+++ /dev/null
-libpng.txt - A description on how to use and modify libpng
-
- libpng version 1.4.3 - June 26, 2010
- Updated and distributed by Glenn Randers-Pehrson
- <glennrp at users.sourceforge.net>
- Copyright (c) 1998-2009 Glenn Randers-Pehrson
-
- This document is released under the libpng license.
- For conditions of distribution and use, see the disclaimer
- and license in png.h
-
- Based on:
-
- libpng versions 0.97, January 1998, through 1.4.3 - June 26, 2010
- Updated and distributed by Glenn Randers-Pehrson
- Copyright (c) 1998-2009 Glenn Randers-Pehrson
-
- libpng 1.0 beta 6 version 0.96 May 28, 1997
- Updated and distributed by Andreas Dilger
- Copyright (c) 1996, 1997 Andreas Dilger
-
- libpng 1.0 beta 2 - version 0.88 January 26, 1996
- For conditions of distribution and use, see copyright
- notice in png.h. Copyright (c) 1995, 1996 Guy Eric
- Schalnat, Group 42, Inc.
-
- Updated/rewritten per request in the libpng FAQ
- Copyright (c) 1995, 1996 Frank J. T. Wojcik
- December 18, 1995 & January 20, 1996
-
-I. Introduction
-
-This file describes how to use and modify the PNG reference library
-(known as libpng) for your own use. There are five sections to this
-file: introduction, structures, reading, writing, and modification and
-configuration notes for various special platforms. In addition to this
-file, example.c is a good starting point for using the library, as
-it is heavily commented and should include everything most people
-will need. We assume that libpng is already installed; see the
-INSTALL file for instructions on how to install libpng.
-
-For examples of libpng usage, see the files "example.c", "pngtest.c",
-and the files in the "contrib" directory, all of which are included in
-the libpng distribution.
-
-Libpng was written as a companion to the PNG specification, as a way
-of reducing the amount of time and effort it takes to support the PNG
-file format in application programs.
-
-The PNG specification (second edition), November 2003, is available as
-a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at
-<http://www.w3.org/TR/2003/REC-PNG-20031110/
-The W3C and ISO documents have identical technical content.
-
-The PNG-1.2 specification is available at
-<http://www.libpng.org/pub/png/documents/>. It is technically equivalent
-to the PNG specification (second edition) but has some additional material.
-
-The PNG-1.0 specification is available
-as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
-W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
-
-Some additional chunks are described in the special-purpose public chunks
-documents at <http://www.libpng.org/pub/png/documents/>.
-
-Other information
-about PNG, and the latest version of libpng, can be found at the PNG home
-page, <http://www.libpng.org/pub/png/>.
-
-Most users will not have to modify the library significantly; advanced
-users may want to modify it more. All attempts were made to make it as
-complete as possible, while keeping the code easy to understand.
-Currently, this library only supports C. Support for other languages
-is being considered.
-
-Libpng has been designed to handle multiple sessions at one time,
-to be easily modifiable, to be portable to the vast majority of
-machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
-to use. The ultimate goal of libpng is to promote the acceptance of
-the PNG file format in whatever way possible. While there is still
-work to be done (see the TODO file), libpng should cover the
-majority of the needs of its users.
-
-Libpng uses zlib for its compression and decompression of PNG files.
-Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
-The zlib compression utility is a general purpose utility that is
-useful for more than PNG files, and can be used without libpng.
-See the documentation delivered with zlib for more details.
-You can usually find the source files for the zlib utility wherever you
-find the libpng source files.
-
-Libpng is thread safe, provided the threads are using different
-instances of the structures. Each thread should have its own
-png_struct and png_info instances, and thus its own image.
-Libpng does not protect itself against two threads using the
-same instance of a structure.
-
-II. Structures
-
-There are two main structures that are important to libpng, png_struct
-and png_info. The first, png_struct, is an internal structure that
-will not, for the most part, be used by a user except as the first
-variable passed to every libpng function call.
-
-The png_info structure is designed to provide information about the
-PNG file. At one time, the fields of png_info were intended to be
-directly accessible to the user. However, this tended to cause problems
-with applications using dynamically loaded libraries, and as a result
-a set of interface functions for png_info (the png_get_*() and png_set_*()
-functions) was developed. The fields of png_info are still available for
-older applications, but it is suggested that applications use the new
-interfaces if at all possible.
-
-Applications that do make direct access to the members of png_struct (except
-for png_ptr->jmpbuf) must be recompiled whenever the library is updated,
-and applications that make direct access to the members of png_info must
-be recompiled if they were compiled or loaded with libpng version 1.0.6,
-in which the members were in a different order. In version 1.0.7, the
-members of the png_info structure reverted to the old order, as they were
-in versions 0.97c through 1.0.5. Starting with version 2.0.0, both
-structures are going to be hidden, and the contents of the structures will
-only be accessible through the png_get/png_set functions.
-
-The png.h header file is an invaluable reference for programming with libpng.
-And while I'm on the topic, make sure you include the libpng header file:
-
-#include <png.h>
-
-III. Reading
-
-We'll now walk you through the possible functions to call when reading
-in a PNG file sequentially, briefly explaining the syntax and purpose
-of each one. See example.c and png.h for more detail. While
-progressive reading is covered in the next section, you will still
-need some of the functions discussed in this section to read a PNG
-file.
-
-Setup
-
-You will want to do the I/O initialization(*) before you get into libpng,
-so if it doesn't work, you don't have much to undo. Of course, you
-will also want to insure that you are, in fact, dealing with a PNG
-file. Libpng provides a simple check to see if a file is a PNG file.
-To use it, pass in the first 1 to 8 bytes of the file to the function
-png_sig_cmp(), and it will return 0 (false) if the bytes match the
-corresponding bytes of the PNG signature, or nonzero (true) otherwise.
-Of course, the more bytes you pass in, the greater the accuracy of the
-prediction.
-
-If you are intending to keep the file pointer open for use in libpng,
-you must ensure you don't read more than 8 bytes from the beginning
-of the file, and you also have to make a call to png_set_sig_bytes_read()
-with the number of bytes you read from the beginning. Libpng will
-then only check the bytes (if any) that your program didn't read.
-
-(*): If you are not using the standard I/O functions, you will need
-to replace them with custom functions. See the discussion under
-Customizing libpng.
-
-
- FILE *fp = fopen(file_name, "rb");
- if (!fp)
- {
- return (ERROR);
- }
- fread(header, 1, number, fp);
- is_png = !png_sig_cmp(header, 0, number);
- if (!is_png)
- {
- return (NOT_PNG);
- }
-
-
-Next, png_struct and png_info need to be allocated and initialized. In
-order to ensure that the size of these structures is correct even with a
-dynamically linked libpng, there are functions to initialize and
-allocate the structures. We also pass the library version, optional
-pointers to error handling functions, and a pointer to a data struct for
-use by the error functions, if necessary (the pointer and functions can
-be NULL if the default error handlers are to be used). See the section
-on Changes to Libpng below regarding the old initialization functions.
-The structure allocation functions quietly return NULL if they fail to
-create the structure, so your application should check for that.
-
- png_structp png_ptr = png_create_read_struct
- (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
- user_error_fn, user_warning_fn);
- if (!png_ptr)
- return (ERROR);
-
- png_infop info_ptr = png_create_info_struct(png_ptr);
- if (!info_ptr)
- {
- png_destroy_read_struct(&png_ptr,
- (png_infopp)NULL, (png_infopp)NULL);
- return (ERROR);
- }
-
- png_infop end_info = png_create_info_struct(png_ptr);
- if (!end_info)
- {
- png_destroy_read_struct(&png_ptr, &info_ptr,
- (png_infopp)NULL);
- return (ERROR);
- }
-
-If you want to use your own memory allocation routines,
-define PNG_USER_MEM_SUPPORTED and use
-png_create_read_struct_2() instead of png_create_read_struct():
-
- png_structp png_ptr = png_create_read_struct_2
- (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
- user_error_fn, user_warning_fn, (png_voidp)
- user_mem_ptr, user_malloc_fn, user_free_fn);
-
-The error handling routines passed to png_create_read_struct()
-and the memory alloc/free routines passed to png_create_struct_2()
-are only necessary if you are not using the libpng supplied error
-handling and memory alloc/free functions.
-
-When libpng encounters an error, it expects to longjmp back
-to your routine. Therefore, you will need to call setjmp and pass
-your png_jmpbuf(png_ptr). If you read the file from different
-routines, you will need to update the jmpbuf field every time you enter
-a new routine that will call a png_*() function.
-
-See your documentation of setjmp/longjmp for your compiler for more
-information on setjmp/longjmp. See the discussion on libpng error
-handling in the Customizing Libpng section below for more information
-on the libpng error handling. If an error occurs, and libpng longjmp's
-back to your setjmp, you will want to call png_destroy_read_struct() to
-free any memory.
-
- if (setjmp(png_jmpbuf(png_ptr)))
- {
- png_destroy_read_struct(&png_ptr, &info_ptr,
- &end_info);
- fclose(fp);
- return (ERROR);
- }
-
-If you would rather avoid the complexity of setjmp/longjmp issues,
-you can compile libpng with PNG_NO_SETJMP, in which case
-errors will result in a call to PNG_ABORT() which defaults to abort().
-
-You can #define PNG_ABORT() to a function that does something
-more useful than abort(), as long as your function does not
-return.
-
-Now you need to set up the input code. The default for libpng is to
-use the C function fread(). If you use this, you will need to pass a
-valid FILE * in the function png_init_io(). Be sure that the file is
-opened in binary mode. If you wish to handle reading data in another
-way, you need not call the png_init_io() function, but you must then
-implement the libpng I/O methods discussed in the Customizing Libpng
-section below.
-
- png_init_io(png_ptr, fp);
-
-If you had previously opened the file and read any of the signature from
-the beginning in order to see if this was a PNG file, you need to let
-libpng know that there are some bytes missing from the start of the file.
-
- png_set_sig_bytes(png_ptr, number);
-
-You can change the zlib compression buffer size to be used while
-reading compressed data with
-
- png_set_compression_buffer_size(png_ptr, buffer_size);
-
-where the default size is 8192 bytes. Note that the buffer size
-is changed immediately and the buffer is reallocated immediately,
-instead of setting a flag to be acted upon later.
-
-Setting up callback code
-
-You can set up a callback function to handle any unknown chunks in the
-input stream. You must supply the function
-
- read_chunk_callback(png_ptr ptr,
- png_unknown_chunkp chunk);
- {
- /* The unknown chunk structure contains your
- chunk data, along with similar data for any other
- unknown chunks: */
-
- png_byte name[5];
- png_byte *data;
- png_size_t size;
-
- /* Note that libpng has already taken care of
- the CRC handling */
-
- /* put your code here. Search for your chunk in the
- unknown chunk structure, process it, and return one
- of the following: */
-
- return (-n); /* chunk had an error */
- return (0); /* did not recognize */
- return (n); /* success */
- }
-
-(You can give your function another name that you like instead of
-"read_chunk_callback")
-
-To inform libpng about your function, use
-
- png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
- read_chunk_callback);
-
-This names not only the callback function, but also a user pointer that
-you can retrieve with
-
- png_get_user_chunk_ptr(png_ptr);
-
-If you call the png_set_read_user_chunk_fn() function, then all unknown
-chunks will be saved when read, in case your callback function will need
-one or more of them. This behavior can be changed with the
-png_set_keep_unknown_chunks() function, described below.
-
-At this point, you can set up a callback function that will be
-called after each row has been read, which you can use to control
-a progress meter or the like. It's demonstrated in pngtest.c.
-You must supply a function
-
- void read_row_callback(png_ptr ptr, png_uint_32 row,
- int pass);
- {
- /* put your code here */
- }
-
-(You can give it another name that you like instead of "read_row_callback")
-
-To inform libpng about your function, use
-
- png_set_read_status_fn(png_ptr, read_row_callback);
-
-Unknown-chunk handling
-
-Now you get to set the way the library processes unknown chunks in the
-input PNG stream. Both known and unknown chunks will be read. Normal
-behavior is that known chunks will be parsed into information in
-various info_ptr members while unknown chunks will be discarded. This
-behavior can be wasteful if your application will never use some known
-chunk types. To change this, you can call:
-
- png_set_keep_unknown_chunks(png_ptr, keep,
- chunk_list, num_chunks);
- keep - 0: default unknown chunk handling
- 1: ignore; do not keep
- 2: keep only if safe-to-copy
- 3: keep even if unsafe-to-copy
- You can use these definitions:
- PNG_HANDLE_CHUNK_AS_DEFAULT 0
- PNG_HANDLE_CHUNK_NEVER 1
- PNG_HANDLE_CHUNK_IF_SAFE 2
- PNG_HANDLE_CHUNK_ALWAYS 3
- chunk_list - list of chunks affected (a byte string,
- five bytes per chunk, NULL or '\0' if
- num_chunks is 0)
- num_chunks - number of chunks affected; if 0, all
- unknown chunks are affected. If nonzero,
- only the chunks in the list are affected
-
-Unknown chunks declared in this way will be saved as raw data onto a
-list of png_unknown_chunk structures. If a chunk that is normally
-known to libpng is named in the list, it will be handled as unknown,
-according to the "keep" directive. If a chunk is named in successive
-instances of png_set_keep_unknown_chunks(), the final instance will
-take precedence. The IHDR and IEND chunks should not be named in
-chunk_list; if they are, libpng will process them normally anyway.
-
-Here is an example of the usage of png_set_keep_unknown_chunks(),
-where the private "vpAg" chunk will later be processed by a user chunk
-callback function:
-
- png_byte vpAg[5]={118, 112, 65, 103, (png_byte) '\0'};
-
- #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
- png_byte unused_chunks[]=
- {
- 104, 73, 83, 84, (png_byte) '\0', /* hIST */
- 105, 84, 88, 116, (png_byte) '\0', /* iTXt */
- 112, 67, 65, 76, (png_byte) '\0', /* pCAL */
- 115, 67, 65, 76, (png_byte) '\0', /* sCAL */
- 115, 80, 76, 84, (png_byte) '\0', /* sPLT */
- 116, 73, 77, 69, (png_byte) '\0', /* tIME */
- };
- #endif
-
- ...
-
- #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
- /* ignore all unknown chunks: */
- png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
- /* except for vpAg: */
- png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
- /* also ignore unused known chunks: */
- png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
- (int)sizeof(unused_chunks)/5);
- #endif
-
-User limits
-
-The PNG specification allows the width and height of an image to be as
-large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
-Since very few applications really need to process such large images,
-we have imposed an arbitrary 1-million limit on rows and columns.
-Larger images will be rejected immediately with a png_error() call. If
-you wish to override this limit, you can use
-
- png_set_user_limits(png_ptr, width_max, height_max);
-
-to set your own limits, or use width_max = height_max = 0x7fffffffL
-to allow all valid dimensions (libpng may reject some very large images
-anyway because of potential buffer overflow conditions).
-
-You should put this statement after you create the PNG structure and
-before calling png_read_info(), png_read_png(), or png_process_data().
-If you need to retrieve the limits that are being applied, use
-
- width_max = png_get_user_width_max(png_ptr);
- height_max = png_get_user_height_max(png_ptr);
-
-The PNG specification sets no limit on the number of ancillary chunks
-allowed in a PNG datastream. You can impose a limit on the total number
-of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
-
- png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
-
-where 0x7fffffffL means unlimited. You can retrieve this limit with
-
- chunk_cache_max = png_get_chunk_cache_max(png_ptr);
-
-This limit also applies to the number of buffers that can be allocated
-by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks.
-
-You can also set a limit on the amount of memory that a compressed chunk
-other than IDAT can occupy, with
-
- png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
-
-and you can retrieve the limit with
-
- chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
-
-Any chunks that would cause either of these limits to be exceeded will
-be ignored.
-
-The high-level read interface
-
-At this point there are two ways to proceed; through the high-level
-read interface, or through a sequence of low-level read operations.
-You can use the high-level interface if (a) you are willing to read
-the entire image into memory, and (b) the input transformations
-you want to do are limited to the following set:
-
- PNG_TRANSFORM_IDENTITY No transformation
- PNG_TRANSFORM_STRIP_16 Strip 16-bit samples to
- 8 bits
- PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
- PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
- samples to bytes
- PNG_TRANSFORM_PACKSWAP Change order of packed
- pixels to LSB first
- PNG_TRANSFORM_EXPAND Perform set_expand()
- PNG_TRANSFORM_INVERT_MONO Invert monochrome images
- PNG_TRANSFORM_SHIFT Normalize pixels to the
- sBIT depth
- PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
- to BGRA
- PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
- to AG
- PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
- to transparency
- PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
- PNG_TRANSFORM_GRAY_TO_RGB Expand grayscale samples
- to RGB (or GA to RGBA)
-
-(This excludes setting a background color, doing gamma transformation,
-quantizing, and setting filler.) If this is the case, simply do this:
-
- png_read_png(png_ptr, info_ptr, png_transforms, NULL)
-
-where png_transforms is an integer containing the bitwise OR of some
-set of transformation flags. This call is equivalent to png_read_info(),
-followed the set of transformations indicated by the transform mask,
-then png_read_image(), and finally png_read_end().
-
-(The final parameter of this call is not yet used. Someday it might point
-to transformation parameters required by some future input transform.)
-
-You must use png_transforms and not call any png_set_transform() functions
-when you use png_read_png().
-
-After you have called png_read_png(), you can retrieve the image data
-with
-
- row_pointers = png_get_rows(png_ptr, info_ptr);
-
-where row_pointers is an array of pointers to the pixel data for each row:
-
- png_bytep row_pointers[height];
-
-If you know your image size and pixel size ahead of time, you can allocate
-row_pointers prior to calling png_read_png() with
-
- if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
- png_error (png_ptr,
- "Image is too tall to process in memory");
- if (width > PNG_UINT_32_MAX/pixel_size)
- png_error (png_ptr,
- "Image is too wide to process in memory");
- row_pointers = png_malloc(png_ptr,
- height*png_sizeof(png_bytep));
- for (int i=0; i<height, i++)
- row_pointers[i]=NULL; /* security precaution */
- for (int i=0; i<height, i++)
- row_pointers[i]=png_malloc(png_ptr,
- width*pixel_size);
- png_set_rows(png_ptr, info_ptr, &row_pointers);
-
-Alternatively you could allocate your image in one big block and define
-row_pointers[i] to point into the proper places in your block.
-
-If you use png_set_rows(), the application is responsible for freeing
-row_pointers (and row_pointers[i], if they were separately allocated).
-
-If you don't allocate row_pointers ahead of time, png_read_png() will
-do it, and it'll be free'ed when you call png_destroy_*().
-
-The low-level read interface
-
-If you are going the low-level route, you are now ready to read all
-the file information up to the actual image data. You do this with a
-call to png_read_info().
-
- png_read_info(png_ptr, info_ptr);
-
-This will process all chunks up to but not including the image data.
-
-Querying the info structure
-
-Functions are used to get the information from the info_ptr once it
-has been read. Note that these fields may not be completely filled
-in until png_read_end() has read the chunk data following the image.
-
- png_get_IHDR(png_ptr, info_ptr, &width, &height,
- &bit_depth, &color_type, &interlace_type,
- &compression_type, &filter_method);
-
- width - holds the width of the image
- in pixels (up to 2^31).
- height - holds the height of the image
- in pixels (up to 2^31).
- bit_depth - holds the bit depth of one of the
- image channels. (valid values are
- 1, 2, 4, 8, 16 and depend also on
- the color_type. See also
- significant bits (sBIT) below).
- color_type - describes which color/alpha channels
- are present.
- PNG_COLOR_TYPE_GRAY
- (bit depths 1, 2, 4, 8, 16)
- PNG_COLOR_TYPE_GRAY_ALPHA
- (bit depths 8, 16)
- PNG_COLOR_TYPE_PALETTE
- (bit depths 1, 2, 4, 8)
- PNG_COLOR_TYPE_RGB
- (bit_depths 8, 16)
- PNG_COLOR_TYPE_RGB_ALPHA
- (bit_depths 8, 16)
-
- PNG_COLOR_MASK_PALETTE
- PNG_COLOR_MASK_COLOR
- PNG_COLOR_MASK_ALPHA
-
- filter_method - (must be PNG_FILTER_TYPE_BASE
- for PNG 1.0, and can also be
- PNG_INTRAPIXEL_DIFFERENCING if
- the PNG datastream is embedded in
- a MNG-1.0 datastream)
- compression_type - (must be PNG_COMPRESSION_TYPE_BASE
- for PNG 1.0)
- interlace_type - (PNG_INTERLACE_NONE or
- PNG_INTERLACE_ADAM7)
-
- Any or all of interlace_type, compression_type, or
- filter_method can be NULL if you are
- not interested in their values.
-
- Note that png_get_IHDR() returns 32-bit data into
- the application's width and height variables.
- This is an unsafe situation if these are 16-bit
- variables. In such situations, the
- png_get_image_width() and png_get_image_height()
- functions described below are safer.
-
- width = png_get_image_width(png_ptr,
- info_ptr);
- height = png_get_image_height(png_ptr,
- info_ptr);
- bit_depth = png_get_bit_depth(png_ptr,
- info_ptr);
- color_type = png_get_color_type(png_ptr,
- info_ptr);
- filter_method = png_get_filter_type(png_ptr,
- info_ptr);
- compression_type = png_get_compression_type(png_ptr,
- info_ptr);
- interlace_type = png_get_interlace_type(png_ptr,
- info_ptr);
-
- channels = png_get_channels(png_ptr, info_ptr);
- channels - number of channels of info for the
- color type (valid values are 1 (GRAY,
- PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
- 4 (RGB_ALPHA or RGB + filler byte))
- rowbytes = png_get_rowbytes(png_ptr, info_ptr);
- rowbytes - number of bytes needed to hold a row
-
- signature = png_get_signature(png_ptr, info_ptr);
- signature - holds the signature read from the
- file (if any). The data is kept in
- the same offset it would be if the
- whole signature were read (i.e. if an
- application had already read in 4
- bytes of signature before starting
- libpng, the remaining 4 bytes would
- be in signature[4] through signature[7]
- (see png_set_sig_bytes())).
-
-These are also important, but their validity depends on whether the chunk
-has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
-png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
-data has been read, or zero if it is missing. The parameters to the
-png_get_<chunk> are set directly if they are simple data types, or a
-pointer into the info_ptr is returned for any complex types.
-
- png_get_PLTE(png_ptr, info_ptr, &palette,
- &num_palette);
- palette - the palette for the file
- (array of png_color)
- num_palette - number of entries in the palette
-
- png_get_gAMA(png_ptr, info_ptr, &gamma);
- gamma - the gamma the file is written
- at (PNG_INFO_gAMA)
-
- png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
- srgb_intent - the rendering intent (PNG_INFO_sRGB)
- The presence of the sRGB chunk
- means that the pixel data is in the
- sRGB color space. This chunk also
- implies specific values of gAMA and
- cHRM.
-
- png_get_iCCP(png_ptr, info_ptr, &name,
- &compression_type, &profile, &proflen);
- name - The profile name.
- compression - The compression type; always
- PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
- You may give NULL to this argument to
- ignore it.
- profile - International Color Consortium color
- profile data. May contain NULs.
- proflen - length of profile data in bytes.
-
- png_get_sBIT(png_ptr, info_ptr, &sig_bit);
- sig_bit - the number of significant bits for
- (PNG_INFO_sBIT) each of the gray,
- red, green, and blue channels,
- whichever are appropriate for the
- given color type (png_color_16)
-
- png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
- &num_trans, &trans_color);
- trans_alpha - array of alpha (transparency)
- entries for palette (PNG_INFO_tRNS)
- trans_color - graylevel or color sample values of
- the single transparent color for
- non-paletted images (PNG_INFO_tRNS)
- num_trans - number of transparent entries
- (PNG_INFO_tRNS)
-
- png_get_hIST(png_ptr, info_ptr, &hist);
- (PNG_INFO_hIST)
- hist - histogram of palette (array of
- png_uint_16)
-
- png_get_tIME(png_ptr, info_ptr, &mod_time);
- mod_time - time image was last modified
- (PNG_VALID_tIME)
-
- png_get_bKGD(png_ptr, info_ptr, &background);
- background - background color (PNG_VALID_bKGD)
- valid 16-bit red, green and blue
- values, regardless of color_type
-
- num_comments = png_get_text(png_ptr, info_ptr,
- &text_ptr, &num_text);
- num_comments - number of comments
- text_ptr - array of png_text holding image
- comments
- text_ptr[i].compression - type of compression used
- on "text" PNG_TEXT_COMPRESSION_NONE
- PNG_TEXT_COMPRESSION_zTXt
- PNG_ITXT_COMPRESSION_NONE
- PNG_ITXT_COMPRESSION_zTXt
- text_ptr[i].key - keyword for comment. Must contain
- 1-79 characters.
- text_ptr[i].text - text comments for current
- keyword. Can be empty.
- text_ptr[i].text_length - length of text string,
- after decompression, 0 for iTXt
- text_ptr[i].itxt_length - length of itxt string,
- after decompression, 0 for tEXt/zTXt
- text_ptr[i].lang - language of comment (empty
- string for unknown).
- text_ptr[i].lang_key - keyword in UTF-8
- (empty string for unknown).
- Note that the itxt_length, lang, and lang_key
- members of the text_ptr structure only exist
- when the library is built with iTXt chunk support.
-
- num_text - number of comments (same as
- num_comments; you can put NULL here
- to avoid the duplication)
- Note while png_set_text() will accept text, language,
- and translated keywords that can be NULL pointers, the
- structure returned by png_get_text will always contain
- regular zero-terminated C strings. They might be
- empty strings but they will never be NULL pointers.
-
- num_spalettes = png_get_sPLT(png_ptr, info_ptr,
- &palette_ptr);
- palette_ptr - array of palette structures holding
- contents of one or more sPLT chunks
- read.
- num_spalettes - number of sPLT chunks read.
-
- png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
- &unit_type);
- offset_x - positive offset from the left edge
- of the screen
- offset_y - positive offset from the top edge
- of the screen
- unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
-
- png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
- &unit_type);
- res_x - pixels/unit physical resolution in
- x direction
- res_y - pixels/unit physical resolution in
- x direction
- unit_type - PNG_RESOLUTION_UNKNOWN,
- PNG_RESOLUTION_METER
-
- png_get_sCAL(png_ptr, info_ptr, &unit, &width,
- &height)
- unit - physical scale units (an integer)
- width - width of a pixel in physical scale units
- height - height of a pixel in physical scale units
- (width and height are doubles)
-
- png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
- &height)
- unit - physical scale units (an integer)
- width - width of a pixel in physical scale units
- height - height of a pixel in physical scale units
- (width and height are strings like "2.54")
-
- num_unknown_chunks = png_get_unknown_chunks(png_ptr,
- info_ptr, &unknowns)
- unknowns - array of png_unknown_chunk
- structures holding unknown chunks
- unknowns[i].name - name of unknown chunk
- unknowns[i].data - data of unknown chunk
- unknowns[i].size - size of unknown chunk's data
- unknowns[i].location - position of chunk in file
-
- The value of "i" corresponds to the order in which the
- chunks were read from the PNG file or inserted with the
- png_set_unknown_chunks() function.
-
-The data from the pHYs chunk can be retrieved in several convenient
-forms:
-
- res_x = png_get_x_pixels_per_meter(png_ptr,
- info_ptr)
- res_y = png_get_y_pixels_per_meter(png_ptr,
- info_ptr)
- res_x_and_y = png_get_pixels_per_meter(png_ptr,
- info_ptr)
- res_x = png_get_x_pixels_per_inch(png_ptr,
- info_ptr)
- res_y = png_get_y_pixels_per_inch(png_ptr,
- info_ptr)
- res_x_and_y = png_get_pixels_per_inch(png_ptr,
- info_ptr)
- aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
- info_ptr)
-
- (Each of these returns 0 [signifying "unknown"] if
- the data is not present or if res_x is 0;
- res_x_and_y is 0 if res_x != res_y)
-
-The data from the oFFs chunk can be retrieved in several convenient
-forms:
-
- x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
- y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
- x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
- y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
-
- (Each of these returns 0 [signifying "unknown" if both
- x and y are 0] if the data is not present or if the
- chunk is present but the unit is the pixel)
-
-For more information, see the png_info definition in png.h and the
-PNG specification for chunk contents. Be careful with trusting
-rowbytes, as some of the transformations could increase the space
-needed to hold a row (expand, filler, gray_to_rgb, etc.).
-See png_read_update_info(), below.
-
-A quick word about text_ptr and num_text. PNG stores comments in
-keyword/text pairs, one pair per chunk, with no limit on the number
-of text chunks, and a 2^31 byte limit on their size. While there are
-suggested keywords, there is no requirement to restrict the use to these
-strings. It is strongly suggested that keywords and text be sensible
-to humans (that's the point), so don't use abbreviations. Non-printing
-symbols are not allowed. See the PNG specification for more details.
-There is also no requirement to have text after the keyword.
-
-Keywords should be limited to 79 Latin-1 characters without leading or
-trailing spaces, but non-consecutive spaces are allowed within the
-keyword. It is possible to have the same keyword any number of times.
-The text_ptr is an array of png_text structures, each holding a
-pointer to a language string, a pointer to a keyword and a pointer to
-a text string. The text string, language code, and translated
-keyword may be empty or NULL pointers. The keyword/text
-pairs are put into the array in the order that they are received.
-However, some or all of the text chunks may be after the image, so, to
-make sure you have read all the text chunks, don't mess with these
-until after you read the stuff after the image. This will be
-mentioned again below in the discussion that goes with png_read_end().
-
-Input transformations
-
-After you've read the header information, you can set up the library
-to handle any special transformations of the image data. The various
-ways to transform the data will be described in the order that they
-should occur. This is important, as some of these change the color
-type and/or bit depth of the data, and some others only work on
-certain color types and bit depths. Even though each transformation
-checks to see if it has data that it can do something with, you should
-make sure to only enable a transformation if it will be valid for the
-data. For example, don't swap red and blue on grayscale data.
-
-The colors used for the background and transparency values should be
-supplied in the same format/depth as the current image data. They
-are stored in the same format/depth as the image data in a bKGD or tRNS
-chunk, so this is what libpng expects for this data. The colors are
-transformed to keep in sync with the image data when an application
-calls the png_read_update_info() routine (see below).
-
-Data will be decoded into the supplied row buffers packed into bytes
-unless the library has been told to transform it into another format.
-For example, 4 bit/pixel paletted or grayscale data will be returned
-2 pixels/byte with the leftmost pixel in the high-order bits of the
-byte, unless png_set_packing() is called. 8-bit RGB data will be stored
-in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
-is called to insert filler bytes, either before or after each RGB triplet.
-16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
-byte of the color value first, unless png_set_strip_16() is called to
-transform it to regular RGB RGB triplets, or png_set_filler() or
-png_set_add alpha() is called to insert filler bytes, either before or
-after each RRGGBB triplet. Similarly, 8-bit or 16-bit grayscale data can
-be modified with
-png_set_filler(), png_set_add_alpha(), or png_set_strip_16().
-
-The following code transforms grayscale images of less than 8 to 8 bits,
-changes paletted images to RGB, and adds a full alpha channel if there is
-transparency information in a tRNS chunk. This is most useful on
-grayscale images with bit depths of 2 or 4 or if there is a multiple-image
-viewing application that wishes to treat all images in the same way.
-
- if (color_type == PNG_COLOR_TYPE_PALETTE)
- png_set_palette_to_rgb(png_ptr);
-
- if (color_type == PNG_COLOR_TYPE_GRAY &&
- bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
-
- if (png_get_valid(png_ptr, info_ptr,
- PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
-
-These three functions are actually aliases for png_set_expand(), added
-in libpng version 1.0.4, with the function names expanded to improve code
-readability. In some future version they may actually do different
-things.
-
-As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
-added. It expands the sample depth without changing tRNS to alpha.
-
-As of libpng version 1.4.3, not all possible expansions are supported.
-
-In the following table, the 01 means grayscale with depth<8, 31 means
-indexed with depth<8, other numerals represent the color type, "T" means
-the tRNS chunk is present, A means an alpha channel is present, and O
-means tRNS or alpha is present but all pixels in the image are opaque.
-
- FROM 01 31 0 0T 0O 2 2T 2O 3 3T 3O 4A 4O 6A 6O
- TO
- 01 -
- 31 -
- 0 1 -
- 0T -
- 0O -
- 2 GX -
- 2T -
- 2O -
- 3 1 -
- 3T -
- 3O -
- 4A T -
- 4O -
- 6A GX TX TX -
- 6O GX TX -
-
-Within the matrix,
- "-" means the transformation is not supported.
- "X" means the transformation is obtained by png_set_expand().
- "1" means the transformation is obtained by
- png_set_expand_gray_1_2_4_to_8
- "G" means the transformation is obtained by
- png_set_gray_to_rgb().
- "P" means the transformation is obtained by
- png_set_expand_palette_to_rgb().
- "T" means the transformation is obtained by
- png_set_tRNS_to_alpha().
-
-PNG can have files with 16 bits per channel. If you only can handle
-8 bits per channel, this will strip the pixels down to 8 bit.
-
- if (bit_depth == 16)
- png_set_strip_16(png_ptr);
-
-If, for some reason, you don't need the alpha channel on an image,
-and you want to remove it rather than combining it with the background
-(but the image author certainly had in mind that you *would* combine
-it with the background, so that's what you should probably do):
-
- if (color_type & PNG_COLOR_MASK_ALPHA)
- png_set_strip_alpha(png_ptr);
-
-In PNG files, the alpha channel in an image
-is the level of opacity. If you need the alpha channel in an image to
-be the level of transparency instead of opacity, you can invert the
-alpha channel (or the tRNS chunk data) after it's read, so that 0 is
-fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
-images) is fully transparent, with
-
- png_set_invert_alpha(png_ptr);
-
-PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
-they can, resulting in, for example, 8 pixels per byte for 1 bit
-files. This code expands to 1 pixel per byte without changing the
-values of the pixels:
-
- if (bit_depth < 8)
- png_set_packing(png_ptr);
-
-PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels
-stored in a PNG image have been "scaled" or "shifted" up to the next
-higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
-to 8 bits/sample in the range [0, 255]). However, it is also possible
-to convert the PNG pixel data back to the original bit depth of the
-image. This call reduces the pixels back down to the original bit depth:
-
- png_color_8p sig_bit;
-
- if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
- png_set_shift(png_ptr, sig_bit);
-
-PNG files store 3-color pixels in red, green, blue order. This code
-changes the storage of the pixels to blue, green, red:
-
- if (color_type == PNG_COLOR_TYPE_RGB ||
- color_type == PNG_COLOR_TYPE_RGB_ALPHA)
- png_set_bgr(png_ptr);
-
-PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
-into 4 or 8 bytes for windowing systems that need them in this format:
-
- if (color_type == PNG_COLOR_TYPE_RGB)
- png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
-
-where "filler" is the 8 or 16-bit number to fill with, and the location is
-either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
-you want the filler before the RGB or after. This transformation
-does not affect images that already have full alpha channels. To add an
-opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
-will generate RGBA pixels.
-
-Note that png_set_filler() does not change the color type. If you want
-to do that, you can add a true alpha channel with
-
- if (color_type == PNG_COLOR_TYPE_RGB ||
- color_type == PNG_COLOR_TYPE_GRAY)
- png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
-
-where "filler" contains the alpha value to assign to each pixel.
-This function was added in libpng-1.2.7.
-
-If you are reading an image with an alpha channel, and you need the
-data as ARGB instead of the normal PNG format RGBA:
-
- if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
- png_set_swap_alpha(png_ptr);
-
-For some uses, you may want a grayscale image to be represented as
-RGB. This code will do that conversion:
-
- if (color_type == PNG_COLOR_TYPE_GRAY ||
- color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
- png_set_gray_to_rgb(png_ptr);
-
-Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
-with alpha.
-
- if (color_type == PNG_COLOR_TYPE_RGB ||
- color_type == PNG_COLOR_TYPE_RGB_ALPHA)
- png_set_rgb_to_gray_fixed(png_ptr, error_action,
- int red_weight, int green_weight);
-
- error_action = 1: silently do the conversion
- error_action = 2: issue a warning if the original
- image has any pixel where
- red != green or red != blue
- error_action = 3: issue an error and abort the
- conversion if the original
- image has any pixel where
- red != green or red != blue
-
- red_weight: weight of red component times 100000
- green_weight: weight of green component times 100000
- If either weight is negative, default
- weights (21268, 71514) are used.
-
-If you have set error_action = 1 or 2, you can
-later check whether the image really was gray, after processing
-the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
-It will return a png_byte that is zero if the image was gray or
-1 if there were any non-gray pixels. bKGD and sBIT data
-will be silently converted to grayscale, using the green channel
-data, regardless of the error_action setting.
-
-With red_weight+green_weight<=100000,
-the normalized graylevel is computed:
-
- int rw = red_weight * 65536;
- int gw = green_weight * 65536;
- int bw = 65536 - (rw + gw);
- gray = (rw*red + gw*green + bw*blue)/65536;
-
-The default values approximate those recommended in the Charles
-Poynton's Color FAQ, <http://www.inforamp.net/~poynton/>
-Copyright (c) 1998-01-04 Charles Poynton <poynton at inforamp.net>
-
- Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
-
-Libpng approximates this with
-
- Y = 0.21268 * R + 0.7151 * G + 0.07217 * B
-
-which can be expressed with integers as
-
- Y = (6969 * R + 23434 * G + 2365 * B)/32768
-
-The calculation is done in a linear colorspace, if the image gamma
-is known.
-
-If you have a grayscale and you are using png_set_expand_depth(),
-png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to
-a higher bit-depth, you must either supply the background color as a gray
-value at the original file bit-depth (need_expand = 1) or else supply the
-background color as an RGB triplet at the final, expanded bit depth
-(need_expand = 0). Similarly, if you are reading a paletted image, you
-must either supply the background color as a palette index (need_expand = 1)
-or as an RGB triplet that may or may not be in the palette (need_expand = 0).
-
- png_color_16 my_background;
- png_color_16p image_background;
-
- if (png_get_bKGD(png_ptr, info_ptr, &image_background))
- png_set_background(png_ptr, image_background,
- PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
- else
- png_set_background(png_ptr, &my_background,
- PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
-
-The png_set_background() function tells libpng to composite images
-with alpha or simple transparency against the supplied background
-color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
-you may use this color, or supply another color more suitable for
-the current display (e.g., the background color from a web page). You
-need to tell libpng whether the color is in the gamma space of the
-display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
-(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
-that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
-know why anyone would use this, but it's here).
-
-To properly display PNG images on any kind of system, the application needs
-to know what the display gamma is. Ideally, the user will know this, and
-the application will allow them to set it. One method of allowing the user
-to set the display gamma separately for each system is to check for a
-SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which will hopefully be
-correctly set.
-
-Note that display_gamma is the overall gamma correction required to produce
-pleasing results, which depends on the lighting conditions in the surrounding
-environment. In a dim or brightly lit room, no compensation other than
-the physical gamma exponent of the monitor is needed, while in a dark room
-a slightly smaller exponent is better.
-
- double gamma, screen_gamma;
-
- if (/* We have a user-defined screen
- gamma value */)
- {
- screen_gamma = user_defined_screen_gamma;
- }
- /* One way that applications can share the same
- screen gamma value */
- else if ((gamma_str = getenv("SCREEN_GAMMA"))
- != NULL)
- {
- screen_gamma = (double)atof(gamma_str);
- }
- /* If we don't have another value */
- else
- {
- screen_gamma = 2.2; /* A good guess for a
- PC monitor in a bright office or a dim room */
- screen_gamma = 2.0; /* A good guess for a
- PC monitor in a dark room */
- screen_gamma = 1.7 or 1.0; /* A good
- guess for Mac systems */
- }
-
-The png_set_gamma() function handles gamma transformations of the data.
-Pass both the file gamma and the current screen_gamma. If the file does
-not have a gamma value, you can pass one anyway if you have an idea what
-it is (usually 0.45455 is a good guess for GIF images on PCs). Note
-that file gammas are inverted from screen gammas. See the discussions
-on gamma in the PNG specification for an excellent description of what
-gamma is, and why all applications should support it. It is strongly
-recommended that PNG viewers support gamma correction.
-
- if (png_get_gAMA(png_ptr, info_ptr, &gamma))
- png_set_gamma(png_ptr, screen_gamma, gamma);
- else
- png_set_gamma(png_ptr, screen_gamma, 0.45455);
-
-If you need to reduce an RGB file to a paletted file, or if a paletted
-file has more entries then will fit on your screen, png_set_quantize()
-will do that. Note that this is a simple match dither that merely
-finds the closest color available. This should work fairly well with
-optimized palettes, and fairly badly with linear color cubes. If you
-pass a palette that is larger then maximum_colors, the file will
-reduce the number of colors in the palette so it will fit into
-maximum_colors. If there is a histogram, it will use it to make
-more intelligent choices when reducing the palette. If there is no
-histogram, it may not do as good a job.
-
- if (color_type & PNG_COLOR_MASK_COLOR)
- {
- if (png_get_valid(png_ptr, info_ptr,
- PNG_INFO_PLTE))
- {
- png_uint_16p histogram = NULL;
-
- png_get_hIST(png_ptr, info_ptr,
- &histogram);
- png_set_quantize(png_ptr, palette, num_palette,
- max_screen_colors, histogram, 1);
- }
- else
- {
- png_color std_color_cube[MAX_SCREEN_COLORS] =
- { ... colors ... };
-
- png_set_quantize(png_ptr, std_color_cube,
- MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
- NULL,0);
- }
- }
-
-PNG files describe monochrome as black being zero and white being one.
-The following code will reverse this (make black be one and white be
-zero):
-
- if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
- png_set_invert_mono(png_ptr);
-
-This function can also be used to invert grayscale and gray-alpha images:
-
- if (color_type == PNG_COLOR_TYPE_GRAY ||
- color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
- png_set_invert_mono(png_ptr);
-
-PNG files store 16 bit pixels in network byte order (big-endian,
-ie. most significant bits first). This code changes the storage to the
-other way (little-endian, i.e. least significant bits first, the
-way PCs store them):
-
- if (bit_depth == 16)
- png_set_swap(png_ptr);
-
-If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
-need to change the order the pixels are packed into bytes, you can use:
-
- if (bit_depth < 8)
- png_set_packswap(png_ptr);
-
-Finally, you can write your own transformation function if none of
-the existing ones meets your needs. This is done by setting a callback
-with
-
- png_set_read_user_transform_fn(png_ptr,
- read_transform_fn);
-
-You must supply the function
-
- void read_transform_fn(png_ptr ptr, row_info_ptr
- row_info, png_bytep data)
-
-See pngtest.c for a working example. Your function will be called
-after all of the other transformations have been processed.
-
-You can also set up a pointer to a user structure for use by your
-callback function, and you can inform libpng that your transform
-function will change the number of channels or bit depth with the
-function
-
- png_set_user_transform_info(png_ptr, user_ptr,
- user_depth, user_channels);
-
-The user's application, not libpng, is responsible for allocating and
-freeing any memory required for the user structure.
-
-You can retrieve the pointer via the function
-png_get_user_transform_ptr(). For example:
-
- voidp read_user_transform_ptr =
- png_get_user_transform_ptr(png_ptr);
-
-The last thing to handle is interlacing; this is covered in detail below,
-but you must call the function here if you want libpng to handle expansion
-of the interlaced image.
-
- number_of_passes = png_set_interlace_handling(png_ptr);
-
-After setting the transformations, libpng can update your png_info
-structure to reflect any transformations you've requested with this
-call. This is most useful to update the info structure's rowbytes
-field so you can use it to allocate your image memory. This function
-will also update your palette with the correct screen_gamma and
-background if these have been given with the calls above.
-
- png_read_update_info(png_ptr, info_ptr);
-
-After you call png_read_update_info(), you can allocate any
-memory you need to hold the image. The row data is simply
-raw byte data for all forms of images. As the actual allocation
-varies among applications, no example will be given. If you
-are allocating one large chunk, you will need to build an
-array of pointers to each row, as it will be needed for some
-of the functions below.
-
-Reading image data
-
-After you've allocated memory, you can read the image data.
-The simplest way to do this is in one function call. If you are
-allocating enough memory to hold the whole image, you can just
-call png_read_image() and libpng will read in all the image data
-and put it in the memory area supplied. You will need to pass in
-an array of pointers to each row.
-
-This function automatically handles interlacing, so you don't need
-to call png_set_interlace_handling() or call this function multiple
-times, or any of that other stuff necessary with png_read_rows().
-
- png_read_image(png_ptr, row_pointers);
-
-where row_pointers is:
-
- png_bytep row_pointers[height];
-
-You can point to void or char or whatever you use for pixels.
-
-If you don't want to read in the whole image at once, you can
-use png_read_rows() instead. If there is no interlacing (check
-interlace_type == PNG_INTERLACE_NONE), this is simple:
-
- png_read_rows(png_ptr, row_pointers, NULL,
- number_of_rows);
-
-where row_pointers is the same as in the png_read_image() call.
-
-If you are doing this just one row at a time, you can do this with
-a single row_pointer instead of an array of row_pointers:
-
- png_bytep row_pointer = row;
- png_read_row(png_ptr, row_pointer, NULL);
-
-If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
-get somewhat harder. The only current (PNG Specification version 1.2)
-interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7)
-is a somewhat complicated 2D interlace scheme, known as Adam7, that
-breaks down an image into seven smaller images of varying size, based
-on an 8x8 grid.
-
-libpng can fill out those images or it can give them to you "as is".
-If you want them filled out, there are two ways to do that. The one
-mentioned in the PNG specification is to expand each pixel to cover
-those pixels that have not been read yet (the "rectangle" method).
-This results in a blocky image for the first pass, which gradually
-smooths out as more pixels are read. The other method is the "sparkle"
-method, where pixels are drawn only in their final locations, with the
-rest of the image remaining whatever colors they were initialized to
-before the start of the read. The first method usually looks better,
-but tends to be slower, as there are more pixels to put in the rows.
-
-If you don't want libpng to handle the interlacing details, just call
-png_read_rows() seven times to read in all seven images. Each of the
-images is a valid image by itself, or they can all be combined on an
-8x8 grid to form a single image (although if you intend to combine them
-you would be far better off using the libpng interlace handling).
-
-The first pass will return an image 1/8 as wide as the entire image
-(every 8th column starting in column 0) and 1/8 as high as the original
-(every 8th row starting in row 0), the second will be 1/8 as wide
-(starting in column 4) and 1/8 as high (also starting in row 0). The
-third pass will be 1/4 as wide (every 4th pixel starting in column 0) and
-1/8 as high (every 8th row starting in row 4), and the fourth pass will
-be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
-and every 4th row starting in row 0). The fifth pass will return an
-image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
-while the sixth pass will be 1/2 as wide and 1/2 as high as the original
-(starting in column 1 and row 0). The seventh and final pass will be as
-wide as the original, and 1/2 as high, containing all of the odd
-numbered scanlines. Phew!
-
-If you want libpng to expand the images, call this before calling
-png_start_read_image() or png_read_update_info():
-
- if (interlace_type == PNG_INTERLACE_ADAM7)
- number_of_passes
- = png_set_interlace_handling(png_ptr);
-
-This will return the number of passes needed. Currently, this
-is seven, but may change if another interlace type is added.
-This function can be called even if the file is not interlaced,
-where it will return one pass.
-
-If you are not going to display the image after each pass, but are
-going to wait until the entire image is read in, use the sparkle
-effect. This effect is faster and the end result of either method
-is exactly the same. If you are planning on displaying the image
-after each pass, the "rectangle" effect is generally considered the
-better looking one.
-
-If you only want the "sparkle" effect, just call png_read_rows() as
-normal, with the third parameter NULL. Make sure you make pass over
-the image number_of_passes times, and you don't change the data in the
-rows between calls. You can change the locations of the data, just
-not the data. Each pass only writes the pixels appropriate for that
-pass, and assumes the data from previous passes is still valid.
-
- png_read_rows(png_ptr, row_pointers, NULL,
- number_of_rows);
-
-If you only want the first effect (the rectangles), do the same as
-before except pass the row buffer in the third parameter, and leave
-the second parameter NULL.
-
- png_read_rows(png_ptr, NULL, row_pointers,
- number_of_rows);
-
-Finishing a sequential read
-
-After you are finished reading the image through the
-low-level interface, you can finish reading the file. If you are
-interested in comments or time, which may be stored either before or
-after the image data, you should pass the separate png_info struct if
-you want to keep the comments from before and after the image
-separate. If you are not interested, you can pass NULL.
-
- png_read_end(png_ptr, end_info);
-
-When you are done, you can free all memory allocated by libpng like this:
-
- png_destroy_read_struct(&png_ptr, &info_ptr,
- &end_info);
-
-It is also possible to individually free the info_ptr members that
-point to libpng-allocated storage with the following function:
-
- png_free_data(png_ptr, info_ptr, mask, seq)
- mask - identifies data to be freed, a mask
- containing the bitwise OR of one or
- more of
- PNG_FREE_PLTE, PNG_FREE_TRNS,
- PNG_FREE_HIST, PNG_FREE_ICCP,
- PNG_FREE_PCAL, PNG_FREE_ROWS,
- PNG_FREE_SCAL, PNG_FREE_SPLT,
- PNG_FREE_TEXT, PNG_FREE_UNKN,
- or simply PNG_FREE_ALL
- seq - sequence number of item to be freed
- (-1 for all items)
-
-This function may be safely called when the relevant storage has
-already been freed, or has not yet been allocated, or was allocated
-by the user and not by libpng, and will in those cases do nothing.
-The "seq" parameter is ignored if only one item of the selected data
-type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
-are allowed for the data type identified in the mask, such as text or
-sPLT, only the n'th item in the structure is freed, where n is "seq".
-
-The default behavior is only to free data that was allocated internally
-by libpng. This can be changed, so that libpng will not free the data,
-or so that it will free data that was allocated by the user with png_malloc()
-or png_zalloc() and passed in via a png_set_*() function, with
-
- png_data_freer(png_ptr, info_ptr, freer, mask)
- mask - which data elements are affected
- same choices as in png_free_data()
- freer - one of
- PNG_DESTROY_WILL_FREE_DATA
- PNG_SET_WILL_FREE_DATA
- PNG_USER_WILL_FREE_DATA
-
-This function only affects data that has already been allocated.
-You can call this function after reading the PNG data but before calling
-any png_set_*() functions, to control whether the user or the png_set_*()
-function is responsible for freeing any existing data that might be present,
-and again after the png_set_*() functions to control whether the user
-or png_destroy_*() is supposed to free the data. When the user assumes
-responsibility for libpng-allocated data, the application must use
-png_free() to free it, and when the user transfers responsibility to libpng
-for data that the user has allocated, the user must have used png_malloc()
-or png_zalloc() to allocate it.
-
-If you allocated your row_pointers in a single block, as suggested above in
-the description of the high level read interface, you must not transfer
-responsibility for freeing it to the png_set_rows or png_read_destroy function,
-because they would also try to free the individual row_pointers[i].
-
-If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
-separately, do not transfer responsibility for freeing text_ptr to libpng,
-because when libpng fills a png_text structure it combines these members with
-the key member, and png_free_data() will free only text_ptr.key. Similarly,
-if you transfer responsibility for free'ing text_ptr from libpng to your
-application, your application must not separately free those members.
-
-The png_free_data() function will turn off the "valid" flag for anything
-it frees. If you need to turn the flag off for a chunk that was freed by
-your application instead of by libpng, you can use
-
- png_set_invalid(png_ptr, info_ptr, mask);
- mask - identifies the chunks to be made invalid,
- containing the bitwise OR of one or
- more of
- PNG_INFO_gAMA, PNG_INFO_sBIT,
- PNG_INFO_cHRM, PNG_INFO_PLTE,
- PNG_INFO_tRNS, PNG_INFO_bKGD,
- PNG_INFO_hIST, PNG_INFO_pHYs,
- PNG_INFO_oFFs, PNG_INFO_tIME,
- PNG_INFO_pCAL, PNG_INFO_sRGB,
- PNG_INFO_iCCP, PNG_INFO_sPLT,
- PNG_INFO_sCAL, PNG_INFO_IDAT
-
-For a more compact example of reading a PNG image, see the file example.c.
-
-Reading PNG files progressively
-
-The progressive reader is slightly different then the non-progressive
-reader. Instead of calling png_read_info(), png_read_rows(), and
-png_read_end(), you make one call to png_process_data(), which calls
-callbacks when it has the info, a row, or the end of the image. You
-set up these callbacks with png_set_progressive_read_fn(). You don't
-have to worry about the input/output functions of libpng, as you are
-giving the library the data directly in png_process_data(). I will
-assume that you have read the section on reading PNG files above,
-so I will only highlight the differences (although I will show
-all of the code).
-
-png_structp png_ptr;
-png_infop info_ptr;
-
- /* An example code fragment of how you would
- initialize the progressive reader in your
- application. */
- int
- initialize_png_reader()
- {
- png_ptr = png_create_read_struct
- (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
- user_error_fn, user_warning_fn);
- if (!png_ptr)
- return (ERROR);
- info_ptr = png_create_info_struct(png_ptr);
- if (!info_ptr)
- {
- png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
- (png_infopp)NULL);
- return (ERROR);
- }
-
- if (setjmp(png_jmpbuf(png_ptr)))
- {
- png_destroy_read_struct(&png_ptr, &info_ptr,
- (png_infopp)NULL);
- return (ERROR);
- }
-
- /* This one's new. You can provide functions
- to be called when the header info is valid,
- when each row is completed, and when the image
- is finished. If you aren't using all functions,
- you can specify NULL parameters. Even when all
- three functions are NULL, you need to call
- png_set_progressive_read_fn(). You can use
- any struct as the user_ptr (cast to a void pointer
- for the function call), and retrieve the pointer
- from inside the callbacks using the function
-
- png_get_progressive_ptr(png_ptr);
-
- which will return a void pointer, which you have
- to cast appropriately.
- */
- png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
- info_callback, row_callback, end_callback);
-
- return 0;
- }
-
- /* A code fragment that you call as you receive blocks
- of data */
- int
- process_data(png_bytep buffer, png_uint_32 length)
- {
- if (setjmp(png_jmpbuf(png_ptr)))
- {
- png_destroy_read_struct(&png_ptr, &info_ptr,
- (png_infopp)NULL);
- return (ERROR);
- }
-
- /* This one's new also. Simply give it a chunk
- of data from the file stream (in order, of
- course). On machines with segmented memory
- models machines, don't give it any more than
- 64K. The library seems to run fine with sizes
- of 4K. Although you can give it much less if
- necessary (I assume you can give it chunks of
- 1 byte, I haven't tried less then 256 bytes
- yet). When this function returns, you may
- want to display any rows that were generated
- in the row callback if you don't already do
- so there.
- */
- png_process_data(png_ptr, info_ptr, buffer, length);
- return 0;
- }
-
- /* This function is called (as set by
- png_set_progressive_read_fn() above) when enough data
- has been supplied so all of the header has been
- read.
- */
- void
- info_callback(png_structp png_ptr, png_infop info)
- {
- /* Do any setup here, including setting any of
- the transformations mentioned in the Reading
- PNG files section. For now, you _must_ call
- either png_start_read_image() or
- png_read_update_info() after all the
- transformations are set (even if you don't set
- any). You may start getting rows before
- png_process_data() returns, so this is your
- last chance to prepare for that.
- */
- }
-
- /* This function is called when each row of image
- data is complete */
- void
- row_callback(png_structp png_ptr, png_bytep new_row,
- png_uint_32 row_num, int pass)
- {
- /* If the image is interlaced, and you turned
- on the interlace handler, this function will
- be called for every row in every pass. Some
- of these rows will not be changed from the
- previous pass. When the row is not changed,
- the new_row variable will be NULL. The rows
- and passes are called in order, so you don't
- really need the row_num and pass, but I'm
- supplying them because it may make your life
- easier.
-
- For the non-NULL rows of interlaced images,
- you must call png_progressive_combine_row()
- passing in the row and the old row. You can
- call this function for NULL rows (it will just
- return) and for non-interlaced images (it just
- does the memcpy for you) if it will make the
- code easier. Thus, you can just do this for
- all cases:
- */
-
- png_progressive_combine_row(png_ptr, old_row,
- new_row);
-
- /* where old_row is what was displayed for
- previously for the row. Note that the first
- pass (pass == 0, really) will completely cover
- the old row, so the rows do not have to be
- initialized. After the first pass (and only
- for interlaced images), you will have to pass
- the current row, and the function will combine
- the old row and the new row.
- */
- }
-
- void
- end_callback(png_structp png_ptr, png_infop info)
- {
- /* This function is called after the whole image
- has been read, including any chunks after the
- image (up to and including the IEND). You
- will usually have the same info chunk as you
- had in the header, although some data may have
- been added to the comments and time fields.
-
- Most people won't do much here, perhaps setting
- a flag that marks the image as finished.
- */
- }
-
-
-
-IV. Writing
-
-Much of this is very similar to reading. However, everything of
-importance is repeated here, so you won't have to constantly look
-back up in the reading section to understand writing.
-
-Setup
-
-You will want to do the I/O initialization before you get into libpng,
-so if it doesn't work, you don't have anything to undo. If you are not
-using the standard I/O functions, you will need to replace them with
-custom writing functions. See the discussion under Customizing libpng.
-
- FILE *fp = fopen(file_name, "wb");
- if (!fp)
- {
- return (ERROR);
- }
-
-Next, png_struct and png_info need to be allocated and initialized.
-As these can be both relatively large, you may not want to store these
-on the stack, unless you have stack space to spare. Of course, you
-will want to check if they return NULL. If you are also reading,
-you won't want to name your read structure and your write structure
-both "png_ptr"; you can call them anything you like, such as
-"read_ptr" and "write_ptr". Look at pngtest.c, for example.
-
- png_structp png_ptr = png_create_write_struct
- (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
- user_error_fn, user_warning_fn);
- if (!png_ptr)
- return (ERROR);
-
- png_infop info_ptr = png_create_info_struct(png_ptr);
- if (!info_ptr)
- {
- png_destroy_write_struct(&png_ptr,
- (png_infopp)NULL);
- return (ERROR);
- }
-
-If you want to use your own memory allocation routines,
-define PNG_USER_MEM_SUPPORTED and use
-png_create_write_struct_2() instead of png_create_write_struct():
-
- png_structp png_ptr = png_create_write_struct_2
- (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
- user_error_fn, user_warning_fn, (png_voidp)
- user_mem_ptr, user_malloc_fn, user_free_fn);
-
-After you have these structures, you will need to set up the
-error handling. When libpng encounters an error, it expects to
-longjmp() back to your routine. Therefore, you will need to call
-setjmp() and pass the png_jmpbuf(png_ptr). If you
-write the file from different routines, you will need to update
-the png_jmpbuf(png_ptr) every time you enter a new routine that will
-call a png_*() function. See your documentation of setjmp/longjmp
-for your compiler for more information on setjmp/longjmp. See
-the discussion on libpng error handling in the Customizing Libpng
-section below for more information on the libpng error handling.
-
- if (setjmp(png_jmpbuf(png_ptr)))
- {
- png_destroy_write_struct(&png_ptr, &info_ptr);
- fclose(fp);
- return (ERROR);
- }
- ...
- return;
-
-If you would rather avoid the complexity of setjmp/longjmp issues,
-you can compile libpng with PNG_NO_SETJMP, in which case
-errors will result in a call to PNG_ABORT() which defaults to abort().
-
-You can #define PNG_ABORT() to a function that does something
-more useful than abort(), as long as your function does not
-return.
-
-Now you need to set up the output code. The default for libpng is to
-use the C function fwrite(). If you use this, you will need to pass a
-valid FILE * in the function png_init_io(). Be sure that the file is
-opened in binary mode. Again, if you wish to handle writing data in
-another way, see the discussion on libpng I/O handling in the Customizing
-Libpng section below.
-
- png_init_io(png_ptr, fp);
-
-If you are embedding your PNG into a datastream such as MNG, and don't
-want libpng to write the 8-byte signature, or if you have already
-written the signature in your application, use
-
- png_set_sig_bytes(png_ptr, 8);
-
-to inform libpng that it should not write a signature.
-
-Write callbacks
-
-At this point, you can set up a callback function that will be
-called after each row has been written, which you can use to control
-a progress meter or the like. It's demonstrated in pngtest.c.
-You must supply a function
-
- void write_row_callback(png_ptr, png_uint_32 row,
- int pass);
- {
- /* put your code here */
- }
-
-(You can give it another name that you like instead of "write_row_callback")
-
-To inform libpng about your function, use
-
- png_set_write_status_fn(png_ptr, write_row_callback);
-
-You now have the option of modifying how the compression library will
-run. The following functions are mainly for testing, but may be useful
-in some cases, like if you need to write PNG files extremely fast and
-are willing to give up some compression, or if you want to get the
-maximum possible compression at the expense of slower writing. If you
-have no special needs in this area, let the library do what it wants by
-not calling this function at all, as it has been tuned to deliver a good
-speed/compression ratio. The second parameter to png_set_filter() is
-the filter method, for which the only valid values are 0 (as of the
-July 1999 PNG specification, version 1.2) or 64 (if you are writing
-a PNG datastream that is to be embedded in a MNG datastream). The third
-parameter is a flag that indicates which filter type(s) are to be tested
-for each scanline. See the PNG specification for details on the specific
-filter types.
-
-
- /* turn on or off filtering, and/or choose
- specific filters. You can use either a single
- PNG_FILTER_VALUE_NAME or the bitwise OR of one
- or more PNG_FILTER_NAME masks. */
- png_set_filter(png_ptr, 0,
- PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
- PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
- PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
- PNG_FILTER_AVG | PNG_FILTER_VALUE_AVG |
- PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
- PNG_ALL_FILTERS);
-
-If an application
-wants to start and stop using particular filters during compression,
-it should start out with all of the filters (to ensure that the previous
-row of pixels will be stored in case it's needed later), and then add
-and remove them after the start of compression.
-
-If you are writing a PNG datastream that is to be embedded in a MNG
-datastream, the second parameter can be either 0 or 64.
-
-The png_set_compression_*() functions interface to the zlib compression
-library, and should mostly be ignored unless you really know what you are
-doing. The only generally useful call is png_set_compression_level()
-which changes how much time zlib spends on trying to compress the image
-data. See the Compression Library (zlib.h and algorithm.txt, distributed
-with zlib) for details on the compression levels.
-
- /* set the zlib compression level */
- png_set_compression_level(png_ptr,
- Z_BEST_COMPRESSION);
-
- /* set other zlib parameters */
- png_set_compression_mem_level(png_ptr, 8);
- png_set_compression_strategy(png_ptr,
- Z_DEFAULT_STRATEGY);
- png_set_compression_window_bits(png_ptr, 15);
- png_set_compression_method(png_ptr, 8);
- png_set_compression_buffer_size(png_ptr, 8192)
-
-extern PNG_EXPORT(void,png_set_zbuf_size)
-
-Setting the contents of info for output
-
-You now need to fill in the png_info structure with all the data you
-wish to write before the actual image. Note that the only thing you
-are allowed to write after the image is the text chunks and the time
-chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
-the latest PNG specification for more information on that. If you
-wish to write them before the image, fill them in now, and flag that
-data as being valid. If you want to wait until after the data, don't
-fill them until png_write_end(). For all the fields in png_info and
-their data types, see png.h. For explanations of what the fields
-contain, see the PNG specification.
-
-Some of the more important parts of the png_info are:
-
- png_set_IHDR(png_ptr, info_ptr, width, height,
- bit_depth, color_type, interlace_type,
- compression_type, filter_method)
- width - holds the width of the image
- in pixels (up to 2^31).
- height - holds the height of the image
- in pixels (up to 2^31).
- bit_depth - holds the bit depth of one of the
- image channels.
- (valid values are 1, 2, 4, 8, 16
- and depend also on the
- color_type. See also significant
- bits (sBIT) below).
- color_type - describes which color/alpha
- channels are present.
- PNG_COLOR_TYPE_GRAY
- (bit depths 1, 2, 4, 8, 16)
- PNG_COLOR_TYPE_GRAY_ALPHA
- (bit depths 8, 16)
- PNG_COLOR_TYPE_PALETTE
- (bit depths 1, 2, 4, 8)
- PNG_COLOR_TYPE_RGB
- (bit_depths 8, 16)
- PNG_COLOR_TYPE_RGB_ALPHA
- (bit_depths 8, 16)
-
- PNG_COLOR_MASK_PALETTE
- PNG_COLOR_MASK_COLOR
- PNG_COLOR_MASK_ALPHA
-
- interlace_type - PNG_INTERLACE_NONE or
- PNG_INTERLACE_ADAM7
- compression_type - (must be
- PNG_COMPRESSION_TYPE_DEFAULT)
- filter_method - (must be PNG_FILTER_TYPE_DEFAULT
- or, if you are writing a PNG to
- be embedded in a MNG datastream,
- can also be
- PNG_INTRAPIXEL_DIFFERENCING)
-
-If you call png_set_IHDR(), the call must appear before any of the
-other png_set_*() functions, because they might require access to some of
-the IHDR settings. The remaining png_set_*() functions can be called
-in any order.
-
-If you wish, you can reset the compression_type, interlace_type, or
-filter_method later by calling png_set_IHDR() again; if you do this, the
-width, height, bit_depth, and color_type must be the same in each call.
-
- png_set_PLTE(png_ptr, info_ptr, palette,
- num_palette);
- palette - the palette for the file
- (array of png_color)
- num_palette - number of entries in the palette
-
- png_set_gAMA(png_ptr, info_ptr, gamma);
- gamma - the gamma the image was created
- at (PNG_INFO_gAMA)
-
- png_set_sRGB(png_ptr, info_ptr, srgb_intent);
- srgb_intent - the rendering intent
- (PNG_INFO_sRGB) The presence of
- the sRGB chunk means that the pixel
- data is in the sRGB color space.
- This chunk also implies specific
- values of gAMA and cHRM. Rendering
- intent is the CSS-1 property that
- has been defined by the International
- Color Consortium
- (http://www.color.org).
- It can be one of
- PNG_sRGB_INTENT_SATURATION,
- PNG_sRGB_INTENT_PERCEPTUAL,
- PNG_sRGB_INTENT_ABSOLUTE, or
- PNG_sRGB_INTENT_RELATIVE.
-
-
- png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
- srgb_intent);
- srgb_intent - the rendering intent
- (PNG_INFO_sRGB) The presence of the
- sRGB chunk means that the pixel
- data is in the sRGB color space.
- This function also causes gAMA and
- cHRM chunks with the specific values
- that are consistent with sRGB to be
- written.
-
- png_set_iCCP(png_ptr, info_ptr, name, compression_type,
- profile, proflen);
- name - The profile name.
- compression - The compression type; always
- PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
- You may give NULL to this argument to
- ignore it.
- profile - International Color Consortium color
- profile data. May contain NULs.
- proflen - length of profile data in bytes.
-
- png_set_sBIT(png_ptr, info_ptr, sig_bit);
- sig_bit - the number of significant bits for
- (PNG_INFO_sBIT) each of the gray, red,
- green, and blue channels, whichever are
- appropriate for the given color type
- (png_color_16)
-
- png_set_tRNS(png_ptr, info_ptr, trans_alpha,
- num_trans, trans_color);
- trans_alpha - array of alpha (transparency)
- entries for palette (PNG_INFO_tRNS)
- trans_color - graylevel or color sample values
- (in order red, green, blue) of the
- single transparent color for
- non-paletted images (PNG_INFO_tRNS)
- num_trans - number of transparent entries
- (PNG_INFO_tRNS)
-
- png_set_hIST(png_ptr, info_ptr, hist);
- (PNG_INFO_hIST)
- hist - histogram of palette (array of
- png_uint_16)
-
- png_set_tIME(png_ptr, info_ptr, mod_time);
- mod_time - time image was last modified
- (PNG_VALID_tIME)
-
- png_set_bKGD(png_ptr, info_ptr, background);
- background - background color (PNG_VALID_bKGD)
-
- png_set_text(png_ptr, info_ptr, text_ptr, num_text);
- text_ptr - array of png_text holding image
- comments
- text_ptr[i].compression - type of compression used
- on "text" PNG_TEXT_COMPRESSION_NONE
- PNG_TEXT_COMPRESSION_zTXt
- PNG_ITXT_COMPRESSION_NONE
- PNG_ITXT_COMPRESSION_zTXt
- text_ptr[i].key - keyword for comment. Must contain
- 1-79 characters.
- text_ptr[i].text - text comments for current
- keyword. Can be NULL or empty.
- text_ptr[i].text_length - length of text string,
- after decompression, 0 for iTXt
- text_ptr[i].itxt_length - length of itxt string,
- after decompression, 0 for tEXt/zTXt
- text_ptr[i].lang - language of comment (NULL or
- empty for unknown).
- text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
- or empty for unknown).
- Note that the itxt_length, lang, and lang_key
- members of the text_ptr structure only exist
- when the library is built with iTXt chunk support.
-
- num_text - number of comments
-
- png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
- num_spalettes);
- palette_ptr - array of png_sPLT_struct structures
- to be added to the list of palettes
- in the info structure.
- num_spalettes - number of palette structures to be
- added.
-
- png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
- unit_type);
- offset_x - positive offset from the left
- edge of the screen
- offset_y - positive offset from the top
- edge of the screen
- unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
-
- png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
- unit_type);
- res_x - pixels/unit physical resolution
- in x direction
- res_y - pixels/unit physical resolution
- in y direction
- unit_type - PNG_RESOLUTION_UNKNOWN,
- PNG_RESOLUTION_METER
-
- png_set_sCAL(png_ptr, info_ptr, unit, width, height)
- unit - physical scale units (an integer)
- width - width of a pixel in physical scale units
- height - height of a pixel in physical scale units
- (width and height are doubles)
-
- png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
- unit - physical scale units (an integer)
- width - width of a pixel in physical scale units
- height - height of a pixel in physical scale units
- (width and height are strings like "2.54")
-
- png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
- num_unknowns)
- unknowns - array of png_unknown_chunk
- structures holding unknown chunks
- unknowns[i].name - name of unknown chunk
- unknowns[i].data - data of unknown chunk
- unknowns[i].size - size of unknown chunk's data
- unknowns[i].location - position to write chunk in file
- 0: do not write chunk
- PNG_HAVE_IHDR: before PLTE
- PNG_HAVE_PLTE: before IDAT
- PNG_AFTER_IDAT: after IDAT
-
-The "location" member is set automatically according to
-what part of the output file has already been written.
-You can change its value after calling png_set_unknown_chunks()
-as demonstrated in pngtest.c. Within each of the "locations",
-the chunks are sequenced according to their position in the
-structure (that is, the value of "i", which is the order in which
-the chunk was either read from the input file or defined with
-png_set_unknown_chunks).
-
-A quick word about text and num_text. text is an array of png_text
-structures. num_text is the number of valid structures in the array.
-Each png_text structure holds a language code, a keyword, a text value,
-and a compression type.
-
-The compression types have the same valid numbers as the compression
-types of the image data. Currently, the only valid number is zero.
-However, you can store text either compressed or uncompressed, unlike
-images, which always have to be compressed. So if you don't want the
-text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
-Because tEXt and zTXt chunks don't have a language field, if you
-specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
-any language code or translated keyword will not be written out.
-
-Until text gets around 1000 bytes, it is not worth compressing it.
-After the text has been written out to the file, the compression type
-is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
-so that it isn't written out again at the end (in case you are calling
-png_write_end() with the same struct.
-
-The keywords that are given in the PNG Specification are:
-
- Title Short (one line) title or
- caption for image
- Author Name of image's creator
- Description Description of image (possibly long)
- Copyright Copyright notice
- Creation Time Time of original image creation
- (usually RFC 1123 format, see below)
- Software Software used to create the image
- Disclaimer Legal disclaimer
- Warning Warning of nature of content
- Source Device used to create the image
- Comment Miscellaneous comment; conversion
- from other image format
-
-The keyword-text pairs work like this. Keywords should be short
-simple descriptions of what the comment is about. Some typical
-keywords are found in the PNG specification, as is some recommendations
-on keywords. You can repeat keywords in a file. You can even write
-some text before the image and some after. For example, you may want
-to put a description of the image before the image, but leave the
-disclaimer until after, so viewers working over modem connections
-don't have to wait for the disclaimer to go over the modem before
-they start seeing the image. Finally, keywords should be full
-words, not abbreviations. Keywords and text are in the ISO 8859-1
-(Latin-1) character set (a superset of regular ASCII) and can not
-contain NUL characters, and should not contain control or other
-unprintable characters. To make the comments widely readable, stick
-with basic ASCII, and avoid machine specific character set extensions
-like the IBM-PC character set. The keyword must be present, but
-you can leave off the text string on non-compressed pairs.
-Compressed pairs must have a text string, as only the text string
-is compressed anyway, so the compression would be meaningless.
-
-PNG supports modification time via the png_time structure. Two
-conversion routines are provided, png_convert_from_time_t() for
-time_t and png_convert_from_struct_tm() for struct tm. The
-time_t routine uses gmtime(). You don't have to use either of
-these, but if you wish to fill in the png_time structure directly,
-you should provide the time in universal time (GMT) if possible
-instead of your local time. Note that the year number is the full
-year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
-that months start with 1.
-
-If you want to store the time of the original image creation, you should
-use a plain tEXt chunk with the "Creation Time" keyword. This is
-necessary because the "creation time" of a PNG image is somewhat vague,
-depending on whether you mean the PNG file, the time the image was
-created in a non-PNG format, a still photo from which the image was
-scanned, or possibly the subject matter itself. In order to facilitate
-machine-readable dates, it is recommended that the "Creation Time"
-tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
-although this isn't a requirement. Unlike the tIME chunk, the
-"Creation Time" tEXt chunk is not expected to be automatically changed
-by the software. To facilitate the use of RFC 1123 dates, a function
-png_convert_to_rfc1123(png_timep) is provided to convert from PNG
-time to an RFC 1123 format string.
-
-Writing unknown chunks
-
-You can use the png_set_unknown_chunks function to queue up chunks
-for writing. You give it a chunk name, raw data, and a size; that's
-all there is to it. The chunks will be written by the next following
-png_write_info_before_PLTE, png_write_info, or png_write_end function.
-Any chunks previously read into the info structure's unknown-chunk
-list will also be written out in a sequence that satisfies the PNG
-specification's ordering rules.
-
-The high-level write interface
-
-At this point there are two ways to proceed; through the high-level
-write interface, or through a sequence of low-level write operations.
-You can use the high-level interface if your image data is present
-in the info structure. All defined output
-transformations are permitted, enabled by the following masks.
-
- PNG_TRANSFORM_IDENTITY No transformation
- PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
- PNG_TRANSFORM_PACKSWAP Change order of packed
- pixels to LSB first
- PNG_TRANSFORM_INVERT_MONO Invert monochrome images
- PNG_TRANSFORM_SHIFT Normalize pixels to the
- sBIT depth
- PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
- to BGRA
- PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
- to AG
- PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
- to transparency
- PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
- PNG_TRANSFORM_STRIP_FILLER Strip out filler
- bytes (deprecated).
- PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
- filler bytes
- PNG_TRANSFORM_STRIP_FILLER_AFTER Strip out trailing
- filler bytes
-
-If you have valid image data in the info structure (you can use
-png_set_rows() to put image data in the info structure), simply do this:
-
- png_write_png(png_ptr, info_ptr, png_transforms, NULL)
-
-where png_transforms is an integer containing the bitwise OR of some set of
-transformation flags. This call is equivalent to png_write_info(),
-followed the set of transformations indicated by the transform mask,
-then png_write_image(), and finally png_write_end().
-
-(The final parameter of this call is not yet used. Someday it might point
-to transformation parameters required by some future output transform.)
-
-You must use png_transforms and not call any png_set_transform() functions
-when you use png_write_png().
-
-The low-level write interface
-
-If you are going the low-level route instead, you are now ready to
-write all the file information up to the actual image data. You do
-this with a call to png_write_info().
-
- png_write_info(png_ptr, info_ptr);
-
-Note that there is one transformation you may need to do before
-png_write_info(). In PNG files, the alpha channel in an image is the
-level of opacity. If your data is supplied as a level of transparency,
-you can invert the alpha channel before you write it, so that 0 is
-fully transparent and 255 (in 8-bit or paletted images) or 65535
-(in 16-bit images) is fully opaque, with
-
- png_set_invert_alpha(png_ptr);
-
-This must appear before png_write_info() instead of later with the
-other transformations because in the case of paletted images the tRNS
-chunk data has to be inverted before the tRNS chunk is written. If
-your image is not a paletted image, the tRNS data (which in such cases
-represents a single color to be rendered as transparent) won't need to
-be changed, and you can safely do this transformation after your
-png_write_info() call.
-
-If you need to write a private chunk that you want to appear before
-the PLTE chunk when PLTE is present, you can write the PNG info in
-two steps, and insert code to write your own chunk between them:
-
- png_write_info_before_PLTE(png_ptr, info_ptr);
- png_set_unknown_chunks(png_ptr, info_ptr, ...);
- png_write_info(png_ptr, info_ptr);
-
-After you've written the file information, you can set up the library
-to handle any special transformations of the image data. The various
-ways to transform the data will be described in the order that they
-should occur. This is important, as some of these change the color
-type and/or bit depth of the data, and some others only work on
-certain color types and bit depths. Even though each transformation
-checks to see if it has data that it can do something with, you should
-make sure to only enable a transformation if it will be valid for the
-data. For example, don't swap red and blue on grayscale data.
-
-PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
-the library to strip input data that has 4 or 8 bytes per pixel down
-to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
-bytes per pixel).
-
- png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
-
-where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
-PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
-is stored XRGB or RGBX.
-
-PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
-they can, resulting in, for example, 8 pixels per byte for 1 bit files.
-If the data is supplied at 1 pixel per byte, use this code, which will
-correctly pack the pixels into a single byte:
-
- png_set_packing(png_ptr);
-
-PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
-data is of another bit depth, you can write an sBIT chunk into the
-file so that decoders can recover the original data if desired.
-
- /* Set the true bit depth of the image data */
- if (color_type & PNG_COLOR_MASK_COLOR)
- {
- sig_bit.red = true_bit_depth;
- sig_bit.green = true_bit_depth;
- sig_bit.blue = true_bit_depth;
- }
- else
- {
- sig_bit.gray = true_bit_depth;
- }
- if (color_type & PNG_COLOR_MASK_ALPHA)
- {
- sig_bit.alpha = true_bit_depth;
- }
-
- png_set_sBIT(png_ptr, info_ptr, &sig_bit);
-
-If the data is stored in the row buffer in a bit depth other than
-one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
-this will scale the values to appear to be the correct bit depth as
-is required by PNG.
-
- png_set_shift(png_ptr, &sig_bit);
-
-PNG files store 16 bit pixels in network byte order (big-endian,
-ie. most significant bits first). This code would be used if they are
-supplied the other way (little-endian, i.e. least significant bits
-first, the way PCs store them):
-
- if (bit_depth > 8)
- png_set_swap(png_ptr);
-
-If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
-need to change the order the pixels are packed into bytes, you can use:
-
- if (bit_depth < 8)
- png_set_packswap(png_ptr);
-
-PNG files store 3 color pixels in red, green, blue order. This code
-would be used if they are supplied as blue, green, red:
-
- png_set_bgr(png_ptr);
-
-PNG files describe monochrome as black being zero and white being
-one. This code would be used if the pixels are supplied with this reversed
-(black being one and white being zero):
-
- png_set_invert_mono(png_ptr);
-
-Finally, you can write your own transformation function if none of
-the existing ones meets your needs. This is done by setting a callback
-with
-
- png_set_write_user_transform_fn(png_ptr,
- write_transform_fn);
-
-You must supply the function
-
- void write_transform_fn(png_ptr ptr, row_info_ptr
- row_info, png_bytep data)
-
-See pngtest.c for a working example. Your function will be called
-before any of the other transformations are processed.
-
-You can also set up a pointer to a user structure for use by your
-callback function.
-
- png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
-
-The user_channels and user_depth parameters of this function are ignored
-when writing; you can set them to zero as shown.
-
-You can retrieve the pointer via the function png_get_user_transform_ptr().
-For example:
-
- voidp write_user_transform_ptr =
- png_get_user_transform_ptr(png_ptr);
-
-It is possible to have libpng flush any pending output, either manually,
-or automatically after a certain number of lines have been written. To
-flush the output stream a single time call:
-
- png_write_flush(png_ptr);
-
-and to have libpng flush the output stream periodically after a certain
-number of scanlines have been written, call:
-
- png_set_flush(png_ptr, nrows);
-
-Note that the distance between rows is from the last time png_write_flush()
-was called, or the first row of the image if it has never been called.
-So if you write 50 lines, and then png_set_flush 25, it will flush the
-output on the next scanline, and every 25 lines thereafter, unless
-png_write_flush() is called before 25 more lines have been written.
-If nrows is too small (less than about 10 lines for a 640 pixel wide
-RGB image) the image compression may decrease noticeably (although this
-may be acceptable for real-time applications). Infrequent flushing will
-only degrade the compression performance by a few percent over images
-that do not use flushing.
-
-Writing the image data
-
-That's it for the transformations. Now you can write the image data.
-The simplest way to do this is in one function call. If you have the
-whole image in memory, you can just call png_write_image() and libpng
-will write the image. You will need to pass in an array of pointers to
-each row. This function automatically handles interlacing, so you don't
-need to call png_set_interlace_handling() or call this function multiple
-times, or any of that other stuff necessary with png_write_rows().
-
- png_write_image(png_ptr, row_pointers);
-
-where row_pointers is:
-
- png_byte *row_pointers[height];
-
-You can point to void or char or whatever you use for pixels.
-
-If you don't want to write the whole image at once, you can
-use png_write_rows() instead. If the file is not interlaced,
-this is simple:
-
- png_write_rows(png_ptr, row_pointers,
- number_of_rows);
-
-row_pointers is the same as in the png_write_image() call.
-
-If you are just writing one row at a time, you can do this with
-a single row_pointer instead of an array of row_pointers:
-
- png_bytep row_pointer = row;
-
- png_write_row(png_ptr, row_pointer);
-
-When the file is interlaced, things can get a good deal more complicated.
-The only currently (as of the PNG Specification version 1.2, dated July
-1999) defined interlacing scheme for PNG files is the "Adam7" interlace
-scheme, that breaks down an image into seven smaller images of varying
-size. libpng will build these images for you, or you can do them
-yourself. If you want to build them yourself, see the PNG specification
-for details of which pixels to write when.
-
-If you don't want libpng to handle the interlacing details, just
-use png_set_interlace_handling() and call png_write_rows() the
-correct number of times to write all seven sub-images.
-
-If you want libpng to build the sub-images, call this before you start
-writing any rows:
-
- number_of_passes =
- png_set_interlace_handling(png_ptr);
-
-This will return the number of passes needed. Currently, this is seven,
-but may change if another interlace type is added.
-
-Then write the complete image number_of_passes times.
-
- png_write_rows(png_ptr, row_pointers,
- number_of_rows);
-
-As some of these rows are not used, and thus return immediately, you may
-want to read about interlacing in the PNG specification, and only update
-the rows that are actually used.
-
-Finishing a sequential write
-
-After you are finished writing the image, you should finish writing
-the file. If you are interested in writing comments or time, you should
-pass an appropriately filled png_info pointer. If you are not interested,
-you can pass NULL.
-
- png_write_end(png_ptr, info_ptr);
-
-When you are done, you can free all memory used by libpng like this:
-
- png_destroy_write_struct(&png_ptr, &info_ptr);
-
-It is also possible to individually free the info_ptr members that
-point to libpng-allocated storage with the following function:
-
- png_free_data(png_ptr, info_ptr, mask, seq)
- mask - identifies data to be freed, a mask
- containing the bitwise OR of one or
- more of
- PNG_FREE_PLTE, PNG_FREE_TRNS,
- PNG_FREE_HIST, PNG_FREE_ICCP,
- PNG_FREE_PCAL, PNG_FREE_ROWS,
- PNG_FREE_SCAL, PNG_FREE_SPLT,
- PNG_FREE_TEXT, PNG_FREE_UNKN,
- or simply PNG_FREE_ALL
- seq - sequence number of item to be freed
- (-1 for all items)
-
-This function may be safely called when the relevant storage has
-already been freed, or has not yet been allocated, or was allocated
-by the user and not by libpng, and will in those cases do nothing.
-The "seq" parameter is ignored if only one item of the selected data
-type, such as PLTE, is allowed. If "seq" is not -1, and multiple items
-are allowed for the data type identified in the mask, such as text or
-sPLT, only the n'th item in the structure is freed, where n is "seq".
-
-If you allocated data such as a palette that you passed in to libpng
-with png_set_*, you must not free it until just before the call to
-png_destroy_write_struct().
-
-The default behavior is only to free data that was allocated internally
-by libpng. This can be changed, so that libpng will not free the data,
-or so that it will free data that was allocated by the user with png_malloc()
-or png_zalloc() and passed in via a png_set_*() function, with
-
- png_data_freer(png_ptr, info_ptr, freer, mask)
- mask - which data elements are affected
- same choices as in png_free_data()
- freer - one of
- PNG_DESTROY_WILL_FREE_DATA
- PNG_SET_WILL_FREE_DATA
- PNG_USER_WILL_FREE_DATA
-
-For example, to transfer responsibility for some data from a read structure
-to a write structure, you could use
-
- png_data_freer(read_ptr, read_info_ptr,
- PNG_USER_WILL_FREE_DATA,
- PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
- png_data_freer(write_ptr, write_info_ptr,
- PNG_DESTROY_WILL_FREE_DATA,
- PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
-
-thereby briefly reassigning responsibility for freeing to the user but
-immediately afterwards reassigning it once more to the write_destroy
-function. Having done this, it would then be safe to destroy the read
-structure and continue to use the PLTE, tRNS, and hIST data in the write
-structure.
-
-This function only affects data that has already been allocated.
-You can call this function before calling after the png_set_*() functions
-to control whether the user or png_destroy_*() is supposed to free the data.
-When the user assumes responsibility for libpng-allocated data, the
-application must use
-png_free() to free it, and when the user transfers responsibility to libpng
-for data that the user has allocated, the user must have used png_malloc()
-or png_zalloc() to allocate it.
-
-If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
-separately, do not transfer responsibility for freeing text_ptr to libpng,
-because when libpng fills a png_text structure it combines these members with
-the key member, and png_free_data() will free only text_ptr.key. Similarly,
-if you transfer responsibility for free'ing text_ptr from libpng to your
-application, your application must not separately free those members.
-For a more compact example of writing a PNG image, see the file example.c.
-
-V. Modifying/Customizing libpng:
-
-There are two issues here. The first is changing how libpng does
-standard things like memory allocation, input/output, and error handling.
-The second deals with more complicated things like adding new chunks,
-adding new transformations, and generally changing how libpng works.
-Both of those are compile-time issues; that is, they are generally
-determined at the time the code is written, and there is rarely a need
-to provide the user with a means of changing them.
-
-Memory allocation, input/output, and error handling
-
-All of the memory allocation, input/output, and error handling in libpng
-goes through callbacks that are user-settable. The default routines are
-in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change
-these functions, call the appropriate png_set_*_fn() function.
-
-Memory allocation is done through the functions png_malloc(), png_calloc(),
-and png_free(). These currently just call the standard C functions.
-png_calloc() calls png_malloc() and then png_memset() to clear the newly
-allocated memory to zero. If your pointers can't access more then 64K
-at a time, you will want to set MAXSEG_64K in zlib.h. Since it is
-unlikely that the method of handling memory allocation on a platform
-will change between applications, these functions must be modified in
-the library at compile time. If you prefer to use a different method
-of allocating and freeing data, you can use png_create_read_struct_2() or
-png_create_write_struct_2() to register your own functions as described
-above. These functions also provide a void pointer that can be retrieved
-via
-
- mem_ptr=png_get_mem_ptr(png_ptr);
-
-Your replacement memory functions must have prototypes as follows:
-
- png_voidp malloc_fn(png_structp png_ptr,
- png_alloc_size_t size);
- void free_fn(png_structp png_ptr, png_voidp ptr);
-
-Your malloc_fn() must return NULL in case of failure. The png_malloc()
-function will normally call png_error() if it receives a NULL from the
-system memory allocator or from your replacement malloc_fn().
-
-Your free_fn() will never be called with a NULL ptr, since libpng's
-png_free() checks for NULL before calling free_fn().
-
-Input/Output in libpng is done through png_read() and png_write(),
-which currently just call fread() and fwrite(). The FILE * is stored in
-png_struct and is initialized via png_init_io(). If you wish to change
-the method of I/O, the library supplies callbacks that you can set
-through the function png_set_read_fn() and png_set_write_fn() at run
-time, instead of calling the png_init_io() function. These functions
-also provide a void pointer that can be retrieved via the function
-png_get_io_ptr(). For example:
-
- png_set_read_fn(png_structp read_ptr,
- voidp read_io_ptr, png_rw_ptr read_data_fn)
-
- png_set_write_fn(png_structp write_ptr,
- voidp write_io_ptr, png_rw_ptr write_data_fn,
- png_flush_ptr output_flush_fn);
-
- voidp read_io_ptr = png_get_io_ptr(read_ptr);
- voidp write_io_ptr = png_get_io_ptr(write_ptr);
-
-The replacement I/O functions must have prototypes as follows:
-
- void user_read_data(png_structp png_ptr,
- png_bytep data, png_size_t length);
- void user_write_data(png_structp png_ptr,
- png_bytep data, png_size_t length);
- void user_flush_data(png_structp png_ptr);
-
-The user_read_data() function is responsible for detecting and
-handling end-of-data errors.
-
-Supplying NULL for the read, write, or flush functions sets them back
-to using the default C stream functions, which expect the io_ptr to
-point to a standard *FILE structure. It is probably a mistake
-to use NULL for one of write_data_fn and output_flush_fn but not both
-of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
-It is an error to read from a write stream, and vice versa.
-
-Error handling in libpng is done through png_error() and png_warning().
-Errors handled through png_error() are fatal, meaning that png_error()
-should never return to its caller. Currently, this is handled via
-setjmp() and longjmp() (unless you have compiled libpng with
-PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
-but you could change this to do things like exit() if you should wish,
-as long as your function does not return.
-
-On non-fatal errors, png_warning() is called
-to print a warning message, and then control returns to the calling code.
-By default png_error() and png_warning() print a message on stderr via
-fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
-(because you don't want the messages) or PNG_NO_STDIO defined (because
-fprintf() isn't available). If you wish to change the behavior of the error
-functions, you will need to set up your own message callbacks. These
-functions are normally supplied at the time that the png_struct is created.
-It is also possible to redirect errors and warnings to your own replacement
-functions after png_create_*_struct() has been called by calling:
-
- png_set_error_fn(png_structp png_ptr,
- png_voidp error_ptr, png_error_ptr error_fn,
- png_error_ptr warning_fn);
-
- png_voidp error_ptr = png_get_error_ptr(png_ptr);
-
-If NULL is supplied for either error_fn or warning_fn, then the libpng
-default function will be used, calling fprintf() and/or longjmp() if a
-problem is encountered. The replacement error functions should have
-parameters as follows:
-
- void user_error_fn(png_structp png_ptr,
- png_const_charp error_msg);
- void user_warning_fn(png_structp png_ptr,
- png_const_charp warning_msg);
-
-The motivation behind using setjmp() and longjmp() is the C++ throw and
-catch exception handling methods. This makes the code much easier to write,
-as there is no need to check every return code of every function call.
-However, there are some uncertainties about the status of local variables
-after a longjmp, so the user may want to be careful about doing anything
-after setjmp returns non-zero besides returning itself. Consult your
-compiler documentation for more details. For an alternative approach, you
-may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net).
-
-Custom chunks
-
-If you need to read or write custom chunks, you may need to get deeper
-into the libpng code. The library now has mechanisms for storing
-and writing chunks of unknown type; you can even declare callbacks
-for custom chunks. However, this may not be good enough if the
-library code itself needs to know about interactions between your
-chunk and existing `intrinsic' chunks.
-
-If you need to write a new intrinsic chunk, first read the PNG
-specification. Acquire a first level of understanding of how it works.
-Pay particular attention to the sections that describe chunk names,
-and look at how other chunks were designed, so you can do things
-similarly. Second, check out the sections of libpng that read and
-write chunks. Try to find a chunk that is similar to yours and use
-it as a template. More details can be found in the comments inside
-the code. It is best to handle unknown chunks in a generic method,
-via callback functions, instead of by modifying libpng functions.
-
-If you wish to write your own transformation for the data, look through
-the part of the code that does the transformations, and check out some of
-the simpler ones to get an idea of how they work. Try to find a similar
-transformation to the one you want to add and copy off of it. More details
-can be found in the comments inside the code itself.
-
-Configuring for 16 bit platforms
-
-You will want to look into zconf.h to tell zlib (and thus libpng) that
-it cannot allocate more then 64K at a time. Even if you can, the memory
-won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
-
-Configuring for DOS
-
-For DOS users who only have access to the lower 640K, you will
-have to limit zlib's memory usage via a png_set_compression_mem_level()
-call. See zlib.h or zconf.h in the zlib library for more information.
-
-Configuring for Medium Model
-
-Libpng's support for medium model has been tested on most of the popular
-compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
-defined, and FAR gets defined to far in pngconf.h, and you should be
-all set. Everything in the library (except for zlib's structure) is
-expecting far data. You must use the typedefs with the p or pp on
-the end for pointers (or at least look at them and be careful). Make
-note that the rows of data are defined as png_bytepp, which is an
-unsigned char far * far *.
-
-Configuring for gui/windowing platforms:
-
-You will need to write new error and warning functions that use the GUI
-interface, as described previously, and set them to be the error and
-warning functions at the time that png_create_*_struct() is called,
-in order to have them available during the structure initialization.
-They can be changed later via png_set_error_fn(). On some compilers,
-you may also have to change the memory allocators (png_malloc, etc.).
-
-Configuring for compiler xxx:
-
-All includes for libpng are in pngconf.h. If you need to add, change
-or delete an include, this is the place to do it.
-The includes that are not needed outside libpng are placed in pngpriv.h,
-which is only used by the routines inside libpng itself.
-The files in libpng proper only include pngpriv.h and png.h, which
-in turn includes pngconf.h.
-
-Configuring zlib:
-
-There are special functions to configure the compression. Perhaps the
-most useful one changes the compression level, which currently uses
-input compression values in the range 0 - 9. The library normally
-uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
-have shown that for a large majority of images, compression values in
-the range 3-6 compress nearly as well as higher levels, and do so much
-faster. For online applications it may be desirable to have maximum speed
-(Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
-specify no compression (Z_NO_COMPRESSION = 0), but this would create
-files larger than just storing the raw bitmap. You can specify the
-compression level by calling:
-
- png_set_compression_level(png_ptr, level);
-
-Another useful one is to reduce the memory level used by the library.
-The memory level defaults to 8, but it can be lowered if you are
-short on memory (running DOS, for example, where you only have 640K).
-Note that the memory level does have an effect on compression; among
-other things, lower levels will result in sections of incompressible
-data being emitted in smaller stored blocks, with a correspondingly
-larger relative overhead of up to 15% in the worst case.
-
- png_set_compression_mem_level(png_ptr, level);
-
-The other functions are for configuring zlib. They are not recommended
-for normal use and may result in writing an invalid PNG file. See
-zlib.h for more information on what these mean.
-
- png_set_compression_strategy(png_ptr,
- strategy);
- png_set_compression_window_bits(png_ptr,
- window_bits);
- png_set_compression_method(png_ptr, method);
- png_set_compression_buffer_size(png_ptr, size);
-
-Controlling row filtering
-
-If you want to control whether libpng uses filtering or not, which
-filters are used, and how it goes about picking row filters, you
-can call one of these functions. The selection and configuration
-of row filters can have a significant impact on the size and
-encoding speed and a somewhat lesser impact on the decoding speed
-of an image. Filtering is enabled by default for RGB and grayscale
-images (with and without alpha), but not for paletted images nor
-for any images with bit depths less than 8 bits/pixel.
-
-The 'method' parameter sets the main filtering method, which is
-currently only '0' in the PNG 1.2 specification. The 'filters'
-parameter sets which filter(s), if any, should be used for each
-scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
-to turn filtering on and off, respectively.
-
-Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
-PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
-ORed together with '|' to specify one or more filters to use.
-These filters are described in more detail in the PNG specification.
-If you intend to change the filter type during the course of writing
-the image, you should start with flags set for all of the filters
-you intend to use so that libpng can initialize its internal
-structures appropriately for all of the filter types. (Note that this
-means the first row must always be adaptively filtered, because libpng
-currently does not allocate the filter buffers until png_write_row()
-is called for the first time.)
-
- filters = PNG_FILTER_NONE | PNG_FILTER_SUB
- PNG_FILTER_UP | PNG_FILTER_AVG |
- PNG_FILTER_PAETH | PNG_ALL_FILTERS;
-
- png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
- filters);
- The second parameter can also be
- PNG_INTRAPIXEL_DIFFERENCING if you are
- writing a PNG to be embedded in a MNG
- datastream. This parameter must be the
- same as the value of filter_method used
- in png_set_IHDR().
-
-It is also possible to influence how libpng chooses from among the
-available filters. This is done in one or both of two ways - by
-telling it how important it is to keep the same filter for successive
-rows, and by telling it the relative computational costs of the filters.
-
- double weights[3] = {1.5, 1.3, 1.1},
- costs[PNG_FILTER_VALUE_LAST] =
- {1.0, 1.3, 1.3, 1.5, 1.7};
-
- png_set_filter_heuristics(png_ptr,
- PNG_FILTER_HEURISTIC_WEIGHTED, 3,
- weights, costs);
-
-The weights are multiplying factors that indicate to libpng that the
-row filter should be the same for successive rows unless another row filter
-is that many times better than the previous filter. In the above example,
-if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
-"sum of absolute differences" 1.5 x 1.3 times higher than other filters
-and still be chosen, while the NONE filter could have a sum 1.1 times
-higher than other filters and still be chosen. Unspecified weights are
-taken to be 1.0, and the specified weights should probably be declining
-like those above in order to emphasize recent filters over older filters.
-
-The filter costs specify for each filter type a relative decoding cost
-to be considered when selecting row filters. This means that filters
-with higher costs are less likely to be chosen over filters with lower
-costs, unless their "sum of absolute differences" is that much smaller.
-The costs do not necessarily reflect the exact computational speeds of
-the various filters, since this would unduly influence the final image
-size.
-
-Note that the numbers above were invented purely for this example and
-are given only to help explain the function usage. Little testing has
-been done to find optimum values for either the costs or the weights.
-
-Removing unwanted object code
-
-There are a bunch of #define's in pngconf.h that control what parts of
-libpng are compiled. All the defines end in _SUPPORTED. If you are
-never going to use a capability, you can change the #define to #undef
-before recompiling libpng and save yourself code and data space, or
-you can turn off individual capabilities with defines that begin with
-PNG_NO_.
-
-You can also turn all of the transforms and ancillary chunk capabilities
-off en masse with compiler directives that define
-PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
-or all four,
-along with directives to turn on any of the capabilities that you do
-want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra
-transformations but still leave the library fully capable of reading
-and writing PNG files with all known public chunks. Use of the
-PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library
-that is incapable of reading or writing ancillary chunks. If you are
-not using the progressive reading capability, you can turn that off
-with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING
-capability, which you'll still have).
-
-All the reading and writing specific code are in separate files, so the
-linker should only grab the files it needs. However, if you want to
-make sure, or if you are building a stand alone library, all the
-reading files start with pngr and all the writing files start with
-pngw. The files that don't match either (like png.c, pngtrans.c, etc.)
-are used for both reading and writing, and always need to be included.
-The progressive reader is in pngpread.c
-
-If you are creating or distributing a dynamically linked library (a .so
-or DLL file), you should not remove or disable any parts of the library,
-as this will cause applications linked with different versions of the
-library to fail if they call functions not available in your library.
-The size of the library itself should not be an issue, because only
-those sections that are actually used will be loaded into memory.
-
-Requesting debug printout
-
-The macro definition PNG_DEBUG can be used to request debugging
-printout. Set it to an integer value in the range 0 to 3. Higher
-numbers result in increasing amounts of debugging information. The
-information is printed to the "stderr" file, unless another file
-name is specified in the PNG_DEBUG_FILE macro definition.
-
-When PNG_DEBUG > 0, the following functions (macros) become available:
-
- png_debug(level, message)
- png_debug1(level, message, p1)
- png_debug2(level, message, p1, p2)
-
-in which "level" is compared to PNG_DEBUG to decide whether to print
-the message, "message" is the formatted string to be printed,
-and p1 and p2 are parameters that are to be embedded in the string
-according to printf-style formatting directives. For example,
-
- png_debug1(2, "foo=%d\n", foo);
-
-is expanded to
-
- if(PNG_DEBUG > 2)
- fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
-
-When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
-can still use PNG_DEBUG to control your own debugging:
-
- #ifdef PNG_DEBUG
- fprintf(stderr, ...
- #endif
-
-When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
-having level = 0 will be printed. There aren't any such statements in
-this version of libpng, but if you insert some they will be printed.
-
-VI. MNG support
-
-The MNG specification (available at http://www.libpng.org/pub/mng) allows
-certain extensions to PNG for PNG images that are embedded in MNG datastreams.
-Libpng can support some of these extensions. To enable them, use the
-png_permit_mng_features() function:
-
- feature_set = png_permit_mng_features(png_ptr, mask)
- mask is a png_uint_32 containing the bitwise OR of the
- features you want to enable. These include
- PNG_FLAG_MNG_EMPTY_PLTE
- PNG_FLAG_MNG_FILTER_64
- PNG_ALL_MNG_FEATURES
- feature_set is a png_uint_32 that is the bitwise AND of
- your mask with the set of MNG features that is
- supported by the version of libpng that you are using.
-
-It is an error to use this function when reading or writing a standalone
-PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped
-in a MNG datastream. As a minimum, it must have the MNG 8-byte signature
-and the MHDR and MEND chunks. Libpng does not provide support for these
-or any other MNG chunks; your application must provide its own support for
-them. You may wish to consider using libmng (available at
-http://www.libmng.com) instead.
-
-VII. Changes to Libpng from version 0.88
-
-It should be noted that versions of libpng later than 0.96 are not
-distributed by the original libpng author, Guy Schalnat, nor by
-Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
-distributed versions 0.89 through 0.96, but rather by another member
-of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
-still alive and well, but they have moved on to other things.
-
-The old libpng functions png_read_init(), png_write_init(),
-png_info_init(), png_read_destroy(), and png_write_destroy() have been
-moved to PNG_INTERNAL in version 0.95 to discourage their use. These
-functions will be removed from libpng version 2.0.0.
-
-The preferred method of creating and initializing the libpng structures is
-via the png_create_read_struct(), png_create_write_struct(), and
-png_create_info_struct() because they isolate the size of the structures
-from the application, allow version error checking, and also allow the
-use of custom error handling routines during the initialization, which
-the old functions do not. The functions png_read_destroy() and
-png_write_destroy() do not actually free the memory that libpng
-allocated for these structs, but just reset the data structures, so they
-can be used instead of png_destroy_read_struct() and
-png_destroy_write_struct() if you feel there is too much system overhead
-allocating and freeing the png_struct for each image read.
-
-Setting the error callbacks via png_set_message_fn() before
-png_read_init() as was suggested in libpng-0.88 is no longer supported
-because this caused applications that do not use custom error functions
-to fail if the png_ptr was not initialized to zero. It is still possible
-to set the error callbacks AFTER png_read_init(), or to change them with
-png_set_error_fn(), which is essentially the same function, but with a new
-name to force compilation errors with applications that try to use the old
-method.
-
-Starting with version 1.0.7, you can find out which version of the library
-you are using at run-time:
-
- png_uint_32 libpng_vn = png_access_version_number();
-
-The number libpng_vn is constructed from the major version, minor
-version with leading zero, and release number with leading zero,
-(e.g., libpng_vn for version 1.0.7 is 10007).
-
-You can also check which version of png.h you used when compiling your
-application:
-
- png_uint_32 application_vn = PNG_LIBPNG_VER;
-
-VIII. Changes to Libpng from version 1.0.x to 1.2.x
-
-Support for user memory management was enabled by default. To
-accomplish this, the functions png_create_read_struct_2(),
-png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
-png_malloc_default(), and png_free_default() were added.
-
-Support for the iTXt chunk has been enabled by default as of
-version 1.2.41.
-
-Support for certain MNG features was enabled.
-
-Support for numbered error messages was added. However, we never got
-around to actually numbering the error messages. The function
-png_set_strip_error_numbers() was added (Note: the prototype for this
-function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
-builds of libpng-1.2.15. It was restored in libpng-1.2.36).
-
-The png_malloc_warn() function was added at libpng-1.2.3. This issues
-a png_warning and returns NULL instead of aborting when it fails to
-acquire the requested memory allocation.
-
-Support for setting user limits on image width and height was enabled
-by default. The functions png_set_user_limits(), png_get_user_width_max(),
-and png_get_user_height_max() were added at libpng-1.2.6.
-
-The png_set_add_alpha() function was added at libpng-1.2.7.
-
-The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.
-Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the
-tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is
-deprecated.
-
-A number of macro definitions in support of runtime selection of
-assembler code features (especially Intel MMX code support) were
-added at libpng-1.2.0:
-
- PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
- PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
- PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
- PNG_ASM_FLAG_MMX_READ_INTERLACE
- PNG_ASM_FLAG_MMX_READ_FILTER_SUB
- PNG_ASM_FLAG_MMX_READ_FILTER_UP
- PNG_ASM_FLAG_MMX_READ_FILTER_AVG
- PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
- PNG_ASM_FLAGS_INITIALIZED
- PNG_MMX_READ_FLAGS
- PNG_MMX_FLAGS
- PNG_MMX_WRITE_FLAGS
- PNG_MMX_FLAGS
-
-We added the following functions in support of runtime
-selection of assembler code features:
-
- png_get_mmx_flagmask()
- png_set_mmx_thresholds()
- png_get_asm_flags()
- png_get_mmx_bitdepth_threshold()
- png_get_mmx_rowbytes_threshold()
- png_set_asm_flags()
-
-We replaced all of these functions with simple stubs in libpng-1.2.20,
-when the Intel assembler code was removed due to a licensing issue.
-
-These macros are deprecated:
-
- PNG_READ_TRANSFORMS_NOT_SUPPORTED
- PNG_PROGRESSIVE_READ_NOT_SUPPORTED
- PNG_NO_SEQUENTIAL_READ_SUPPORTED
- PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
- PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
- PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED
-
-They have been replaced, respectively, by:
-
- PNG_NO_READ_TRANSFORMS
- PNG_NO_PROGRESSIVE_READ
- PNG_NO_SEQUENTIAL_READ
- PNG_NO_WRITE_TRANSFORMS
- PNG_NO_READ_ANCILLARY_CHUNKS
- PNG_NO_WRITE_ANCILLARY_CHUNKS
-
-PNG_MAX_UINT was replaced with PNG_UINT_31_MAX. It has been
-deprecated since libpng-1.0.16 and libpng-1.2.6.
-
-The function
- png_check_sig(sig, num)
-was replaced with
- !png_sig_cmp(sig, 0, num)
-It has been deprecated since libpng-0.90.
-
-The function
- png_set_gray_1_2_4_to_8()
-which also expands tRNS to alpha was replaced with
- png_set_expand_gray_1_2_4_to_8()
-which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
-
-IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
-
-Private libpng prototypes and macro definitions were moved from
-png.h and pngconf.h into a new pngpriv.h header file.
-
-Functions png_set_benign_errors(), png_benign_error(), and
-png_chunk_benign_error() were added.
-
-Support for setting the maximum amount of memory that the application
-will allocate for reading chunks was added, as a security measure.
-The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
-were added to the library.
-
-We implemented support for I/O states by adding png_ptr member io_state
-and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c
-
-We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level
-input transforms.
-
-Checking for and reporting of errors in the IHDR chunk is more thorough.
-
-Support for global arrays was removed, to improve thread safety.
-
-Some obsolete/deprecated macros and functions have been removed.
-
-Typecasted NULL definitions such as
- #define png_voidp_NULL (png_voidp)NULL
-were eliminated. If you used these in your application, just use
-NULL instead.
-
-The png_struct and info_struct members "trans" and "trans_values" were
-changed to "trans_alpha" and "trans_color", respectively.
-
-The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles
-were removed.
-
-The PNG_1_0_X and PNG_1_2_X macros were eliminated.
-
-The PNG_LEGACY_SUPPORTED macro was eliminated.
-
-Many WIN32_WCE #ifdefs were removed.
-
-The functions png_read_init(info_ptr), png_write_init(info_ptr),
-png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
-have been removed. They have been deprecated since libpng-0.95.
-
-The png_permit_empty_plte() was removed. It has been deprecated
-since libpng-1.0.9. Use png_permit_mng_features() instead.
-
-We removed the obsolete stub functions png_get_mmx_flagmask(),
-png_set_mmx_thresholds(), png_get_asm_flags(),
-png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
-png_set_asm_flags(), and png_mmx_supported()
-
-We removed the obsolete png_check_sig(), png_memcpy_check(), and
-png_memset_check() functions. Instead use !png_sig_cmp(), png_memcpy(),
-and png_memset(), respectively.
-
-The function png_set_gray_1_2_4_to_8() was removed. It has been
-deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
-png_set_expand_gray_1_2_4_to_8() because the former function also
-expanded palette images.
-
-We changed the prototype for png_malloc() from
- png_malloc(png_structp png_ptr, png_uint_32 size)
-to
- png_malloc(png_structp png_ptr, png_alloc_size_t size)
-
-This also applies to the prototype for the user replacement malloc_fn().
-
-The png_calloc() function was added and is used in place of
-of "png_malloc(); png_memset();" except in the case in png_read_png()
-where the array consists of pointers; in this case a "for" loop is used
-after the png_malloc() to set the pointers to NULL, to give robust.
-behavior in case the application runs out of memory part-way through
-the process.
-
-We changed the prototypes of png_get_compression_buffer_size() and
-png_set_compression_buffer_size() to work with png_size_t instead of
-png_uint_32.
-
-Support for numbered error messages was removed by default, since we
-never got around to actually numbering the error messages. The function
-png_set_strip_error_numbers() was removed from the library by default.
-
-The png_zalloc() and png_zfree() functions are no longer exported.
-The png_zalloc() function no longer zeroes out the memory that it
-allocates.
-
-Support for dithering was disabled by default in libpng-1.4.0, because
-been well tested and doesn't actually "dither". The code was not
-removed, however, and could be enabled by building libpng with
-PNG_READ_DITHER_SUPPORTED defined. In libpng-1.4.2, this support
-was reenabled, but the function was renamed png_set_quantize() to
-reflect more accurately what it actually does. At the same time,
-the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
-PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS.
-
-We removed the trailing '.' from the warning and error messages.
-
-X. Detecting libpng
-
-The png_get_io_ptr() function has been present since libpng-0.88, has never
-changed, and is unaffected by conditional compilation macros. It is the
-best choice for use in configure scripts for detecting the presence of any
-libpng version since 0.88. In an autoconf "configure.in" you could use
-
- AC_CHECK_LIB(png, png_get_io_ptr, ...
-
-XI. Source code repository
-
-Since about February 2009, version 1.2.34, libpng has been under "git" source
-control. The git repository was built from old libpng-x.y.z.tar.gz files
-going back to version 0.70. You can access the git repository (read only)
-at
-
- git://libpng.git.sourceforge.net/gitroot/libpng
-
-or you can browse it via "gitweb" at
-
- http://libpng.git.sourceforge.net/git/gitweb.cgi?p=libpng
-
-Patches can be sent to glennrp at users.sourceforge.net or to
-png-mng-implement at lists.sourceforge.net or you can upload them to
-the libpng bug tracker at
-
- http://libpng.sourceforge.net
-
-XII. Coding style
-
-Our coding style is similar to the "Allman" style, with curly
-braces on separate lines:
-
- if (condition)
- {
- action;
- }
-
- else if (another condition)
- {
- another action;
- }
-
-The braces can be omitted from simple one-line actions:
-
- if (condition)
- return (0);
-
-We use 3-space indentation, except for continued statements which
-are usually indented the same as the first line of the statement
-plus four more spaces.
-
-For macro definitions we use 2-space indentation, always leaving the "#"
-in the first column.
-
- #ifndef PNG_NO_FEATURE
- # ifndef PNG_FEATURE_SUPPORTED
- # define PNG_FEATURE_SUPPORTED
- # endif
- #endif
-
-Comments appear with the leading "/*" at the same indentation as
-the statement that follows the comment:
-
- /* Single-line comment */
- statement;
-
- /* This is a multiple-line
- * comment.
- */
- statement;
-
-Very short comments can be placed after the end of the statement
-to which they pertain:
-
- statement; /* comment */
-
-We don't use C++ style ("//") comments. We have, however,
-used them in the past in some now-abandoned MMX assembler
-code.
-
-Functions and their curly braces are not indented, and
-exported functions are marked with PNGAPI:
-
- /* This is a public function that is visible to
- * application programers. It does thus-and-so.
- */
- void PNGAPI
- png_exported_function(png_ptr, png_info, foo)
- {
- body;
- }
-
-The prototypes for all exported functions appear in png.h,
-above the comment that says
-
- /* Maintainer: Put new public prototypes here ... */
-
-We mark all non-exported functions with "/* PRIVATE */"":
-
- void /* PRIVATE */
- png_non_exported_function(png_ptr, png_info, foo)
- {
- body;
- }
-
-The prototypes for non-exported functions (except for those in
-pngtest) appear in
-pngpriv.h
-above the comment that says
-
- /* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */
-
-The names of all exported functions and variables begin
-with "png_", and all publicly visible C preprocessor
-macros begin with "PNG_".
-
-We put a space after each comma and after each semicolon
-in "for" statments, and we put spaces before and after each
-C binary operator and after "for" or "while", and before
-"?". We don't put a space between a typecast and the expression
-being cast, nor do we put one between a function name and the
-left parenthesis that follows it:
-
- for (i = 2; i > 0; --i)
- y[i] = a(x) + (int)b;
-
-We prefer #ifdef and #ifndef to #if defined() and if !defined()
-when there is only one macro being tested.
-
-We do not use the TAB character for indentation in the C sources.
-
-Lines do not exceed 80 characters.
-
-Other rules can be inferred by inspecting the libpng source.
-
-XIII. Y2K Compliance in libpng
-
-June 26, 2010
-
-Since the PNG Development group is an ad-hoc body, we can't make
-an official declaration.
-
-This is your unofficial assurance that libpng from version 0.71 and
-upward through 1.4.3 are Y2K compliant. It is my belief that earlier
-versions were also Y2K compliant.
-
-Libpng only has three year fields. One is a 2-byte unsigned integer that
-will hold years up to 65535. The other two hold the date in text
-format, and will hold years up to 9999.
-
-The integer is
- "png_uint_16 year" in png_time_struct.
-
-The strings are
- "png_charp time_buffer" in png_struct and
- "near_time_buffer", which is a local character string in png.c.
-
-There are seven time-related functions:
-
- png_convert_to_rfc_1123() in png.c
- (formerly png_convert_to_rfc_1152() in error)
- png_convert_from_struct_tm() in pngwrite.c, called
- in pngwrite.c
- png_convert_from_time_t() in pngwrite.c
- png_get_tIME() in pngget.c
- png_handle_tIME() in pngrutil.c, called in pngread.c
- png_set_tIME() in pngset.c
- png_write_tIME() in pngwutil.c, called in pngwrite.c
-
-All appear to handle dates properly in a Y2K environment. The
-png_convert_from_time_t() function calls gmtime() to convert from system
-clock time, which returns (year - 1900), which we properly convert to
-the full 4-digit year. There is a possibility that applications using
-libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
-function, or that they are incorrectly passing only a 2-digit year
-instead of "year - 1900" into the png_convert_from_struct_tm() function,
-but this is not under our control. The libpng documentation has always
-stated that it works with 4-digit years, and the APIs have been
-documented as such.
-
-The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
-integer to hold the year, and can hold years as large as 65535.
-
-zlib, upon which libpng depends, is also Y2K compliant. It contains
-no date-related code.
-
-
- Glenn Randers-Pehrson
- libpng maintainer
- PNG Development Group
projects / reactos.git / blobdiff