1100
1101 Calling png_set_background before the PNG file header is read will not work
1102 prior to libpng-1.5.4. Because the failure may result in unexpected warnings or
1103 errors it is therefore much safer to call png_set_background after the head has
1104 been read. Unfortunately this means that prior to libpng-1.5.4 it cannot be
1105 used with the high level interface.
1106
1107 The high-level read interface
1108
1109 At this point there are two ways to proceed; through the high-level
1110 read interface, or through a sequence of low-level read operations.
1111 You can use the high-level interface if (a) you are willing to read
1112 the entire image into memory, and (b) the input transformations
1113 you want to do are limited to the following set:
1114
1115 PNG_TRANSFORM_IDENTITY No transformation
1116 PNG_TRANSFORM_SCALE_16 Strip 16-bit samples to
1117 8-bit accurately
1118 PNG_TRANSFORM_STRIP_16 Chop 16-bit samples to
1119 8-bit less accurately
1120 PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
1121 PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
1122 samples to bytes
1123 PNG_TRANSFORM_PACKSWAP Change order of packed
1124 pixels to LSB first
1125 PNG_TRANSFORM_EXPAND Perform set_expand()
1126 PNG_TRANSFORM_INVERT_MONO Invert monochrome images
1127 PNG_TRANSFORM_SHIFT Normalize pixels to the
1128 sBIT depth
1129 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
1130 to BGRA
1131 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
1132 to AG
1133 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
1134 to transparency
1135 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
1136 PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples
1137 to RGB (or GA to RGBA)
1138 PNG_TRANSFORM_EXPAND_16 Expand samples to 16 bits
1139
1140 (This excludes setting a background color, doing gamma transformation,
1141 quantizing, and setting filler.) If this is the case, simply do this:
1142
1143 png_read_png(png_ptr, info_ptr, png_transforms, NULL)
1144
1145 where png_transforms is an integer containing the bitwise OR of some
1146 set of transformation flags. This call is equivalent to png_read_info(),
1147 followed the set of transformations indicated by the transform mask,
1148 then png_read_image(), and finally png_read_end().
1149
1150 (The final parameter of this call is not yet used. Someday it might point
1151 to transformation parameters required by some future input transform.)
1152
1153 You must use png_transforms and not call any png_set_transform() functions
1154 when you use png_read_png().
1155
1156 After you have called png_read_png(), you can retrieve the image data
1157 with
1158
1159 row_pointers = png_get_rows(png_ptr, info_ptr);
1160
1161 where row_pointers is an array of pointers to the pixel data for each row:
1162
1163 png_bytep row_pointers[height];
1164
1165 If you know your image size and pixel size ahead of time, you can allocate
1166 row_pointers prior to calling png_read_png() with
1167
1168 if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
1169 png_error (png_ptr,
1170 "Image is too tall to process in memory");
1171
1172 if (width > PNG_UINT_32_MAX/pixel_size)
1173 png_error (png_ptr,
1174 "Image is too wide to process in memory");
1175
1176 row_pointers = png_malloc(png_ptr,
1177 height*(sizeof (png_bytep)));
1178
1179 for (int i=0; i<height, i++)
1180 row_pointers[i]=NULL; /* security precaution */
1181
1182 for (int i=0; i<height, i++)
1183 row_pointers[i]=png_malloc(png_ptr,
1184 width*pixel_size);
1185
1186 png_set_rows(png_ptr, info_ptr, &row_pointers);
1187
1188 Alternatively you could allocate your image in one big block and define
1189 row_pointers[i] to point into the proper places in your block.
1190
1191 If you use png_set_rows(), the application is responsible for freeing
1192 row_pointers (and row_pointers[i], if they were separately allocated).
1193
1194 If you don't allocate row_pointers ahead of time, png_read_png() will
1195 do it, and it'll be free'ed by libpng when you call png_destroy_*().
1196
1197 The low-level read interface
1198
1199 If you are going the low-level route, you are now ready to read all
1200 the file information up to the actual image data. You do this with a
1201 call to png_read_info().
1202
1203 png_read_info(png_ptr, info_ptr);
1204
1205 This will process all chunks up to but not including the image data.
1206
1207 This also copies some of the data from the PNG file into the decode structure
1208 for use in later transformations. Important information copied in is:
1209
1210 1) The PNG file gamma from the gAMA chunk. This overwrites the default value
1211 provided by an earlier call to png_set_gamma or png_set_alpha_mode.
1212
1213 2) Prior to libpng-1.5.4 the background color from a bKGd chunk. This
1214 damages the information provided by an earlier call to png_set_background
1215 resulting in unexpected behavior. Libpng-1.5.4 no longer does this.
1216
1217 3) The number of significant bits in each component value. Libpng uses this to
1218 optimize gamma handling by reducing the internal lookup table sizes.
1219
1220 4) The transparent color information from a tRNS chunk. This can be modified by
1221 a later call to png_set_tRNS.
1222
1223 Querying the info structure
1224
1225 Functions are used to get the information from the info_ptr once it
1226 has been read. Note that these fields may not be completely filled
1227 in until png_read_end() has read the chunk data following the image.
1228
1229 png_get_IHDR(png_ptr, info_ptr, &width, &height,
1230 &bit_depth, &color_type, &interlace_type,
1231 &compression_type, &filter_method);
1232
1233 width - holds the width of the image
1234 in pixels (up to 2^31).
1235
1236 height - holds the height of the image
1237 in pixels (up to 2^31).
1238
1239 bit_depth - holds the bit depth of one of the
1240 image channels. (valid values are
1241 1, 2, 4, 8, 16 and depend also on
1242 the color_type. See also
1243 significant bits (sBIT) below).
1244
1245 color_type - describes which color/alpha channels
1246 are present.
1247 PNG_COLOR_TYPE_GRAY
1248 (bit depths 1, 2, 4, 8, 16)
1249 PNG_COLOR_TYPE_GRAY_ALPHA
1250 (bit depths 8, 16)
1251 PNG_COLOR_TYPE_PALETTE
1252 (bit depths 1, 2, 4, 8)
1253 PNG_COLOR_TYPE_RGB
1254 (bit_depths 8, 16)
1255 PNG_COLOR_TYPE_RGB_ALPHA
1256 (bit_depths 8, 16)
1257
1258 PNG_COLOR_MASK_PALETTE
1259 PNG_COLOR_MASK_COLOR
1260 PNG_COLOR_MASK_ALPHA
1261
1262 interlace_type - (PNG_INTERLACE_NONE or
1263 PNG_INTERLACE_ADAM7)
1264
1265 compression_type - (must be PNG_COMPRESSION_TYPE_BASE
1266 for PNG 1.0)
1267
1268 filter_method - (must be PNG_FILTER_TYPE_BASE
1269 for PNG 1.0, and can also be
1270 PNG_INTRAPIXEL_DIFFERENCING if
1271 the PNG datastream is embedded in
1272 a MNG-1.0 datastream)
1273
1274 Any of width, height, color_type, bit_depth,
1275 interlace_type, compression_type, or filter_method can
1276 be NULL if you are not interested in their values.
1277
1278 Note that png_get_IHDR() returns 32-bit data into
1279 the application's width and height variables.
1280 This is an unsafe situation if these are not png_uint_32
1281 variables. In such situations, the
1282 png_get_image_width() and png_get_image_height()
1283 functions described below are safer.
1284
1285 width = png_get_image_width(png_ptr,
1286 info_ptr);
1287
1288 height = png_get_image_height(png_ptr,
1289 info_ptr);
1290
1291 bit_depth = png_get_bit_depth(png_ptr,
1292 info_ptr);
1293
1294 color_type = png_get_color_type(png_ptr,
1295 info_ptr);
1296
1297 interlace_type = png_get_interlace_type(png_ptr,
1298 info_ptr);
1299
1300 compression_type = png_get_compression_type(png_ptr,
1301 info_ptr);
1302
1303 filter_method = png_get_filter_type(png_ptr,
1304 info_ptr);
1305
1306 channels = png_get_channels(png_ptr, info_ptr);
1307
1308 channels - number of channels of info for the
1309 color type (valid values are 1 (GRAY,
1310 PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
1311 4 (RGB_ALPHA or RGB + filler byte))
1312
1313 rowbytes = png_get_rowbytes(png_ptr, info_ptr);
1314
1315 rowbytes - number of bytes needed to hold a row
1316
1317 signature = png_get_signature(png_ptr, info_ptr);
1318
1319 signature - holds the signature read from the
1320 file (if any). The data is kept in
1321 the same offset it would be if the
1322 whole signature were read (i.e. if an
1323 application had already read in 4
1324 bytes of signature before starting
1325 libpng, the remaining 4 bytes would
1326 be in signature[4] through signature[7]
1327 (see png_set_sig_bytes())).
1328
1329 These are also important, but their validity depends on whether the chunk
1330 has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
1331 png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
1332 data has been read, or zero if it is missing. The parameters to the
1333 png_get_<chunk> are set directly if they are simple data types, or a
1334 pointer into the info_ptr is returned for any complex types.
1335
1336 The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks
1337 is simply returned to give the application information about how the
1338 image was encoded. Libpng itself only does transformations using the file
1339 gamma when combining semitransparent pixels with the background color, and,
1340 since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels
1341 within the simplified API. Libpng also uses the file gamma when converting
1342 RGB to gray, beginning with libpng-1.0.5, if the application calls
1343 png_set_rgb_to_gray()).
1344
1345 png_get_PLTE(png_ptr, info_ptr, &palette,
1346 &num_palette);
1347
1348 palette - the palette for the file
1349 (array of png_color)
1350
1351 num_palette - number of entries in the palette
1352
1353 png_get_gAMA(png_ptr, info_ptr, &file_gamma);
1354 png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
1355
1356 file_gamma - the gamma at which the file is
1357 written (PNG_INFO_gAMA)
1358
1359 int_file_gamma - 100,000 times the gamma at which the
1360 file is written
1361
1362 png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x,
1363 &red_y, &green_x, &green_y, &blue_x, &blue_y)
1364 png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
1365 &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
1366 &blue_Z)
1367 png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
1368 &int_white_y, &int_red_x, &int_red_y,
1369 &int_green_x, &int_green_y, &int_blue_x,
1370 &int_blue_y)
1371 png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
1372 &int_red_Z, &int_green_X, &int_green_Y,
1373 &int_green_Z, &int_blue_X, &int_blue_Y,
1374 &int_blue_Z)
1375
1376 {white,red,green,blue}_{x,y}
1377 A color space encoding specified using the
1378 chromaticities of the end points and the
1379 white point. (PNG_INFO_cHRM)
1380
1381 {red,green,blue}_{X,Y,Z}
1382 A color space encoding specified using the
1383 encoding end points - the CIE tristimulus
1384 specification of the intended color of the red,
1385 green and blue channels in the PNG RGB data.
1386 The white point is simply the sum of the three
1387 end points. (PNG_INFO_cHRM)
1388
1389 png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
1390
1391 srgb_intent - the rendering intent (PNG_INFO_sRGB)
1392 The presence of the sRGB chunk
1393 means that the pixel data is in the
1394 sRGB color space. This chunk also
1395 implies specific values of gAMA and
1396 cHRM.
1397
1398 png_get_iCCP(png_ptr, info_ptr, &name,
1399 &compression_type, &profile, &proflen);
1400
1401 name - The profile name.
1402
1403 compression_type - The compression type; always
1404 PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
1405 You may give NULL to this argument to
1406 ignore it.
1407
1408 profile - International Color Consortium color
1409 profile data. May contain NULs.
1410
1411 proflen - length of profile data in bytes.
1412
1413 png_get_sBIT(png_ptr, info_ptr, &sig_bit);
1414
1415 sig_bit - the number of significant bits for
1416 (PNG_INFO_sBIT) each of the gray,
1417 red, green, and blue channels,
1418 whichever are appropriate for the
1419 given color type (png_color_16)
1420
1421 png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
1422 &num_trans, &trans_color);
1423
1424 trans_alpha - array of alpha (transparency)
1425 entries for palette (PNG_INFO_tRNS)
1426
1427 num_trans - number of transparent entries
1428 (PNG_INFO_tRNS)
1429
1430 trans_color - graylevel or color sample values of
1431 the single transparent color for
1432 non-paletted images (PNG_INFO_tRNS)
1433
1434 png_get_hIST(png_ptr, info_ptr, &hist);
1435 (PNG_INFO_hIST)
1436
1437 hist - histogram of palette (array of
1438 png_uint_16)
1439
1440 png_get_tIME(png_ptr, info_ptr, &mod_time);
1441
1442 mod_time - time image was last modified
1443 (PNG_VALID_tIME)
1444
1445 png_get_bKGD(png_ptr, info_ptr, &background);
1446
1447 background - background color (of type
1448 png_color_16p) (PNG_VALID_bKGD)
1449 valid 16-bit red, green and blue
1450 values, regardless of color_type
1451
1452 num_comments = png_get_text(png_ptr, info_ptr,
1453 &text_ptr, &num_text);
1454
1455 num_comments - number of comments
1456
1457 text_ptr - array of png_text holding image
1458 comments
1459
1460 text_ptr[i].compression - type of compression used
1461 on "text" PNG_TEXT_COMPRESSION_NONE
1462 PNG_TEXT_COMPRESSION_zTXt
1463 PNG_ITXT_COMPRESSION_NONE
1464 PNG_ITXT_COMPRESSION_zTXt
1465
1466 text_ptr[i].key - keyword for comment. Must contain
1467 1-79 characters.
1468
1469 text_ptr[i].text - text comments for current
1470 keyword. Can be empty.
1471
1472 text_ptr[i].text_length - length of text string,
1473 after decompression, 0 for iTXt
1474
1475 text_ptr[i].itxt_length - length of itxt string,
1476 after decompression, 0 for tEXt/zTXt
1477
1478 text_ptr[i].lang - language of comment (empty
1479 string for unknown).
1480
1481 text_ptr[i].lang_key - keyword in UTF-8
1482 (empty string for unknown).
1483
1484 Note that the itxt_length, lang, and lang_key
1485 members of the text_ptr structure only exist when the
1486 library is built with iTXt chunk support. Prior to
1487 libpng-1.4.0 the library was built by default without
1488 iTXt support. Also note that when iTXt is supported,
1489 they contain NULL pointers when the "compression"
1490 field contains PNG_TEXT_COMPRESSION_NONE or
1491 PNG_TEXT_COMPRESSION_zTXt.
1492
1493 num_text - number of comments (same as
1494 num_comments; you can put NULL here
1495 to avoid the duplication)
1496
1497 Note while png_set_text() will accept text, language,
1498 and translated keywords that can be NULL pointers, the
1499 structure returned by png_get_text will always contain
1500 regular zero-terminated C strings. They might be
1501 empty strings but they will never be NULL pointers.
1502
1503 num_spalettes = png_get_sPLT(png_ptr, info_ptr,
1504 &palette_ptr);
1505
1506 num_spalettes - number of sPLT chunks read.
1507
1508 palette_ptr - array of palette structures holding
1509 contents of one or more sPLT chunks
1510 read.
1511
1512 png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
1513 &unit_type);
1514
1515 offset_x - positive offset from the left edge
1516 of the screen (can be negative)
1517
1518 offset_y - positive offset from the top edge
1519 of the screen (can be negative)
1520
1521 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
1522
1523 png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
1524 &unit_type);
1525
1526 res_x - pixels/unit physical resolution in
1527 x direction
1528
1529 res_y - pixels/unit physical resolution in
1530 x direction
1531
1532 unit_type - PNG_RESOLUTION_UNKNOWN,
1533 PNG_RESOLUTION_METER
1534
1535 png_get_sCAL(png_ptr, info_ptr, &unit, &width,
1536 &height)
1537
1538 unit - physical scale units (an integer)
1539
1540 width - width of a pixel in physical scale units
1541
1542 height - height of a pixel in physical scale units
1543 (width and height are doubles)
1544
1545 png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
1546 &height)
1547
1548 unit - physical scale units (an integer)
1549
1550 width - width of a pixel in physical scale units
1551 (expressed as a string)
1552
1553 height - height of a pixel in physical scale units
1554 (width and height are strings like "2.54")
1555
1556 num_unknown_chunks = png_get_unknown_chunks(png_ptr,
1557 info_ptr, &unknowns)
1558
1559 unknowns - array of png_unknown_chunk
1560 structures holding unknown chunks
1561
1562 unknowns[i].name - name of unknown chunk
1563
1564 unknowns[i].data - data of unknown chunk
1565
1566 unknowns[i].size - size of unknown chunk's data
1567
1568 unknowns[i].location - position of chunk in file
1569
1570 The value of "i" corresponds to the order in which the
1571 chunks were read from the PNG file or inserted with the
1572 png_set_unknown_chunks() function.
1573
1574 The value of "location" is a bitwise "or" of
1575
1576 PNG_HAVE_IHDR (0x01)
1577 PNG_HAVE_PLTE (0x02)
1578 PNG_AFTER_IDAT (0x08)
1579
1580 The data from the pHYs chunk can be retrieved in several convenient
1581 forms:
1582
1583 res_x = png_get_x_pixels_per_meter(png_ptr,
1584 info_ptr)
1585
1586 res_y = png_get_y_pixels_per_meter(png_ptr,
1587 info_ptr)
1588
1589 res_x_and_y = png_get_pixels_per_meter(png_ptr,
1590 info_ptr)
1591
1592 res_x = png_get_x_pixels_per_inch(png_ptr,
1593 info_ptr)
1594
1595 res_y = png_get_y_pixels_per_inch(png_ptr,
1596 info_ptr)
1597
1598 res_x_and_y = png_get_pixels_per_inch(png_ptr,
1599 info_ptr)
1600
1601 aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
1602 info_ptr)
1603
1604 Each of these returns 0 [signifying "unknown"] if
1605 the data is not present or if res_x is 0;
1606 res_x_and_y is 0 if res_x != res_y
1607
1608 Note that because of the way the resolutions are
1609 stored internally, the inch conversions won't
1610 come out to exactly even number. For example,
1611 72 dpi is stored as 0.28346 pixels/meter, and
1612 when this is retrieved it is 71.9988 dpi, so
1613 be sure to round the returned value appropriately
1614 if you want to display a reasonable-looking result.
1615
1616 The data from the oFFs chunk can be retrieved in several convenient
1617 forms:
1618
1619 x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
1620
1621 y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
1622
1623 x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
1624
1625 y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
1626
1627 Each of these returns 0 [signifying "unknown" if both
1628 x and y are 0] if the data is not present or if the
1629 chunk is present but the unit is the pixel. The
1630 remark about inexact inch conversions applies here
1631 as well, because a value in inches can't always be
1632 converted to microns and back without some loss
1633 of precision.
1634
1635 For more information, see the
1636 PNG specification for chunk contents. Be careful with trusting
1637 rowbytes, as some of the transformations could increase the space
1638 needed to hold a row (expand, filler, gray_to_rgb, etc.).
1639 See png_read_update_info(), below.
1640
1641 A quick word about text_ptr and num_text. PNG stores comments in
1642 keyword/text pairs, one pair per chunk, with no limit on the number
1643 of text chunks, and a 2^31 byte limit on their size. While there are
1644 suggested keywords, there is no requirement to restrict the use to these
1645 strings. It is strongly suggested that keywords and text be sensible
1646 to humans (that's the point), so don't use abbreviations. Non-printing
1647 symbols are not allowed. See the PNG specification for more details.
1648 There is also no requirement to have text after the keyword.
1649
1650 Keywords should be limited to 79 Latin-1 characters without leading or
1651 trailing spaces, but non-consecutive spaces are allowed within the
1652 keyword. It is possible to have the same keyword any number of times.
1653 The text_ptr is an array of png_text structures, each holding a
1654 pointer to a language string, a pointer to a keyword and a pointer to
1655 a text string. The text string, language code, and translated
1656 keyword may be empty or NULL pointers. The keyword/text
1657 pairs are put into the array in the order that they are received.
1658 However, some or all of the text chunks may be after the image, so, to
1659 make sure you have read all the text chunks, don't mess with these
1660 until after you read the stuff after the image. This will be
1661 mentioned again below in the discussion that goes with png_read_end().
1662
1663 Input transformations
1664
1665 After you've read the header information, you can set up the library
1666 to handle any special transformations of the image data. The various
1667 ways to transform the data will be described in the order that they
1668 should occur. This is important, as some of these change the color
1669 type and/or bit depth of the data, and some others only work on
1670 certain color types and bit depths.
1671
1672 Transformations you request are ignored if they don't have any meaning for a
1673 particular input data format. However some transformations can have an effect
1674 as a result of a previous transformation. If you specify a contradictory set of
1675 transformations, for example both adding and removing the alpha channel, you
1676 cannot predict the final result.
1677
1678 The color used for the transparency values should be supplied in the same
1679 format/depth as the current image data. It is stored in the same format/depth
1680 as the image data in a tRNS chunk, so this is what libpng expects for this data.
1681
1682 The color used for the background value depends on the need_expand argument as
1683 described below.
1684
1685 Data will be decoded into the supplied row buffers packed into bytes
1686 unless the library has been told to transform it into another format.
1687 For example, 4 bit/pixel paletted or grayscale data will be returned
1688 2 pixels/byte with the leftmost pixel in the high-order bits of the byte,
1689 unless png_set_packing() is called. 8-bit RGB data will be stored
1690 in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
1691 is called to insert filler bytes, either before or after each RGB triplet.
1692
1693 16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
1694 byte of the color value first, unless png_set_scale_16() is called to
1695 transform it to regular RGB RGB triplets, or png_set_filler() or
1696 png_set_add alpha() is called to insert two filler bytes, either before
1697 or after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
1698 be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
1699 or png_set_scale_16().
1700
1701 The following code transforms grayscale images of less than 8 to 8 bits,
1702 changes paletted images to RGB, and adds a full alpha channel if there is
1703 transparency information in a tRNS chunk. This is most useful on
1704 grayscale images with bit depths of 2 or 4 or if there is a multiple-image
1705 viewing application that wishes to treat all images in the same way.
1706
1707 if (color_type == PNG_COLOR_TYPE_PALETTE)
1708 png_set_palette_to_rgb(png_ptr);
1709
1710 if (png_get_valid(png_ptr, info_ptr,
1711 PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
1712
1713 if (color_type == PNG_COLOR_TYPE_GRAY &&
1714 bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
1715
1716 The first two functions are actually aliases for png_set_expand(), added
1717 in libpng version 1.0.4, with the function names expanded to improve code
1718 readability. In some future version they may actually do different
1719 things.
1720
1721 As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
1722 added. It expands the sample depth without changing tRNS to alpha.
1723
1724 As of libpng version 1.5.2, png_set_expand_16() was added. It behaves as
1725 png_set_expand(); however, the resultant channels have 16 bits rather than 8.
1726 Use this when the output color or gray channels are made linear to avoid fairly
1727 severe accuracy loss.
1728
1729 if (bit_depth < 16)
1730 png_set_expand_16(png_ptr);
1731
1732 PNG can have files with 16 bits per channel. If you only can handle
1733 8 bits per channel, this will strip the pixels down to 8-bit.
1734
1735 if (bit_depth == 16)
1736 #if PNG_LIBPNG_VER >= 10504
1737 png_set_scale_16(png_ptr);
1738 #else
1739 png_set_strip_16(png_ptr);
1740 #endif
1741
1742 (The more accurate "png_set_scale_16()" API became available in libpng version
1743 1.5.4).
1744
1745 If you need to process the alpha channel on the image separately from the image
1746 data (for example if you convert it to a bitmap mask) it is possible to have
1747 libpng strip the channel leaving just RGB or gray data:
1748
1749 if (color_type & PNG_COLOR_MASK_ALPHA)
1750 png_set_strip_alpha(png_ptr);
1751
1752 If you strip the alpha channel you need to find some other way of dealing with
1753 the information. If, instead, you want to convert the image to an opaque
1754 version with no alpha channel use png_set_background; see below.
1755
1756 As of libpng version 1.5.2, almost all useful expansions are supported, the
1757 major ommissions are conversion of grayscale to indexed images (which can be
1758 done trivially in the application) and conversion of indexed to grayscale (which
1759 can be done by a trivial manipulation of the palette.)
1760
1761 In the following table, the 01 means grayscale with depth<8, 31 means
1762 indexed with depth<8, other numerals represent the color type, "T" means
1763 the tRNS chunk is present, A means an alpha channel is present, and O
1764 means tRNS or alpha is present but all pixels in the image are opaque.
1765
1766 FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O
1767 TO
1768 01 - [G] - - - - - - - - - - - - -
1769 31 [Q] Q [Q] [Q] [Q] Q Q Q Q Q Q [Q] [Q] Q Q
1770 0 1 G + . . G G G G G G B B GB GB
1771 0T lt Gt t + . Gt G G Gt G G Bt Bt GBt GBt
1772 0O lt Gt t . + Gt Gt G Gt Gt G Bt Bt GBt GBt
1773 2 C P C C C + . . C - - CB CB B B
1774 2T Ct - Ct C C t + t - - - CBt CBt Bt Bt
1775 2O Ct - Ct C C t t + - - - CBt CBt Bt Bt
1776 3 [Q] p [Q] [Q] [Q] Q Q Q + . . [Q] [Q] Q Q
1777 3T [Qt] p [Qt][Q] [Q] Qt Qt Qt t + t [Qt][Qt] Qt Qt
1778 3O [Qt] p [Qt][Q] [Q] Qt Qt Qt t t + [Qt][Qt] Qt Qt
1779 4A lA G A T T GA GT GT GA GT GT + BA G GBA
1780 4O lA GBA A T T GA GT GT GA GT GT BA + GBA G
1781 6A CA PA CA C C A T tT PA P P C CBA + BA
1782 6O CA PBA CA C C A tT T PA P P CBA C BA +
1783
1784 Within the matrix,
1785 "+" identifies entries where 'from' and 'to' are the same.
1786 "-" means the transformation is not supported.
1787 "." means nothing is necessary (a tRNS chunk can just be ignored).
1788 "t" means the transformation is obtained by png_set_tRNS.
1789 "A" means the transformation is obtained by png_set_add_alpha().
1790 "X" means the transformation is obtained by png_set_expand().
1791 "1" means the transformation is obtained by
1792 png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
1793 if there is no transparency in the original or the final
1794 format).
1795 "C" means the transformation is obtained by png_set_gray_to_rgb().
1796 "G" means the transformation is obtained by png_set_rgb_to_gray().
1797 "P" means the transformation is obtained by
1798 png_set_expand_palette_to_rgb().
1799 "p" means the transformation is obtained by png_set_packing().
1800 "Q" means the transformation is obtained by png_set_quantize().
1801 "T" means the transformation is obtained by
1802 png_set_tRNS_to_alpha().
1803 "B" means the transformation is obtained by
1804 png_set_background(), or png_strip_alpha().
1805
1806 When an entry has multiple transforms listed all are required to cause the
1807 right overall transformation. When two transforms are separated by a comma
1808 either will do the job. When transforms are enclosed in [] the transform should
1809 do the job but this is currently unimplemented - a different format will result
1810 if the suggested transformations are used.
1811
1812 In PNG files, the alpha channel in an image
1813 is the level of opacity. If you need the alpha channel in an image to
1814 be the level of transparency instead of opacity, you can invert the
1815 alpha channel (or the tRNS chunk data) after it's read, so that 0 is
1816 fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
1817 images) is fully transparent, with
1818
1819 png_set_invert_alpha(png_ptr);
1820
1821 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
1822 they can, resulting in, for example, 8 pixels per byte for 1 bit
1823 files. This code expands to 1 pixel per byte without changing the
1824 values of the pixels:
1825
1826 if (bit_depth < 8)
1827 png_set_packing(png_ptr);
1828
1829 PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
1830 stored in a PNG image have been "scaled" or "shifted" up to the next
1831 higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
1832 to 8 bits/sample in the range [0, 255]). However, it is also possible
1833 to convert the PNG pixel data back to the original bit depth of the
1834 image. This call reduces the pixels back down to the original bit depth:
1835
1836 png_color_8p sig_bit;
1837
1838 if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
1839 png_set_shift(png_ptr, sig_bit);
1840
1841 PNG files store 3-color pixels in red, green, blue order. This code
1842 changes the storage of the pixels to blue, green, red:
1843
1844 if (color_type == PNG_COLOR_TYPE_RGB ||
1845 color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1846 png_set_bgr(png_ptr);
1847
1848 PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
1849 into 4 or 8 bytes for windowing systems that need them in this format:
1850
1851 if (color_type == PNG_COLOR_TYPE_RGB)
1852 png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
1853
1854 where "filler" is the 8-bit or 16-bit number to fill with, and the location
1855 is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
1856 you want the filler before the RGB or after. When filling an 8-bit pixel,
1857 the least significant 8 bits of the number are used, if a 16-bit number is
1858 supplied. This transformation does not affect images that already have full
1859 alpha channels. To add an opaque alpha channel, use filler=0xffff and
1860 PNG_FILLER_AFTER which will generate RGBA pixels.
1861
1862 Note that png_set_filler() does not change the color type. If you want
1863 to do that, you can add a true alpha channel with
1864
1865 if (color_type == PNG_COLOR_TYPE_RGB ||
1866 color_type == PNG_COLOR_TYPE_GRAY)
1867 png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
1868
1869 where "filler" contains the alpha value to assign to each pixel.
1870 The png_set_add_alpha() function was added in libpng-1.2.7.
1871
1872 If you are reading an image with an alpha channel, and you need the
1873 data as ARGB instead of the normal PNG format RGBA:
1874
1875 if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1876 png_set_swap_alpha(png_ptr);
1877
1878 For some uses, you may want a grayscale image to be represented as
1879 RGB. This code will do that conversion:
1880
1881 if (color_type == PNG_COLOR_TYPE_GRAY ||
1882 color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
1883 png_set_gray_to_rgb(png_ptr);
1884
1885 Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
1886 with alpha.
1887
1888 if (color_type == PNG_COLOR_TYPE_RGB ||
1889 color_type == PNG_COLOR_TYPE_RGB_ALPHA)
1890 png_set_rgb_to_gray(png_ptr, error_action,
1891 double red_weight, double green_weight);
1892
1893 error_action = 1: silently do the conversion
1894
1895 error_action = 2: issue a warning if the original
1896 image has any pixel where
1897 red != green or red != blue
1898
1899 error_action = 3: issue an error and abort the
1900 conversion if the original
1901 image has any pixel where
1902 red != green or red != blue
1903
1904 red_weight: weight of red component
1905
1906 green_weight: weight of green component
1907 If either weight is negative, default
1908 weights are used.
1909
1910 In the corresponding fixed point API the red_weight and green_weight values are
1911 simply scaled by 100,000:
1912
1913 png_set_rgb_to_gray(png_ptr, error_action,
1914 png_fixed_point red_weight,
1915 png_fixed_point green_weight);
1916
1917 If you have set error_action = 1 or 2, you can
1918 later check whether the image really was gray, after processing
1919 the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
1920 It will return a png_byte that is zero if the image was gray or
1921 1 if there were any non-gray pixels. Background and sBIT data
1922 will be silently converted to grayscale, using the green channel
1923 data for sBIT, regardless of the error_action setting.
1924
1925 The default values come from the PNG file cHRM chunk if present; otherwise, the
1926 defaults correspond to the ITU-R recommendation 709, and also the sRGB color
1927 space, as recommended in the Charles Poynton's Colour FAQ,
1928 Copyright (c) 2006-11-28 Charles Poynton, in section 9:
1929
1930 <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
1931
1932 Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
1933
1934 Previous versions of this document, 1998 through 2002, recommended a slightly
1935 different formula:
1936
1937 Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
1938
1939 Libpng uses an integer approximation:
1940
1941 Y = (6968 * R + 23434 * G + 2366 * B)/32768
1942
1943 The calculation is done in a linear colorspace, if the image gamma
1944 can be determined.
1945
1946 The png_set_background() function has been described already; it tells libpng to
1947 composite images with alpha or simple transparency against the supplied
1948 background color. For compatibility with versions of libpng earlier than
1949 libpng-1.5.4 it is recommended that you call the function after reading the file
1950 header, even if you don't want to use the color in a bKGD chunk, if one exists.
1951
1952 If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
1953 you may use this color, or supply another color more suitable for
1954 the current display (e.g., the background color from a web page). You
1955 need to tell libpng how the color is represented, both the format of the
1956 component values in the color (the number of bits) and the gamma encoding of the
1957 color. The function takes two arguments, background_gamma_mode and need_expand
1958 to convey this information; however, only two combinations are likely to be
1959 useful:
1960
1961 png_color_16 my_background;
1962 png_color_16p image_background;
1963
1964 if (png_get_bKGD(png_ptr, info_ptr, &image_background))
1965 png_set_background(png_ptr, image_background,
1966 PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
1967 else
1968 png_set_background(png_ptr, &my_background,
1969 PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);
1970
1971 The second call was described above - my_background is in the format of the
1972 final, display, output produced by libpng. Because you now know the format of
1973 the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
1974 output and to retain palette images (the palette colors will be modified
1975 appropriately and the tRNS chunk removed.) However, if you are doing this,
1976 take great care not to ask for transformations without checking first that
1977 they apply!
1978
1979 In the first call the background color has the original bit depth and color type
1980 of the PNG file. So, for palette images the color is supplied as a palette
1981 index and for low bit greyscale images the color is a reduced bit value in
1982 image_background->gray.
1983
1984 If you didn't call png_set_gamma() before reading the file header, for example
1985 if you need your code to remain compatible with older versions of libpng prior
1986 to libpng-1.5.4, this is the place to call it.
1987
1988 Do not call it if you called png_set_alpha_mode(); doing so will damage the
1989 settings put in place by png_set_alpha_mode(). (If png_set_alpha_mode() is
1990 supported then you can certainly do png_set_gamma() before reading the PNG
1991 header.)
1992
1993 This API unconditionally sets the screen and file gamma values, so it will
1994 override the value in the PNG file unless it is called before the PNG file
1995 reading starts. For this reason you must always call it with the PNG file
1996 value when you call it in this position:
1997
1998 if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
1999 png_set_gamma(png_ptr, screen_gamma, file_gamma);
2000
2001 else
2002 png_set_gamma(png_ptr, screen_gamma, 0.45455);
2003
2004 If you need to reduce an RGB file to a paletted file, or if a paletted
2005 file has more entries than will fit on your screen, png_set_quantize()
2006 will do that. Note that this is a simple match quantization that merely
2007 finds the closest color available. This should work fairly well with
2008 optimized palettes, but fairly badly with linear color cubes. If you
2009 pass a palette that is larger than maximum_colors, the file will
2010 reduce the number of colors in the palette so it will fit into
2011 maximum_colors. If there is a histogram, libpng will use it to make
2012 more intelligent choices when reducing the palette. If there is no
2013 histogram, it may not do as good a job.
2014
2015 if (color_type & PNG_COLOR_MASK_COLOR)
2016 {
2017 if (png_get_valid(png_ptr, info_ptr,
2018 PNG_INFO_PLTE))
2019 {
2020 png_uint_16p histogram = NULL;
2021
2022 png_get_hIST(png_ptr, info_ptr,
2023 &histogram);
2024 png_set_quantize(png_ptr, palette, num_palette,
2025 max_screen_colors, histogram, 1);
2026 }
2027
2028 else
2029 {
2030 png_color std_color_cube[MAX_SCREEN_COLORS] =
2031 { ... colors ... };
2032
2033 png_set_quantize(png_ptr, std_color_cube,
2034 MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
2035 NULL,0);
2036 }
2037 }
2038
2039 PNG files describe monochrome as black being zero and white being one.
2040 The following code will reverse this (make black be one and white be
2041 zero):
2042
2043 if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
2044 png_set_invert_mono(png_ptr);
2045
2046 This function can also be used to invert grayscale and gray-alpha images:
2047
2048 if (color_type == PNG_COLOR_TYPE_GRAY ||
2049 color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
2050 png_set_invert_mono(png_ptr);
2051
2052 PNG files store 16-bit pixels in network byte order (big-endian,
2053 ie. most significant bits first). This code changes the storage to the
2054 other way (little-endian, i.e. least significant bits first, the
2055 way PCs store them):
2056
2057 if (bit_depth == 16)
2058 png_set_swap(png_ptr);
2059
2060 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
2061 need to change the order the pixels are packed into bytes, you can use:
2062
2063 if (bit_depth < 8)
2064 png_set_packswap(png_ptr);
2065
2066 Finally, you can write your own transformation function if none of
2067 the existing ones meets your needs. This is done by setting a callback
2068 with
2069
2070 png_set_read_user_transform_fn(png_ptr,
2071 read_transform_fn);
2072
2073 You must supply the function
2074
2075 void read_transform_fn(png_structp png_ptr, png_row_infop
2076 row_info, png_bytep data)
2077
2078 See pngtest.c for a working example. Your function will be called
2079 after all of the other transformations have been processed. Take care with
2080 interlaced images if you do the interlace yourself - the width of the row is the
2081 width in 'row_info', not the overall image width.
2082
2083 If supported, libpng provides two information routines that you can use to find
2084 where you are in processing the image:
2085
2086 png_get_current_pass_number(png_structp png_ptr);
2087 png_get_current_row_number(png_structp png_ptr);
2088
2089 Don't try using these outside a transform callback - firstly they are only
2090 supported if user transforms are supported, secondly they may well return
2091 unexpected results unless the row is actually being processed at the moment they
2092 are called.
2093
2094 With interlaced
2095 images the value returned is the row in the input sub-image image. Use
2096 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
2097 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
2098
2099 The discussion of interlace handling above contains more information on how to
2100 use these values.
2101
2102 You can also set up a pointer to a user structure for use by your
2103 callback function, and you can inform libpng that your transform
2104 function will change the number of channels or bit depth with the
2105 function
2106
2107 png_set_user_transform_info(png_ptr, user_ptr,
2108 user_depth, user_channels);
2109
2110 The user's application, not libpng, is responsible for allocating and
2111 freeing any memory required for the user structure.
2112
2113 You can retrieve the pointer via the function
2114 png_get_user_transform_ptr(). For example:
2115
2116 voidp read_user_transform_ptr =
2117 png_get_user_transform_ptr(png_ptr);
2118
2119 The last thing to handle is interlacing; this is covered in detail below,
2120 but you must call the function here if you want libpng to handle expansion
2121 of the interlaced image.
2122
2123 number_of_passes = png_set_interlace_handling(png_ptr);
2124
2125 After setting the transformations, libpng can update your png_info
2126 structure to reflect any transformations you've requested with this
2127 call.
2128
2129 png_read_update_info(png_ptr, info_ptr);
2130
2131 This is most useful to update the info structure's rowbytes
2132 field so you can use it to allocate your image memory. This function
2133 will also update your palette with the correct screen_gamma and
2134 background if these have been given with the calls above. You may
2135 only call png_read_update_info() once with a particular info_ptr.
2136
2137 After you call png_read_update_info(), you can allocate any
2138 memory you need to hold the image. The row data is simply
2139 raw byte data for all forms of images. As the actual allocation
2140 varies among applications, no example will be given. If you
2141 are allocating one large chunk, you will need to build an
2142 array of pointers to each row, as it will be needed for some
2143 of the functions below.
2144
2145 Remember: Before you call png_read_update_info(), the png_get_*()
2146 functions return the values corresponding to the original PNG image.
2147 After you call png_read_update_info the values refer to the image
2148 that libpng will output. Consequently you must call all the png_set_
2149 functions before you call png_read_update_info(). This is particularly
2150 important for png_set_interlace_handling() - if you are going to call
2151 png_read_update_info() you must call png_set_interlace_handling() before
2152 it unless you want to receive interlaced output.
2153
2154 Reading image data
2155
2156 After you've allocated memory, you can read the image data.
2157 The simplest way to do this is in one function call. If you are
2158 allocating enough memory to hold the whole image, you can just
2159 call png_read_image() and libpng will read in all the image data
2160 and put it in the memory area supplied. You will need to pass in
2161 an array of pointers to each row.
2162
2163 This function automatically handles interlacing, so you don't
2164 need to call png_set_interlace_handling() (unless you call
2165 png_read_update_info()) or call this function multiple times, or any
2166 of that other stuff necessary with png_read_rows().
2167
2168 png_read_image(png_ptr, row_pointers);
2169
2170 where row_pointers is:
2171
2172 png_bytep row_pointers[height];
2173
2174 You can point to void or char or whatever you use for pixels.
2175
2176 If you don't want to read in the whole image at once, you can
2177 use png_read_rows() instead. If there is no interlacing (check
2178 interlace_type == PNG_INTERLACE_NONE), this is simple:
2179
2180 png_read_rows(png_ptr, row_pointers, NULL,
2181 number_of_rows);
2182
2183 where row_pointers is the same as in the png_read_image() call.
2184
2185 If you are doing this just one row at a time, you can do this with
2186 a single row_pointer instead of an array of row_pointers:
2187
2188 png_bytep row_pointer = row;
2189 png_read_row(png_ptr, row_pointer, NULL);
2190
2191 If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
2192 get somewhat harder. The only current (PNG Specification version 1.2)
2193 interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
2194 a somewhat complicated 2D interlace scheme, known as Adam7, that
2195 breaks down an image into seven smaller images of varying size, based
2196 on an 8x8 grid. This number is defined (from libpng 1.5) as
2197 PNG_INTERLACE_ADAM7_PASSES in png.h
2198
2199 libpng can fill out those images or it can give them to you "as is".
2200 It is almost always better to have libpng handle the interlacing for you.
2201 If you want the images filled out, there are two ways to do that. The one
2202 mentioned in the PNG specification is to expand each pixel to cover
2203 those pixels that have not been read yet (the "rectangle" method).
2204 This results in a blocky image for the first pass, which gradually
2205 smooths out as more pixels are read. The other method is the "sparkle"
2206 method, where pixels are drawn only in their final locations, with the
2207 rest of the image remaining whatever colors they were initialized to
2208 before the start of the read. The first method usually looks better,
2209 but tends to be slower, as there are more pixels to put in the rows.
2210
2211 If, as is likely, you want libpng to expand the images, call this before
2212 calling png_start_read_image() or png_read_update_info():
2213
2214 if (interlace_type == PNG_INTERLACE_ADAM7)
2215 number_of_passes
2216 = png_set_interlace_handling(png_ptr);
2217
2218 This will return the number of passes needed. Currently, this is seven,
2219 but may change if another interlace type is added. This function can be
2220 called even if the file is not interlaced, where it will return one pass.
2221 You then need to read the whole image 'number_of_passes' times. Each time
2222 will distribute the pixels from the current pass to the correct place in
2223 the output image, so you need to supply the same rows to png_read_rows in
2224 each pass.
2225
2226 If you are not going to display the image after each pass, but are
2227 going to wait until the entire image is read in, use the sparkle
2228 effect. This effect is faster and the end result of either method
2229 is exactly the same. If you are planning on displaying the image
2230 after each pass, the "rectangle" effect is generally considered the
2231 better looking one.
2232
2233 If you only want the "sparkle" effect, just call png_read_rows() as
2234 normal, with the third parameter NULL. Make sure you make pass over
2235 the image number_of_passes times, and you don't change the data in the
2236 rows between calls. You can change the locations of the data, just
2237 not the data. Each pass only writes the pixels appropriate for that
2238 pass, and assumes the data from previous passes is still valid.
2239
2240 png_read_rows(png_ptr, row_pointers, NULL,
2241 number_of_rows);
2242
2243 If you only want the first effect (the rectangles), do the same as
2244 before except pass the row buffer in the third parameter, and leave
2245 the second parameter NULL.
2246
2247 png_read_rows(png_ptr, NULL, row_pointers,
2248 number_of_rows);
2249
2250 If you don't want libpng to handle the interlacing details, just call
2251 png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
2252 Each of the images is a valid image by itself; however, you will almost
2253 certainly need to distribute the pixels from each sub-image to the
2254 correct place. This is where everything gets very tricky.
2255
2256 If you want to retrieve the separate images you must pass the correct
2257 number of rows to each successive call of png_read_rows(). The calculation
2258 gets pretty complicated for small images, where some sub-images may
2259 not even exist because either their width or height ends up zero.
2260 libpng provides two macros to help you in 1.5 and later versions:
2261
2262 png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
2263 png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);
2264
2265 Respectively these tell you the width and height of the sub-image
2266 corresponding to the numbered pass. 'pass' is in in the range 0 to 6 -
2267 this can be confusing because the specification refers to the same passes
2268 as 1 to 7! Be careful, you must check both the width and height before
2269 calling png_read_rows() and not call it for that pass if either is zero.
2270
2271 You can, of course, read each sub-image row by row. If you want to
2272 produce optimal code to make a pixel-by-pixel transformation of an
2273 interlaced image this is the best approach; read each row of each pass,
2274 transform it, and write it out to a new interlaced image.
2275
2276 If you want to de-interlace the image yourself libpng provides further
2277 macros to help that tell you where to place the pixels in the output image.
2278 Because the interlacing scheme is rectangular - sub-image pixels are always
2279 arranged on a rectangular grid - all you need to know for each pass is the
2280 starting column and row in the output image of the first pixel plus the
2281 spacing between each pixel. As of libpng 1.5 there are four macros to
2282 retrieve this information:
2283
2284 png_uint_32 x = PNG_PASS_START_COL(pass);
2285 png_uint_32 y = PNG_PASS_START_ROW(pass);
2286 png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
2287 png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);
2288
2289 These allow you to write the obvious loop:
2290
2291 png_uint_32 input_y = 0;
2292 png_uint_32 output_y = PNG_PASS_START_ROW(pass);
2293
2294 while (output_y < output_image_height)
2295 {
2296 png_uint_32 input_x = 0;
2297 png_uint_32 output_x = PNG_PASS_START_COL(pass);
2298
2299 while (output_x < output_image_width)
2300 {
2301 image[output_y][output_x] =
2302 subimage[pass][input_y][input_x++];
2303
2304 output_x += xStep;
2305 }
2306
2307 ++input_y;
2308 output_y += yStep;
2309 }
2310
2311 Notice that the steps between successive output rows and columns are
2312 returned as shifts. This is possible because the pixels in the subimages
2313 are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
2314 image. In practice you may need to directly calculate the output coordinate
2315 given an input coordinate. libpng provides two further macros for this
2316 purpose:
2317
2318 png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
2319 png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);
2320
2321 Finally a pair of macros are provided to tell you if a particular image
2322 row or column appears in a given pass:
2323
2324 int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
2325 int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);
2326
2327 Bear in mind that you will probably also need to check the width and height
2328 of the pass in addition to the above to be sure the pass even exists!
2329
2330 With any luck you are convinced by now that you don't want to do your own
2331 interlace handling. In reality normally the only good reason for doing this
2332 is if you are processing PNG files on a pixel-by-pixel basis and don't want
2333 to load the whole file into memory when it is interlaced.
2334
2335 libpng includes a test program, pngvalid, that illustrates reading and
2336 writing of interlaced images. If you can't get interlacing to work in your
2337 code and don't want to leave it to libpng (the recommended approach), see
2338 how pngvalid.c does it.
2339
2340 Finishing a sequential read
2341
2342 After you are finished reading the image through the
2343 low-level interface, you can finish reading the file.
2344
2345 If you want to use a different crc action for handling CRC errors in
2346 chunks after the image data, you can call png_set_crc_action()
2347 again at this point.
2348
2349 If you are interested in comments or time, which may be stored either
2350 before or after the image data, you should pass the separate png_info
2351 struct if you want to keep the comments from before and after the image
2352 separate.
2353
2354 png_infop end_info = png_create_info_struct(png_ptr);
2355
2356 if (!end_info)
2357 {
2358 png_destroy_read_struct(&png_ptr, &info_ptr,
2359 (png_infopp)NULL);
2360 return (ERROR);
2361 }
2362
2363 png_read_end(png_ptr, end_info);
2364
2365 If you are not interested, you should still call png_read_end()
2366 but you can pass NULL, avoiding the need to create an end_info structure.
2367 If you do this, libpng will not process any chunks after IDAT other than
2368 skipping over them and perhaps (depending on whether you have called
2369 png_set_crc_action) checking their CRCs while looking for the IEND chunk.
2370
2371 png_read_end(png_ptr, (png_infop)NULL);
2372
2373 If you don't call png_read_end(), then your file pointer will be
2374 left pointing to the first chunk after the last IDAT, which is probably
2375 not what you want if you expect to read something beyond the end of
2376 the PNG datastream.
2377
2378 When you are done, you can free all memory allocated by libpng like this:
2379
2380 png_destroy_read_struct(&png_ptr, &info_ptr,
2381 &end_info);
2382
2383 or, if you didn't create an end_info structure,
2384
2385 png_destroy_read_struct(&png_ptr, &info_ptr,
2386 (png_infopp)NULL);
2387
2388 It is also possible to individually free the info_ptr members that
2389 point to libpng-allocated storage with the following function:
2390
2391 png_free_data(png_ptr, info_ptr, mask, seq)
2392
2393 mask - identifies data to be freed, a mask
2394 containing the bitwise OR of one or
2395 more of
2396 PNG_FREE_PLTE, PNG_FREE_TRNS,
2397 PNG_FREE_HIST, PNG_FREE_ICCP,
2398 PNG_FREE_PCAL, PNG_FREE_ROWS,
2399 PNG_FREE_SCAL, PNG_FREE_SPLT,
2400 PNG_FREE_TEXT, PNG_FREE_UNKN,
2401 or simply PNG_FREE_ALL
2402
2403 seq - sequence number of item to be freed
2404 (-1 for all items)
2405
2406 This function may be safely called when the relevant storage has
2407 already been freed, or has not yet been allocated, or was allocated
2408 by the user and not by libpng, and will in those cases do nothing.
2409 The "seq" parameter is ignored if only one item of the selected data
2410 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
2411 are allowed for the data type identified in the mask, such as text or
2412 sPLT, only the n'th item in the structure is freed, where n is "seq".
2413
2414 The default behavior is only to free data that was allocated internally
2415 by libpng. This can be changed, so that libpng will not free the data,
2416 or so that it will free data that was allocated by the user with png_malloc()
2417 or png_calloc() and passed in via a png_set_*() function, with
2418
2419 png_data_freer(png_ptr, info_ptr, freer, mask)
2420
2421 freer - one of
2422 PNG_DESTROY_WILL_FREE_DATA
2423 PNG_SET_WILL_FREE_DATA
2424 PNG_USER_WILL_FREE_DATA
2425
2426 mask - which data elements are affected
2427 same choices as in png_free_data()
2428
2429 This function only affects data that has already been allocated.
2430 You can call this function after reading the PNG data but before calling
2431 any png_set_*() functions, to control whether the user or the png_set_*()
2432 function is responsible for freeing any existing data that might be present,
2433 and again after the png_set_*() functions to control whether the user
2434 or png_destroy_*() is supposed to free the data. When the user assumes
2435 responsibility for libpng-allocated data, the application must use
2436 png_free() to free it, and when the user transfers responsibility to libpng
2437 for data that the user has allocated, the user must have used png_malloc()
2438 or png_calloc() to allocate it.
2439
2440 If you allocated your row_pointers in a single block, as suggested above in
2441 the description of the high level read interface, you must not transfer
2442 responsibility for freeing it to the png_set_rows or png_read_destroy function,
2443 because they would also try to free the individual row_pointers[i].
2444
2445 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
2446 separately, do not transfer responsibility for freeing text_ptr to libpng,
2447 because when libpng fills a png_text structure it combines these members with
2448 the key member, and png_free_data() will free only text_ptr.key. Similarly,
2449 if you transfer responsibility for free'ing text_ptr from libpng to your
2450 application, your application must not separately free those members.
2451
2452 The png_free_data() function will turn off the "valid" flag for anything
2453 it frees. If you need to turn the flag off for a chunk that was freed by
2454 your application instead of by libpng, you can use
2455
2456 png_set_invalid(png_ptr, info_ptr, mask);
2457
2458 mask - identifies the chunks to be made invalid,
2459 containing the bitwise OR of one or
2460 more of
2461 PNG_INFO_gAMA, PNG_INFO_sBIT,
2462 PNG_INFO_cHRM, PNG_INFO_PLTE,
2463 PNG_INFO_tRNS, PNG_INFO_bKGD,
2464 PNG_INFO_hIST, PNG_INFO_pHYs,
2465 PNG_INFO_oFFs, PNG_INFO_tIME,
2466 PNG_INFO_pCAL, PNG_INFO_sRGB,
2467 PNG_INFO_iCCP, PNG_INFO_sPLT,
2468 PNG_INFO_sCAL, PNG_INFO_IDAT
2469
2470 For a more compact example of reading a PNG image, see the file example.c.
2471
2472 Reading PNG files progressively
2473
2474 The progressive reader is slightly different from the non-progressive
2475 reader. Instead of calling png_read_info(), png_read_rows(), and
2476 png_read_end(), you make one call to png_process_data(), which calls
2477 callbacks when it has the info, a row, or the end of the image. You
2478 set up these callbacks with png_set_progressive_read_fn(). You don't
2479 have to worry about the input/output functions of libpng, as you are
2480 giving the library the data directly in png_process_data(). I will
2481 assume that you have read the section on reading PNG files above,
2482 so I will only highlight the differences (although I will show
2483 all of the code).
2484
2485 png_structp png_ptr;
2486 png_infop info_ptr;
2487
2488 /* An example code fragment of how you would
2489 initialize the progressive reader in your
2490 application. */
2491 int
2492 initialize_png_reader()
2493 {
2494 png_ptr = png_create_read_struct
2495 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
2496 user_error_fn, user_warning_fn);
2497
2498 if (!png_ptr)
2499 return (ERROR);
2500
2501 info_ptr = png_create_info_struct(png_ptr);
2502
2503 if (!info_ptr)
2504 {
2505 png_destroy_read_struct(&png_ptr,
2506 (png_infopp)NULL, (png_infopp)NULL);
2507 return (ERROR);
2508 }
2509
2510 if (setjmp(png_jmpbuf(png_ptr)))
2511 {
2512 png_destroy_read_struct(&png_ptr, &info_ptr,
2513 (png_infopp)NULL);
2514 return (ERROR);
2515 }
2516
2517 /* This one's new. You can provide functions
2518 to be called when the header info is valid,
2519 when each row is completed, and when the image
2520 is finished. If you aren't using all functions,
2521 you can specify NULL parameters. Even when all
2522 three functions are NULL, you need to call
2523 png_set_progressive_read_fn(). You can use
2524 any struct as the user_ptr (cast to a void pointer
2525 for the function call), and retrieve the pointer
2526 from inside the callbacks using the function
2527
2528 png_get_progressive_ptr(png_ptr);
2529
2530 which will return a void pointer, which you have
2531 to cast appropriately.
2532 */
2533 png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
2534 info_callback, row_callback, end_callback);
2535
2536 return 0;
2537 }
2538
2539 /* A code fragment that you call as you receive blocks
2540 of data */
2541 int
2542 process_data(png_bytep buffer, png_uint_32 length)
2543 {
2544 if (setjmp(png_jmpbuf(png_ptr)))
2545 {
2546 png_destroy_read_struct(&png_ptr, &info_ptr,
2547 (png_infopp)NULL);
2548 return (ERROR);
2549 }
2550
2551 /* This one's new also. Simply give it a chunk
2552 of data from the file stream (in order, of
2553 course). On machines with segmented memory
2554 models machines, don't give it any more than
2555 64K. The library seems to run fine with sizes
2556 of 4K. Although you can give it much less if
2557 necessary (I assume you can give it chunks of
2558 1 byte, I haven't tried less than 256 bytes
2559 yet). When this function returns, you may
2560 want to display any rows that were generated
2561 in the row callback if you don't already do
2562 so there.
2563 */
2564 png_process_data(png_ptr, info_ptr, buffer, length);
2565
2566 /* At this point you can call png_process_data_skip if
2567 you want to handle data the library will skip yourself;
2568 it simply returns the number of bytes to skip (and stops
2569 libpng skipping that number of bytes on the next
2570 png_process_data call).
2571 return 0;
2572 }
2573
2574 /* This function is called (as set by
2575 png_set_progressive_read_fn() above) when enough data
2576 has been supplied so all of the header has been
2577 read.
2578 */
2579 void
2580 info_callback(png_structp png_ptr, png_infop info)
2581 {
2582 /* Do any setup here, including setting any of
2583 the transformations mentioned in the Reading
2584 PNG files section. For now, you _must_ call
2585 either png_start_read_image() or
2586 png_read_update_info() after all the
2587 transformations are set (even if you don't set
2588 any). You may start getting rows before
2589 png_process_data() returns, so this is your
2590 last chance to prepare for that.
2591
2592 This is where you turn on interlace handling,
2593 assuming you don't want to do it yourself.
2594
2595 If you need to you can stop the processing of
2596 your original input data at this point by calling
2597 png_process_data_pause. This returns the number
2598 of unprocessed bytes from the last png_process_data
2599 call - it is up to you to ensure that the next call
2600 sees these bytes again. If you don't want to bother
2601 with this you can get libpng to cache the unread
2602 bytes by setting the 'save' parameter (see png.h) but
2603 then libpng will have to copy the data internally.
2604 */
2605 }
2606
2607 /* This function is called when each row of image
2608 data is complete */
2609 void
2610 row_callback(png_structp png_ptr, png_bytep new_row,
2611 png_uint_32 row_num, int pass)
2612 {
2613 /* If the image is interlaced, and you turned
2614 on the interlace handler, this function will
2615 be called for every row in every pass. Some
2616 of these rows will not be changed from the
2617 previous pass. When the row is not changed,
2618 the new_row variable will be NULL. The rows
2619 and passes are called in order, so you don't
2620 really need the row_num and pass, but I'm
2621 supplying them because it may make your life
2622 easier.
2623
2624 If you did not turn on interlace handling then
2625 the callback is called for each row of each
2626 sub-image when the image is interlaced. In this
2627 case 'row_num' is the row in the sub-image, not
2628 the row in the output image as it is in all other
2629 cases.
2630
2631 For the non-NULL rows of interlaced images when
2632 you have switched on libpng interlace handling,
2633 you must call png_progressive_combine_row()
2634 passing in the row and the old row. You can
2635 call this function for NULL rows (it will just
2636 return) and for non-interlaced images (it just
2637 does the memcpy for you) if it will make the
2638 code easier. Thus, you can just do this for
2639 all cases if you switch on interlace handling;
2640 */
2641
2642 png_progressive_combine_row(png_ptr, old_row,
2643 new_row);
2644
2645 /* where old_row is what was displayed
2646 previously for the row. Note that the first
2647 pass (pass == 0, really) will completely cover
2648 the old row, so the rows do not have to be
2649 initialized. After the first pass (and only
2650 for interlaced images), you will have to pass
2651 the current row, and the function will combine
2652 the old row and the new row.
2653
2654 You can also call png_process_data_pause in this
2655 callback - see above.
2656 */
2657 }
2658
2659 void
2660 end_callback(png_structp png_ptr, png_infop info)
2661 {
2662 /* This function is called after the whole image
2663 has been read, including any chunks after the
2664 image (up to and including the IEND). You
2665 will usually have the same info chunk as you
2666 had in the header, although some data may have
2667 been added to the comments and time fields.
2668
2669 Most people won't do much here, perhaps setting
2670 a flag that marks the image as finished.
2671 */
2672 }
2673
2674
2675
2676 IV. Writing
2677
2678 Much of this is very similar to reading. However, everything of
2679 importance is repeated here, so you won't have to constantly look
2680 back up in the reading section to understand writing.
2681
2682 Setup
2683
2684 You will want to do the I/O initialization before you get into libpng,
2685 so if it doesn't work, you don't have anything to undo. If you are not
2686 using the standard I/O functions, you will need to replace them with
2687 custom writing functions. See the discussion under Customizing libpng.
2688
2689 FILE *fp = fopen(file_name, "wb");
2690
2691 if (!fp)
2692 return (ERROR);
2693
2694 Next, png_struct and png_info need to be allocated and initialized.
2695 As these can be both relatively large, you may not want to store these
2696 on the stack, unless you have stack space to spare. Of course, you
2697 will want to check if they return NULL. If you are also reading,
2698 you won't want to name your read structure and your write structure
2699 both "png_ptr"; you can call them anything you like, such as
2700 "read_ptr" and "write_ptr". Look at pngtest.c, for example.
2701
2702 png_structp png_ptr = png_create_write_struct
2703 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
2704 user_error_fn, user_warning_fn);
2705
2706 if (!png_ptr)
2707 return (ERROR);
2708
2709 png_infop info_ptr = png_create_info_struct(png_ptr);
2710 if (!info_ptr)
2711 {
2712 png_destroy_write_struct(&png_ptr,
2713 (png_infopp)NULL);
2714 return (ERROR);
2715 }
2716
2717 If you want to use your own memory allocation routines,
2718 define PNG_USER_MEM_SUPPORTED and use
2719 png_create_write_struct_2() instead of png_create_write_struct():
2720
2721 png_structp png_ptr = png_create_write_struct_2
2722 (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
2723 user_error_fn, user_warning_fn, (png_voidp)
2724 user_mem_ptr, user_malloc_fn, user_free_fn);
2725
2726 After you have these structures, you will need to set up the
2727 error handling. When libpng encounters an error, it expects to
2728 longjmp() back to your routine. Therefore, you will need to call
2729 setjmp() and pass the png_jmpbuf(png_ptr). If you
2730 write the file from different routines, you will need to update
2731 the png_jmpbuf(png_ptr) every time you enter a new routine that will
2732 call a png_*() function. See your documentation of setjmp/longjmp
2733 for your compiler for more information on setjmp/longjmp. See
2734 the discussion on libpng error handling in the Customizing Libpng
2735 section below for more information on the libpng error handling.
2736
2737 if (setjmp(png_jmpbuf(png_ptr)))
2738 {
2739 png_destroy_write_struct(&png_ptr, &info_ptr);
2740 fclose(fp);
2741 return (ERROR);
2742 }
2743 ...
2744 return;
2745
2746 If you would rather avoid the complexity of setjmp/longjmp issues,
2747 you can compile libpng with PNG_NO_SETJMP, in which case
2748 errors will result in a call to PNG_ABORT() which defaults to abort().
2749
2750 You can #define PNG_ABORT() to a function that does something
2751 more useful than abort(), as long as your function does not
2752 return.
2753
2754 Checking for invalid palette index on write was added at libpng
2755 1.5.10. If a pixel contains an invalid (out-of-range) index libpng issues
2756 a benign error. This is enabled by default because this condition is an
2757 error according to the PNG specification, Clause 11.3.2, but the error can
2758 be ignored in each png_ptr with
2759
2760 png_set_check_for_invalid_index(png_ptr, 0);
2761
2762 If the error is ignored, or if png_benign_error() treats it as a warning,
2763 any invalid pixels are written as-is by the encoder, resulting in an
2764 invalid PNG datastream as output. In this case the application is
2765 responsible for ensuring that the pixel indexes are in range when it writes
2766 a PLTE chunk with fewer entries than the bit depth would allow.
2767
2768 Now you need to set up the output code. The default for libpng is to
2769 use the C function fwrite(). If you use this, you will need to pass a
2770 valid FILE * in the function png_init_io(). Be sure that the file is
2771 opened in binary mode. Again, if you wish to handle writing data in
2772 another way, see the discussion on libpng I/O handling in the Customizing
2773 Libpng section below.
2774
2775 png_init_io(png_ptr, fp);
2776
2777 If you are embedding your PNG into a datastream such as MNG, and don't
2778 want libpng to write the 8-byte signature, or if you have already
2779 written the signature in your application, use
2780
2781 png_set_sig_bytes(png_ptr, 8);
2782
2783 to inform libpng that it should not write a signature.
2784
2785 Write callbacks
2786
2787 At this point, you can set up a callback function that will be
2788 called after each row has been written, which you can use to control
2789 a progress meter or the like. It's demonstrated in pngtest.c.
2790 You must supply a function
2791
2792 void write_row_callback(png_structp png_ptr, png_uint_32 row,
2793 int pass);
2794 {
2795 /* put your code here */
2796 }
2797
2798 (You can give it another name that you like instead of "write_row_callback")
2799
2800 To inform libpng about your function, use
2801
2802 png_set_write_status_fn(png_ptr, write_row_callback);
2803
2804 When this function is called the row has already been completely processed and
2805 it has also been written out. The 'row' and 'pass' refer to the next row to be
2806 handled. For the
2807 non-interlaced case the row that was just handled is simply one less than the
2808 passed in row number, and pass will always be 0. For the interlaced case the
2809 same applies unless the row value is 0, in which case the row just handled was
2810 the last one from one of the preceding passes. Because interlacing may skip a
2811 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
2812 need to know what the last pass is record (row,pass) from the callback and use
2813 the last recorded value each time.
2814
2815 As with the user transform you can find the output row using the
2816 PNG_ROW_FROM_PASS_ROW macro.
2817
2818 You now have the option of modifying how the compression library will
2819 run. The following functions are mainly for testing, but may be useful
2820 in some cases, like if you need to write PNG files extremely fast and
2821 are willing to give up some compression, or if you want to get the
2822 maximum possible compression at the expense of slower writing. If you
2823 have no special needs in this area, let the library do what it wants by
2824 not calling this function at all, as it has been tuned to deliver a good
2825 speed/compression ratio. The second parameter to png_set_filter() is
2826 the filter method, for which the only valid values are 0 (as of the
2827 July 1999 PNG specification, version 1.2) or 64 (if you are writing
2828 a PNG datastream that is to be embedded in a MNG datastream). The third
2829 parameter is a flag that indicates which filter type(s) are to be tested
2830 for each scanline. See the PNG specification for details on the specific
2831 filter types.
2832
2833
2834 /* turn on or off filtering, and/or choose
2835 specific filters. You can use either a single
2836 PNG_FILTER_VALUE_NAME or the bitwise OR of one
2837 or more PNG_FILTER_NAME masks.
2838 */
2839 png_set_filter(png_ptr, 0,
2840 PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
2841 PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
2842 PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
2843 PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG |
2844 PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
2845 PNG_ALL_FILTERS);
2846
2847 If an application wants to start and stop using particular filters during
2848 compression, it should start out with all of the filters (to ensure that
2849 the previous row of pixels will be stored in case it's needed later),
2850 and then add and remove them after the start of compression.
2851
2852 If you are writing a PNG datastream that is to be embedded in a MNG
2853 datastream, the second parameter can be either 0 or 64.
2854
2855 The png_set_compression_*() functions interface to the zlib compression
2856 library, and should mostly be ignored unless you really know what you are
2857 doing. The only generally useful call is png_set_compression_level()
2858 which changes how much time zlib spends on trying to compress the image
2859 data. See the Compression Library (zlib.h and algorithm.txt, distributed
2860 with zlib) for details on the compression levels.
2861
2862 #include zlib.h
2863
2864 /* Set the zlib compression level */
2865 png_set_compression_level(png_ptr,
2866 Z_BEST_COMPRESSION);
2867
2868 /* Set other zlib parameters for compressing IDAT */
2869 png_set_compression_mem_level(png_ptr, 8);
2870 png_set_compression_strategy(png_ptr,
2871 Z_DEFAULT_STRATEGY);
2872 png_set_compression_window_bits(png_ptr, 15);
2873 png_set_compression_method(png_ptr, 8);
2874 png_set_compression_buffer_size(png_ptr, 8192)
2875
2876 /* Set zlib parameters for text compression
2877 * If you don't call these, the parameters
2878 * fall back on those defined for IDAT chunks
2879 */
2880 png_set_text_compression_mem_level(png_ptr, 8);
2881 png_set_text_compression_strategy(png_ptr,
2882 Z_DEFAULT_STRATEGY);
2883 png_set_text_compression_window_bits(png_ptr, 15);
2884 png_set_text_compression_method(png_ptr, 8);
2885
2886 Setting the contents of info for output
2887
2888 You now need to fill in the png_info structure with all the data you
2889 wish to write before the actual image. Note that the only thing you
2890 are allowed to write after the image is the text chunks and the time
2891 chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
2892 the latest PNG specification for more information on that. If you
2893 wish to write them before the image, fill them in now, and flag that
2894 data as being valid. If you want to wait until after the data, don't
2895 fill them until png_write_end(). For all the fields in png_info and
2896 their data types, see png.h. For explanations of what the fields
2897 contain, see the PNG specification.
2898
2899 Some of the more important parts of the png_info are:
2900
2901 png_set_IHDR(png_ptr, info_ptr, width, height,
2902 bit_depth, color_type, interlace_type,
2903 compression_type, filter_method)
2904
2905 width - holds the width of the image
2906 in pixels (up to 2^31).
2907
2908 height - holds the height of the image
2909 in pixels (up to 2^31).
2910
2911 bit_depth - holds the bit depth of one of the
2912 image channels.
2913 (valid values are 1, 2, 4, 8, 16
2914 and depend also on the
2915 color_type. See also significant
2916 bits (sBIT) below).
2917
2918 color_type - describes which color/alpha
2919 channels are present.
2920 PNG_COLOR_TYPE_GRAY
2921 (bit depths 1, 2, 4, 8, 16)
2922 PNG_COLOR_TYPE_GRAY_ALPHA
2923 (bit depths 8, 16)
2924 PNG_COLOR_TYPE_PALETTE
2925 (bit depths 1, 2, 4, 8)
2926 PNG_COLOR_TYPE_RGB
2927 (bit_depths 8, 16)
2928 PNG_COLOR_TYPE_RGB_ALPHA
2929 (bit_depths 8, 16)
2930
2931 PNG_COLOR_MASK_PALETTE
2932 PNG_COLOR_MASK_COLOR
2933 PNG_COLOR_MASK_ALPHA
2934
2935 interlace_type - PNG_INTERLACE_NONE or
2936 PNG_INTERLACE_ADAM7
2937
2938 compression_type - (must be
2939 PNG_COMPRESSION_TYPE_DEFAULT)
2940
2941 filter_method - (must be PNG_FILTER_TYPE_DEFAULT
2942 or, if you are writing a PNG to
2943 be embedded in a MNG datastream,
2944 can also be
2945 PNG_INTRAPIXEL_DIFFERENCING)
2946
2947 If you call png_set_IHDR(), the call must appear before any of the
2948 other png_set_*() functions, because they might require access to some of
2949 the IHDR settings. The remaining png_set_*() functions can be called
2950 in any order.
2951
2952 If you wish, you can reset the compression_type, interlace_type, or
2953 filter_method later by calling png_set_IHDR() again; if you do this, the
2954 width, height, bit_depth, and color_type must be the same in each call.
2955
2956 png_set_PLTE(png_ptr, info_ptr, palette,
2957 num_palette);
2958
2959 palette - the palette for the file
2960 (array of png_color)
2961 num_palette - number of entries in the palette
2962
2963
2964 png_set_gAMA(png_ptr, info_ptr, file_gamma);
2965 png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
2966
2967 file_gamma - the gamma at which the image was
2968 created (PNG_INFO_gAMA)
2969
2970 int_file_gamma - 100,000 times the gamma at which
2971 the image was created
2972
2973 png_set_cHRM(png_ptr, info_ptr, white_x, white_y, red_x, red_y,
2974 green_x, green_y, blue_x, blue_y)
2975 png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
2976 green_Y, green_Z, blue_X, blue_Y, blue_Z)
2977 png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
2978 int_red_x, int_red_y, int_green_x, int_green_y,
2979 int_blue_x, int_blue_y)
2980 png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
2981 int_red_Z, int_green_X, int_green_Y, int_green_Z,
2982 int_blue_X, int_blue_Y, int_blue_Z)
2983
2984 {white,red,green,blue}_{x,y}
2985 A color space encoding specified using the chromaticities
2986 of the end points and the white point.
2987
2988 {red,green,blue}_{X,Y,Z}
2989 A color space encoding specified using the encoding end
2990 points - the CIE tristimulus specification of the intended
2991 color of the red, green and blue channels in the PNG RGB
2992 data. The white point is simply the sum of the three end
2993 points.
2994
2995 png_set_sRGB(png_ptr, info_ptr, srgb_intent);
2996
2997 srgb_intent - the rendering intent
2998 (PNG_INFO_sRGB) The presence of
2999 the sRGB chunk means that the pixel
3000 data is in the sRGB color space.
3001 This chunk also implies specific
3002 values of gAMA and cHRM. Rendering
3003 intent is the CSS-1 property that
3004 has been defined by the International
3005 Color Consortium
3006 (http://www.color.org).
3007 It can be one of
3008 PNG_sRGB_INTENT_SATURATION,
3009 PNG_sRGB_INTENT_PERCEPTUAL,
3010 PNG_sRGB_INTENT_ABSOLUTE, or
3011 PNG_sRGB_INTENT_RELATIVE.
3012
3013
3014 png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
3015 srgb_intent);
3016
3017 srgb_intent - the rendering intent
3018 (PNG_INFO_sRGB) The presence of the
3019 sRGB chunk means that the pixel
3020 data is in the sRGB color space.
3021 This function also causes gAMA and
3022 cHRM chunks with the specific values
3023 that are consistent with sRGB to be
3024 written.
3025
3026 png_set_iCCP(png_ptr, info_ptr, name, compression_type,
3027 profile, proflen);
3028
3029 name - The profile name.
3030
3031 compression_type - The compression type; always
3032 PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
3033 You may give NULL to this argument to
3034 ignore it.
3035
3036 profile - International Color Consortium color
3037 profile data. May contain NULs.
3038
3039 proflen - length of profile data in bytes.
3040
3041 png_set_sBIT(png_ptr, info_ptr, sig_bit);
3042
3043 sig_bit - the number of significant bits for
3044 (PNG_INFO_sBIT) each of the gray, red,
3045 green, and blue channels, whichever are
3046 appropriate for the given color type
3047 (png_color_16)
3048
3049 png_set_tRNS(png_ptr, info_ptr, trans_alpha,
3050 num_trans, trans_color);
3051
3052 trans_alpha - array of alpha (transparency)
3053 entries for palette (PNG_INFO_tRNS)
3054
3055 num_trans - number of transparent entries
3056 (PNG_INFO_tRNS)
3057
3058 trans_color - graylevel or color sample values
3059 (in order red, green, blue) of the
3060 single transparent color for
3061 non-paletted images (PNG_INFO_tRNS)
3062
3063 png_set_hIST(png_ptr, info_ptr, hist);
3064
3065 hist - histogram of palette (array of
3066 png_uint_16) (PNG_INFO_hIST)
3067
3068 png_set_tIME(png_ptr, info_ptr, mod_time);
3069
3070 mod_time - time image was last modified
3071 (PNG_VALID_tIME)
3072
3073 png_set_bKGD(png_ptr, info_ptr, background);
3074
3075 background - background color (of type
3076 png_color_16p) (PNG_VALID_bKGD)
3077
3078 png_set_text(png_ptr, info_ptr, text_ptr, num_text);
3079
3080 text_ptr - array of png_text holding image
3081 comments
3082
3083 text_ptr[i].compression - type of compression used
3084 on "text" PNG_TEXT_COMPRESSION_NONE
3085 PNG_TEXT_COMPRESSION_zTXt
3086 PNG_ITXT_COMPRESSION_NONE
3087 PNG_ITXT_COMPRESSION_zTXt
3088 text_ptr[i].key - keyword for comment. Must contain
3089 1-79 characters.
3090 text_ptr[i].text - text comments for current
3091 keyword. Can be NULL or empty.
3092 text_ptr[i].text_length - length of text string,
3093 after decompression, 0 for iTXt
3094 text_ptr[i].itxt_length - length of itxt string,
3095 after decompression, 0 for tEXt/zTXt
3096 text_ptr[i].lang - language of comment (NULL or
3097 empty for unknown).
3098 text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
3099 or empty for unknown).
3100
3101 Note that the itxt_length, lang, and lang_key
3102 members of the text_ptr structure only exist when the
3103 library is built with iTXt chunk support. Prior to
3104 libpng-1.4.0 the library was built by default without
3105 iTXt support. Also note that when iTXt is supported,
3106 they contain NULL pointers when the "compression"
3107 field contains PNG_TEXT_COMPRESSION_NONE or
3108 PNG_TEXT_COMPRESSION_zTXt.
3109
3110 num_text - number of comments
3111
3112 png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
3113 num_spalettes);
3114
3115 palette_ptr - array of png_sPLT_struct structures
3116 to be added to the list of palettes
3117 in the info structure.
3118 num_spalettes - number of palette structures to be
3119 added.
3120
3121 png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
3122 unit_type);
3123
3124 offset_x - positive offset from the left
3125 edge of the screen
3126
3127 offset_y - positive offset from the top
3128 edge of the screen
3129
3130 unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
3131
3132 png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
3133 unit_type);
3134
3135 res_x - pixels/unit physical resolution
3136 in x direction
3137
3138 res_y - pixels/unit physical resolution
3139 in y direction
3140
3141 unit_type - PNG_RESOLUTION_UNKNOWN,
3142 PNG_RESOLUTION_METER
3143
3144 png_set_sCAL(png_ptr, info_ptr, unit, width, height)
3145
3146 unit - physical scale units (an integer)
3147
3148 width - width of a pixel in physical scale units
3149
3150 height - height of a pixel in physical scale units
3151 (width and height are doubles)
3152
3153 png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
3154
3155 unit - physical scale units (an integer)
3156
3157 width - width of a pixel in physical scale units
3158 expressed as a string
3159
3160 height - height of a pixel in physical scale units
3161 (width and height are strings like "2.54")
3162
3163 png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
3164 num_unknowns)
3165
3166 unknowns - array of png_unknown_chunk
3167 structures holding unknown chunks
3168 unknowns[i].name - name of unknown chunk
3169 unknowns[i].data - data of unknown chunk
3170 unknowns[i].size - size of unknown chunk's data
3171 unknowns[i].location - position to write chunk in file
3172 0: do not write chunk
3173 PNG_HAVE_IHDR: before PLTE
3174 PNG_HAVE_PLTE: before IDAT
3175 PNG_AFTER_IDAT: after IDAT
3176
3177 The "location" member is set automatically according to
3178 what part of the output file has already been written.
3179 You can change its value after calling png_set_unknown_chunks()
3180 as demonstrated in pngtest.c. Within each of the "locations",
3181 the chunks are sequenced according to their position in the
3182 structure (that is, the value of "i", which is the order in which
3183 the chunk was either read from the input file or defined with
3184 png_set_unknown_chunks).
3185
3186 A quick word about text and num_text. text is an array of png_text
3187 structures. num_text is the number of valid structures in the array.
3188 Each png_text structure holds a language code, a keyword, a text value,
3189 and a compression type.
3190
3191 The compression types have the same valid numbers as the compression
3192 types of the image data. Currently, the only valid number is zero.
3193 However, you can store text either compressed or uncompressed, unlike
3194 images, which always have to be compressed. So if you don't want the
3195 text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
3196 Because tEXt and zTXt chunks don't have a language field, if you
3197 specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
3198 any language code or translated keyword will not be written out.
3199
3200 Until text gets around a few hundred bytes, it is not worth compressing it.
3201 After the text has been written out to the file, the compression type
3202 is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
3203 so that it isn't written out again at the end (in case you are calling
3204 png_write_end() with the same struct).
3205
3206 The keywords that are given in the PNG Specification are:
3207
3208 Title Short (one line) title or
3209 caption for image
3210
3211 Author Name of image's creator
3212
3213 Description Description of image (possibly long)
3214
3215 Copyright Copyright notice
3216
3217 Creation Time Time of original image creation
3218 (usually RFC 1123 format, see below)
3219
3220 Software Software used to create the image
3221
3222 Disclaimer Legal disclaimer
3223
3224 Warning Warning of nature of content
3225
3226 Source Device used to create the image
3227
3228 Comment Miscellaneous comment; conversion
3229 from other image format
3230
3231 The keyword-text pairs work like this. Keywords should be short
3232 simple descriptions of what the comment is about. Some typical
3233 keywords are found in the PNG specification, as is some recommendations
3234 on keywords. You can repeat keywords in a file. You can even write
3235 some text before the image and some after. For example, you may want
3236 to put a description of the image before the image, but leave the
3237 disclaimer until after, so viewers working over modem connections
3238 don't have to wait for the disclaimer to go over the modem before
3239 they start seeing the image. Finally, keywords should be full
3240 words, not abbreviations. Keywords and text are in the ISO 8859-1
3241 (Latin-1) character set (a superset of regular ASCII) and can not
3242 contain NUL characters, and should not contain control or other
3243 unprintable characters. To make the comments widely readable, stick
3244 with basic ASCII, and avoid machine specific character set extensions
3245 like the IBM-PC character set. The keyword must be present, but
3246 you can leave off the text string on non-compressed pairs.
3247 Compressed pairs must have a text string, as only the text string
3248 is compressed anyway, so the compression would be meaningless.
3249
3250 PNG supports modification time via the png_time structure. Two
3251 conversion routines are provided, png_convert_from_time_t() for
3252 time_t and png_convert_from_struct_tm() for struct tm. The
3253 time_t routine uses gmtime(). You don't have to use either of
3254 these, but if you wish to fill in the png_time structure directly,
3255 you should provide the time in universal time (GMT) if possible
3256 instead of your local time. Note that the year number is the full
3257 year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
3258 that months start with 1.
3259
3260 If you want to store the time of the original image creation, you should
3261 use a plain tEXt chunk with the "Creation Time" keyword. This is
3262 necessary because the "creation time" of a PNG image is somewhat vague,
3263 depending on whether you mean the PNG file, the time the image was
3264 created in a non-PNG format, a still photo from which the image was
3265 scanned, or possibly the subject matter itself. In order to facilitate
3266 machine-readable dates, it is recommended that the "Creation Time"
3267 tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
3268 although this isn't a requirement. Unlike the tIME chunk, the
3269 "Creation Time" tEXt chunk is not expected to be automatically changed
3270 by the software. To facilitate the use of RFC 1123 dates, a function
3271 png_convert_to_rfc1123_buffer(buffer, png_timep) is provided to
3272 convert from PNG time to an RFC 1123 format string. The caller must provide
3273 a writeable buffer of at least 29 bytes.
3274
3275 Writing unknown chunks
3276
3277 You can use the png_set_unknown_chunks function to queue up private chunks
3278 for writing. You give it a chunk name, location, raw data, and a size. You
3279 also must use png_set_keep_unknown_chunks() to ensure that libpng will
3280 handle them. That's all there is to it. The chunks will be written by the
3281 next following png_write_info_before_PLTE, png_write_info, or png_write_end
3282 function, depending upon the specified location. Any chunks previously
3283 read into the info structure's unknown-chunk list will also be written out
3284 in a sequence that satisfies the PNG specification's ordering rules.
3285
3286 Here is an example of writing two private chunks, prVt and miNE:
3287
3288 #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
3289 /* Set unknown chunk data */
3290 png_unknown_chunk unk_chunk[2];
3291 strcpy((char *) unk_chunk[0].name, "prVt";
3292 unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
3293 unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
3294 unk_chunk[0].location = PNG_HAVE_IHDR;
3295 strcpy((char *) unk_chunk[1].name, "miNE";
3296 unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
3297 unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
3298 unk_chunk[1].location = PNG_AFTER_IDAT;
3299 png_set_unknown_chunks(write_ptr, write_info_ptr,
3300 unk_chunk, 2);
3301 /* Needed because miNE is not safe-to-copy */
3302 png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
3303 (png_bytep) "miNE", 1);
3304 # if PNG_LIBPNG_VER < 10600
3305 /* Deal with unknown chunk location bug in 1.5.x and earlier */
3306 png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
3307 png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
3308 # endif
3309 # if PNG_LIBPNG_VER < 10500
3310 /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0,
3311 * one before IDAT and another after IDAT, so don't use it; only use
3312 * PNG_HAVE_IHDR location. This call resets the location previously
3313 * set by assignment and png_set_unknown_chunk_location() for chunk 1.
3314 */
3315 png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
3316 # endif
3317 #endif
3318
3319 The high-level write interface
3320
3321 At this point there are two ways to proceed; through the high-level
3322 write interface, or through a sequence of low-level write operations.
3323 You can use the high-level interface if your image data is present
3324 in the info structure. All defined output
3325 transformations are permitted, enabled by the following masks.
3326
3327 PNG_TRANSFORM_IDENTITY No transformation
3328 PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
3329 PNG_TRANSFORM_PACKSWAP Change order of packed
3330 pixels to LSB first
3331 PNG_TRANSFORM_INVERT_MONO Invert monochrome images
3332 PNG_TRANSFORM_SHIFT Normalize pixels to the
3333 sBIT depth
3334 PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
3335 to BGRA
3336 PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
3337 to AG
3338 PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
3339 to transparency
3340 PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
3341 PNG_TRANSFORM_STRIP_FILLER Strip out filler
3342 bytes (deprecated).
3343 PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
3344 filler bytes
3345 PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing
3346 filler bytes
3347
3348 If you have valid image data in the info structure (you can use
3349 png_set_rows() to put image data in the info structure), simply do this:
3350
3351 png_write_png(png_ptr, info_ptr, png_transforms, NULL)
3352
3353 where png_transforms is an integer containing the bitwise OR of some set of
3354 transformation flags. This call is equivalent to png_write_info(),
3355 followed the set of transformations indicated by the transform mask,
3356 then png_write_image(), and finally png_write_end().
3357
3358 (The final parameter of this call is not yet used. Someday it might point
3359 to transformation parameters required by some future output transform.)
3360
3361 You must use png_transforms and not call any png_set_transform() functions
3362 when you use png_write_png().
3363
3364 The low-level write interface
3365
3366 If you are going the low-level route instead, you are now ready to
3367 write all the file information up to the actual image data. You do
3368 this with a call to png_write_info().
3369
3370 png_write_info(png_ptr, info_ptr);
3371
3372 Note that there is one transformation you may need to do before
3373 png_write_info(). In PNG files, the alpha channel in an image is the
3374 level of opacity. If your data is supplied as a level of transparency,
3375 you can invert the alpha channel before you write it, so that 0 is
3376 fully transparent and 255 (in 8-bit or paletted images) or 65535
3377 (in 16-bit images) is fully opaque, with
3378
3379 png_set_invert_alpha(png_ptr);
3380
3381 This must appear before png_write_info() instead of later with the
3382 other transformations because in the case of paletted images the tRNS
3383 chunk data has to be inverted before the tRNS chunk is written. If
3384 your image is not a paletted image, the tRNS data (which in such cases
3385 represents a single color to be rendered as transparent) won't need to
3386 be changed, and you can safely do this transformation after your
3387 png_write_info() call.
3388
3389 If you need to write a private chunk that you want to appear before
3390 the PLTE chunk when PLTE is present, you can write the PNG info in
3391 two steps, and insert code to write your own chunk between them:
3392
3393 png_write_info_before_PLTE(png_ptr, info_ptr);
3394 png_set_unknown_chunks(png_ptr, info_ptr, ...);
3395 png_write_info(png_ptr, info_ptr);
3396
3397 After you've written the file information, you can set up the library
3398 to handle any special transformations of the image data. The various
3399 ways to transform the data will be described in the order that they
3400 should occur. This is important, as some of these change the color
3401 type and/or bit depth of the data, and some others only work on
3402 certain color types and bit depths. Even though each transformation
3403 checks to see if it has data that it can do something with, you should
3404 make sure to only enable a transformation if it will be valid for the
3405 data. For example, don't swap red and blue on grayscale data.
3406
3407 PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
3408 the library to strip input data that has 4 or 8 bytes per pixel down
3409 to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
3410 bytes per pixel).
3411
3412 png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
3413
3414 where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
3415 PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
3416 is stored XRGB or RGBX.
3417
3418 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
3419 they can, resulting in, for example, 8 pixels per byte for 1 bit files.
3420 If the data is supplied at 1 pixel per byte, use this code, which will
3421 correctly pack the pixels into a single byte:
3422
3423 png_set_packing(png_ptr);
3424
3425 PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
3426 data is of another bit depth, you can write an sBIT chunk into the
3427 file so that decoders can recover the original data if desired.
3428
3429 /* Set the true bit depth of the image data */
3430 if (color_type & PNG_COLOR_MASK_COLOR)
3431 {
3432 sig_bit.red = true_bit_depth;
3433 sig_bit.green = true_bit_depth;
3434 sig_bit.blue = true_bit_depth;
3435 }
3436
3437 else
3438 {
3439 sig_bit.gray = true_bit_depth;
3440 }
3441
3442 if (color_type & PNG_COLOR_MASK_ALPHA)
3443 {
3444 sig_bit.alpha = true_bit_depth;
3445 }
3446
3447 png_set_sBIT(png_ptr, info_ptr, &sig_bit);
3448
3449 If the data is stored in the row buffer in a bit depth other than
3450 one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
3451 this will scale the values to appear to be the correct bit depth as
3452 is required by PNG.
3453
3454 png_set_shift(png_ptr, &sig_bit);
3455
3456 PNG files store 16-bit pixels in network byte order (big-endian,
3457 ie. most significant bits first). This code would be used if they are
3458 supplied the other way (little-endian, i.e. least significant bits
3459 first, the way PCs store them):
3460
3461 if (bit_depth > 8)
3462 png_set_swap(png_ptr);
3463
3464 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
3465 need to change the order the pixels are packed into bytes, you can use:
3466
3467 if (bit_depth < 8)
3468 png_set_packswap(png_ptr);
3469
3470 PNG files store 3 color pixels in red, green, blue order. This code
3471 would be used if they are supplied as blue, green, red:
3472
3473 png_set_bgr(png_ptr);
3474
3475 PNG files describe monochrome as black being zero and white being
3476 one. This code would be used if the pixels are supplied with this reversed
3477 (black being one and white being zero):
3478
3479 png_set_invert_mono(png_ptr);
3480
3481 Finally, you can write your own transformation function if none of
3482 the existing ones meets your needs. This is done by setting a callback
3483 with
3484
3485 png_set_write_user_transform_fn(png_ptr,
3486 write_transform_fn);
3487
3488 You must supply the function
3489
3490 void write_transform_fn(png_structp png_ptr, png_row_infop
3491 row_info, png_bytep data)
3492
3493 See pngtest.c for a working example. Your function will be called
3494 before any of the other transformations are processed. If supported
3495 libpng also supplies an information routine that may be called from
3496 your callback:
3497
3498 png_get_current_row_number(png_ptr);
3499 png_get_current_pass_number(png_ptr);
3500
3501 This returns the current row passed to the transform. With interlaced
3502 images the value returned is the row in the input sub-image image. Use
3503 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
3504 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
3505
3506 The discussion of interlace handling above contains more information on how to
3507 use these values.
3508
3509 You can also set up a pointer to a user structure for use by your
3510 callback function.
3511
3512 png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
3513
3514 The user_channels and user_depth parameters of this function are ignored
3515 when writing; you can set them to zero as shown.
3516
3517 You can retrieve the pointer via the function png_get_user_transform_ptr().
3518 For example:
3519
3520 voidp write_user_transform_ptr =
3521 png_get_user_transform_ptr(png_ptr);
3522
3523 It is possible to have libpng flush any pending output, either manually,
3524 or automatically after a certain number of lines have been written. To
3525 flush the output stream a single time call:
3526
3527 png_write_flush(png_ptr);
3528
3529 and to have libpng flush the output stream periodically after a certain
3530 number of scanlines have been written, call:
3531
3532 png_set_flush(png_ptr, nrows);
3533
3534 Note that the distance between rows is from the last time png_write_flush()
3535 was called, or the first row of the image if it has never been called.
3536 So if you write 50 lines, and then png_set_flush 25, it will flush the
3537 output on the next scanline, and every 25 lines thereafter, unless
3538 png_write_flush() is called before 25 more lines have been written.
3539 If nrows is too small (less than about 10 lines for a 640 pixel wide
3540 RGB image) the image compression may decrease noticeably (although this
3541 may be acceptable for real-time applications). Infrequent flushing will
3542 only degrade the compression performance by a few percent over images
3543 that do not use flushing.
3544
3545 Writing the image data
3546
3547 That's it for the transformations. Now you can write the image data.
3548 The simplest way to do this is in one function call. If you have the
3549 whole image in memory, you can just call png_write_image() and libpng
3550 will write the image. You will need to pass in an array of pointers to
3551 each row. This function automatically handles interlacing, so you don't
3552 need to call png_set_interlace_handling() or call this function multiple
3553 times, or any of that other stuff necessary with png_write_rows().
3554
3555 png_write_image(png_ptr, row_pointers);
3556
3557 where row_pointers is:
3558
3559 png_byte *row_pointers[height];
3560
3561 You can point to void or char or whatever you use for pixels.
3562
3563 If you don't want to write the whole image at once, you can
3564 use png_write_rows() instead. If the file is not interlaced,
3565 this is simple:
3566
3567 png_write_rows(png_ptr, row_pointers,
3568 number_of_rows);
3569
3570 row_pointers is the same as in the png_write_image() call.
3571
3572 If you are just writing one row at a time, you can do this with
3573 a single row_pointer instead of an array of row_pointers:
3574
3575 png_bytep row_pointer = row;
3576
3577 png_write_row(png_ptr, row_pointer);
3578
3579 When the file is interlaced, things can get a good deal more complicated.
3580 The only currently (as of the PNG Specification version 1.2, dated July
3581 1999) defined interlacing scheme for PNG files is the "Adam7" interlace
3582 scheme, that breaks down an image into seven smaller images of varying
3583 size. libpng will build these images for you, or you can do them
3584 yourself. If you want to build them yourself, see the PNG specification
3585 for details of which pixels to write when.
3586
3587 If you don't want libpng to handle the interlacing details, just
3588 use png_set_interlace_handling() and call png_write_rows() the
3589 correct number of times to write all the sub-images
3590 (png_set_interlace_handling() returns the number of sub-images.)
3591
3592 If you want libpng to build the sub-images, call this before you start
3593 writing any rows:
3594
3595 number_of_passes = png_set_interlace_handling(png_ptr);
3596
3597 This will return the number of passes needed. Currently, this is seven,
3598 but may change if another interlace type is added.
3599
3600 Then write the complete image number_of_passes times.
3601
3602 png_write_rows(png_ptr, row_pointers, number_of_rows);
3603
3604 Think carefully before you write an interlaced image. Typically code that
3605 reads such images reads all the image data into memory, uncompressed, before
3606 doing any processing. Only code that can display an image on the fly can
3607 take advantage of the interlacing and even then the image has to be exactly
3608 the correct size for the output device, because scaling an image requires
3609 adjacent pixels and these are not available until all the passes have been
3610 read.
3611
3612 If you do write an interlaced image you will hardly ever need to handle
3613 the interlacing yourself. Call png_set_interlace_handling() and use the
3614 approach described above.
3615
3616 The only time it is conceivable that you will really need to write an
3617 interlaced image pass-by-pass is when you have read one pass by pass and
3618 made some pixel-by-pixel transformation to it, as described in the read
3619 code above. In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros
3620 to determine the size of each sub-image in turn and simply write the rows
3621 you obtained from the read code.
3622
3623 Finishing a sequential write
3624
3625 After you are finished writing the image, you should finish writing
3626 the file. If you are interested in writing comments or time, you should
3627 pass an appropriately filled png_info pointer. If you are not interested,
3628 you can pass NULL.
3629
3630 png_write_end(png_ptr, info_ptr);
3631
3632 When you are done, you can free all memory used by libpng like this:
3633
3634 png_destroy_write_struct(&png_ptr, &info_ptr);
3635
3636 It is also possible to individually free the info_ptr members that
3637 point to libpng-allocated storage with the following function:
3638
3639 png_free_data(png_ptr, info_ptr, mask, seq)
3640
3641 mask - identifies data to be freed, a mask
3642 containing the bitwise OR of one or
3643 more of
3644 PNG_FREE_PLTE, PNG_FREE_TRNS,
3645 PNG_FREE_HIST, PNG_FREE_ICCP,
3646 PNG_FREE_PCAL, PNG_FREE_ROWS,
3647 PNG_FREE_SCAL, PNG_FREE_SPLT,
3648 PNG_FREE_TEXT, PNG_FREE_UNKN,
3649 or simply PNG_FREE_ALL
3650
3651 seq - sequence number of item to be freed
3652 (-1 for all items)
3653
3654 This function may be safely called when the relevant storage has
3655 already been freed, or has not yet been allocated, or was allocated
3656 by the user and not by libpng, and will in those cases do nothing.
3657 The "seq" parameter is ignored if only one item of the selected data
3658 type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
3659 are allowed for the data type identified in the mask, such as text or
3660 sPLT, only the n'th item in the structure is freed, where n is "seq".
3661
3662 If you allocated data such as a palette that you passed in to libpng
3663 with png_set_*, you must not free it until just before the call to
3664 png_destroy_write_struct().
3665
3666 The default behavior is only to free data that was allocated internally
3667 by libpng. This can be changed, so that libpng will not free the data,
3668 or so that it will free data that was allocated by the user with png_malloc()
3669 or png_calloc() and passed in via a png_set_*() function, with
3670
3671 png_data_freer(png_ptr, info_ptr, freer, mask)
3672
3673 freer - one of
3674 PNG_DESTROY_WILL_FREE_DATA
3675 PNG_SET_WILL_FREE_DATA
3676 PNG_USER_WILL_FREE_DATA
3677
3678 mask - which data elements are affected
3679 same choices as in png_free_data()
3680
3681 For example, to transfer responsibility for some data from a read structure
3682 to a write structure, you could use
3683
3684 png_data_freer(read_ptr, read_info_ptr,
3685 PNG_USER_WILL_FREE_DATA,
3686 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
3687
3688 png_data_freer(write_ptr, write_info_ptr,
3689 PNG_DESTROY_WILL_FREE_DATA,
3690 PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
3691
3692 thereby briefly reassigning responsibility for freeing to the user but
3693 immediately afterwards reassigning it once more to the write_destroy
3694 function. Having done this, it would then be safe to destroy the read
3695 structure and continue to use the PLTE, tRNS, and hIST data in the write
3696 structure.
3697
3698 This function only affects data that has already been allocated.
3699 You can call this function before calling after the png_set_*() functions
3700 to control whether the user or png_destroy_*() is supposed to free the data.
3701 When the user assumes responsibility for libpng-allocated data, the
3702 application must use
3703 png_free() to free it, and when the user transfers responsibility to libpng
3704 for data that the user has allocated, the user must have used png_malloc()
3705 or png_calloc() to allocate it.
3706
3707 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
3708 separately, do not transfer responsibility for freeing text_ptr to libpng,
3709 because when libpng fills a png_text structure it combines these members with
3710 the key member, and png_free_data() will free only text_ptr.key. Similarly,
3711 if you transfer responsibility for free'ing text_ptr from libpng to your
3712 application, your application must not separately free those members.
3713 For a more compact example of writing a PNG image, see the file example.c.
3714
3715 V. Simplified API
3716
3717 The simplified API, which became available in libpng-1.6.0, hides the details
3718 of both libpng and the PNG file format itself.
3719 It allows PNG files to be read into a very limited number of
3720 in-memory bitmap formats or to be written from the same formats. If these
3721 formats do not accommodate your needs then you can, and should, use the more
3722 sophisticated APIs above - these support a wide variety of in-memory formats
3723 and a wide variety of sophisticated transformations to those formats as well
3724 as a wide variety of APIs to manipulate ancilliary information.
3725
3726 To read a PNG file using the simplified API:
3727
3728 1) Declare a 'png_image' structure (see below) on the stack, set the
3729 version field to PNG_IMAGE_VERSION and the 'opaque' pointer to NULL
3730 (this is REQUIRED, your program may crash if you don't do it.)
3731
3732 2) Call the appropriate png_image_begin_read... function.
3733
3734 3) Set the png_image 'format' member to the required sample format.
3735
3736 4) Allocate a buffer for the image and, if required, the color-map.
3737
3738 5) Call png_image_finish_read to read the image and, if required, the
3739 color-map into your buffers.
3740
3741 There are no restrictions on the format of the PNG input itself; all valid