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+libpng.txt - A description on how to use and modify libpng
+libpng version 1.4.0 - January 3, 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 1.4.0 - January 3, 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_SETJMP_NOT_SUPPORTED, in which case
+errors will result in a call to PNG_ABORT() which defaults to abort().
+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);
+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.
+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,
+dithering, 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 1.4.0, 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);
+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_SETJMP_NOT_SUPPORTED, in which case
+errors will result in a call to PNG_ABORT() which defaults to abort().
+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_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_SETJMP_NOT_SUPPORTED, in which case it is handled via PNG_ABORT()),
+but you could change this to do things like exit() if you should wish.
+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)
+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.
+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;
+/* Multiple-line
+* comment
+*/
+statement;
+Very short comments can be placed at 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".  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
+January 3, 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 1.4.0 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