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-libpng.txt - A description on how to use and modify libpng
-
- libpng version 1.2.40 - September 10, 2009
- 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 1.2.40 - September 10, 2009
- 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 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
-
-(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, of
-    filter_method can be NULL if you are
-    not interested in their values.
-
-    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())).
-
-
-    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);
-
-
-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, &num_trans,
-                     &trans_values);
-    trans          - array of transparent entries for
-                     palette (PNG_INFO_tRNS)
-    trans_values   - 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).
-    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.
-
-PNG can have files with 16 bits per channel.  If you only can handle
-8 bits per channel, this will strip the pixels down to 8 bit.
-
-    if (bit_depth == 16)
-        png_set_strip_16(png_ptr);
-
-If, for some reason, you don't need the alpha channel on an image,
-and you want to remove it rather than combining it with the background
-(but the image author certainly had in mind that you *would* combine
-it with the background, so that's what you should probably do):
-
-    if (color_type & PNG_COLOR_MASK_ALPHA)
-        png_set_strip_alpha(png_ptr);
-
-In PNG files, the alpha channel in an image
-is the level of opacity.  If you need the alpha channel in an image to
-be the level of transparency instead of opacity, you can invert the
-alpha channel (or the tRNS chunk data) after it's read, so that 0 is
-fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
-images) is fully transparent, with
-
-    png_set_invert_alpha(png_ptr);
-
-PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
-they can, resulting in, for example, 8 pixels per byte for 1 bit
-files.  This code expands to 1 pixel per byte without changing the
-values of the pixels:
-
-    if (bit_depth < 8)
-        png_set_packing(png_ptr);
-
-PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
-stored in a PNG image have been "scaled" or "shifted" up to the next
-higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] to
-8 bits/sample in the range [0, 255]).  However, it is also possible to
-convert the PNG pixel data back to the original bit depth of the image.
-This call reduces the pixels back down to the original bit depth:
-
-    png_color_8p sig_bit;
-
-    if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
-        png_set_shift(png_ptr, sig_bit);
-
-PNG files store 3-color pixels in red, green, blue order.  This code
-changes the storage of the pixels to blue, green, red:
-
-    if (color_type == PNG_COLOR_TYPE_RGB ||
-        color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-        png_set_bgr(png_ptr);
-
-PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
-into 4 or 8 bytes for windowing systems that need them in this format:
-
-    if (color_type == PNG_COLOR_TYPE_RGB)
-        png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
-
-where "filler" is the 8 or 16-bit number to fill with, and the location is
-either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
-you want the filler before the RGB or after.  This transformation
-does not affect images that already have full alpha channels.  To add an
-opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
-will generate RGBA pixels.
-
-Note that png_set_filler() does not change the color type.  If you want
-to do that, you can add a true alpha channel with
-
-    if (color_type == PNG_COLOR_TYPE_RGB ||
-           color_type == PNG_COLOR_TYPE_GRAY)
-    png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
-
-where "filler" contains the alpha value to assign to each pixel.
-This function was added in libpng-1.2.7.
-
-If you are reading an image with an alpha channel, and you need the
-data as ARGB instead of the normal PNG format RGBA:
-
-    if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-        png_set_swap_alpha(png_ptr);
-
-For some uses, you may want a grayscale image to be represented as
-RGB.  This code will do that conversion:
-
-    if (color_type == PNG_COLOR_TYPE_GRAY ||
-        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
-          png_set_gray_to_rgb(png_ptr);
-
-Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
-with alpha.
-
-    if (color_type == PNG_COLOR_TYPE_RGB ||
-        color_type == PNG_COLOR_TYPE_RGB_ALPHA)
-          png_set_rgb_to_gray_fixed(png_ptr, error_action,
-             int red_weight, int green_weight);
-
-    error_action = 1: silently do the conversion
-    error_action = 2: issue a warning if the original
-                      image has any pixel where
-                      red != green or red != blue
-    error_action = 3: issue an error and abort the
-                      conversion if the original
-                      image has any pixel where
-                      red != green or red != blue
-
-    red_weight:       weight of red component times 100000
-    green_weight:     weight of green component times 100000
-                      If either weight is negative, default
-                      weights (21268, 71514) are used.
-
-If you have set error_action = 1 or 2, you can
-later check whether the image really was gray, after processing
-the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
-It will return a png_byte that is zero if the image was gray or
-1 if there were any non-gray pixels.  bKGD and sBIT data
-will be silently converted to grayscale, using the green channel
-data, regardless of the error_action setting.
-
-With red_weight+green_weight<=100000,
-the normalized graylevel is computed:
-
-    int rw = red_weight * 65536;
-    int gw = green_weight * 65536;
-    int bw = 65536 - (rw + gw);
-    gray = (rw*red + gw*green + bw*blue)/65536;
-
-The default values approximate those recommended in the Charles
-Poynton's Color FAQ, <http://www.inforamp.net/~poynton/>
-Copyright (c) 1998-01-04 Charles Poynton <poynton at inforamp.net>
-
-    Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
-
-Libpng approximates this with
-
-    Y = 0.21268 * R    + 0.7151 * G    + 0.07217 * B
-
-which can be expressed with integers as
-
-    Y = (6969 * R + 23434 * G + 2365 * B)/32768
-
-The calculation is done in a linear colorspace, if the image gamma
-is known.
-
-If you have a grayscale and you are using png_set_expand_depth(),
-png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to
-a higher bit-depth, you must either supply the background color as a gray
-value at the original file bit-depth (need_expand = 1) or else supply the
-background color as an RGB triplet at the final, expanded bit depth
-(need_expand = 0).  Similarly, if you are reading a paletted image, you
-must either supply the background color as a palette index (need_expand = 1)
-or as an RGB triplet that may or may not be in the palette (need_expand = 0).
-
-    png_color_16 my_background;
-    png_color_16p image_background;
-
-    if (png_get_bKGD(png_ptr, info_ptr, &image_background))
-        png_set_background(png_ptr, image_background,
-          PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
-    else
-        png_set_background(png_ptr, &my_background,
-          PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
-
-The png_set_background() function tells libpng to composite images
-with alpha or simple transparency against the supplied background
-color.  If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
-you may use this color, or supply another color more suitable for
-the current display (e.g., the background color from a web page).  You
-need to tell libpng whether the color is in the gamma space of the
-display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
-(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
-that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
-know why anyone would use this, but it's here).
-
-To properly display PNG images on any kind of system, the application needs
-to know what the display gamma is.  Ideally, the user will know this, and
-the application will allow them to set it.  One method of allowing the user
-to set the display gamma separately for each system is to check for a
-SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which will hopefully be
-correctly set.
-
-Note that display_gamma is the overall gamma correction required to produce
-pleasing results, which depends on the lighting conditions in the surrounding
-environment.  In a dim or brightly lit room, no compensation other than
-the physical gamma exponent of the monitor is needed, while in a dark room
-a slightly smaller exponent is better.
-
-   double gamma, screen_gamma;
-
-   if (/* We have a user-defined screen
-       gamma value */)
-   {
-      screen_gamma = user_defined_screen_gamma;
-   }
-   /* One way that applications can share the same
-      screen gamma value */
-   else if ((gamma_str = getenv("SCREEN_GAMMA"))
-      != NULL)
-   {
-      screen_gamma = (double)atof(gamma_str);
-   }
-   /* If we don't have another value */
-   else
-   {
-      screen_gamma = 2.2; /* A good guess for a
-           PC monitor in a bright office or a dim room */
-      screen_gamma = 2.0; /* A good guess for a
-           PC monitor in a dark room */
-      screen_gamma = 1.7 or 1.0;  /* A good
-           guess for Mac systems */
-   }
-
-The png_set_gamma() function handles gamma transformations of the data.
-Pass both the file gamma and the current screen_gamma.  If the file does
-not have a gamma value, you can pass one anyway if you have an idea what
-it is (usually 0.45455 is a good guess for GIF images on PCs).  Note
-that file gammas are inverted from screen gammas.  See the discussions
-on gamma in the PNG specification for an excellent description of what
-gamma is, and why all applications should support it.  It is strongly
-recommended that PNG viewers support gamma correction.
-
-   if (png_get_gAMA(png_ptr, info_ptr, &gamma))
-      png_set_gamma(png_ptr, screen_gamma, gamma);
-   else
-      png_set_gamma(png_ptr, screen_gamma, 0.45455);
-
-If you need to reduce an RGB file to a paletted file, or if a paletted
-file has more entries then will fit on your screen, png_set_dither()
-will do that.  Note that this is a simple match dither that merely
-finds the closest color available.  This should work fairly well with
-optimized palettes, and fairly badly with linear color cubes.  If you
-pass a palette that is larger then maximum_colors, the file will
-reduce the number of colors in the palette so it will fit into
-maximum_colors.  If there is a histogram, it will use it to make
-more intelligent choices when reducing the palette.  If there is no
-histogram, it may not do as good a job.
-
-   if (color_type & PNG_COLOR_MASK_COLOR)
-   {
-      if (png_get_valid(png_ptr, info_ptr,
-         PNG_INFO_PLTE))
-      {
-         png_uint_16p histogram = NULL;
-
-         png_get_hIST(png_ptr, info_ptr,
-            &histogram);
-         png_set_dither(png_ptr, palette, num_palette,
-            max_screen_colors, histogram, 1);
-      }
-      else
-      {
-         png_color std_color_cube[MAX_SCREEN_COLORS] =
-            { ... colors ... };
-
-         png_set_dither(png_ptr, std_color_cube,
-            MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
-            NULL,0);
-      }
-   }
-
-PNG files describe monochrome as black being zero and white being one.
-The following code will reverse this (make black be one and white be
-zero):
-
-   if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
-      png_set_invert_mono(png_ptr);
-
-This function can also be used to invert grayscale and gray-alpha images:
-
-   if (color_type == PNG_COLOR_TYPE_GRAY ||
-        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
-      png_set_invert_mono(png_ptr);
-
-PNG files store 16 bit pixels in network byte order (big-endian,
-ie. most significant bits first).  This code changes the storage to the
-other way (little-endian, i.e. least significant bits first, the
-way PCs store them):
-
-    if (bit_depth == 16)
-        png_set_swap(png_ptr);
-
-If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
-need to change the order the pixels are packed into bytes, you can use:
-
-    if (bit_depth < 8)
-       png_set_packswap(png_ptr);
-
-Finally, you can write your own transformation function if none of
-the existing ones meets your needs.  This is done by setting a callback
-with
-
-    png_set_read_user_transform_fn(png_ptr,
-       read_transform_fn);
-
-You must supply the function
-
-    void read_transform_fn(png_ptr ptr, row_info_ptr
-       row_info, png_bytep data)
-
-See pngtest.c for a working example.  Your function will be called
-after all of the other transformations have been processed.
-
-You can also set up a pointer to a user structure for use by your
-callback function, and you can inform libpng that your transform
-function will change the number of channels or bit depth with the
-function
-
-    png_set_user_transform_info(png_ptr, user_ptr,
-       user_depth, user_channels);
-
-The user's application, not libpng, is responsible for allocating and
-freeing any memory required for the user structure.
-
-You can retrieve the pointer via the function
-png_get_user_transform_ptr().  For example:
-
-    voidp read_user_transform_ptr =
-       png_get_user_transform_ptr(png_ptr);
-
-The last thing to handle is interlacing; this is covered in detail below,
-but you must call the function here if you want libpng to handle expansion
-of the interlaced image.
-
-    number_of_passes = png_set_interlace_handling(png_ptr);
-
-After setting the transformations, libpng can update your png_info
-structure to reflect any transformations you've requested with this
-call.  This is most useful to update the info structure's rowbytes
-field so you can use it to allocate your image memory.  This function
-will also update your palette with the correct screen_gamma and
-background if these have been given with the calls above.
-
-    png_read_update_info(png_ptr, info_ptr);
-
-After you call png_read_update_info(), you can allocate any
-memory you need to hold the image.  The row data is simply
-raw byte data for all forms of images.  As the actual allocation
-varies among applications, no example will be given.  If you
-are allocating one large chunk, you will need to build an
-array of pointers to each row, as it will be needed for some
-of the functions below.
-
-Reading image data
-
-After you've allocated memory, you can read the image data.
-The simplest way to do this is in one function call.  If you are
-allocating enough memory to hold the whole image, you can just
-call png_read_image() and libpng will read in all the image data
-and put it in the memory area supplied.  You will need to pass in
-an array of pointers to each row.
-
-This function automatically handles interlacing, so you don't need
-to call png_set_interlace_handling() or call this function multiple
-times, or any of that other stuff necessary with png_read_rows().
-
-   png_read_image(png_ptr, row_pointers);
-
-where row_pointers is:
-
-   png_bytep row_pointers[height];
-
-You can point to void or char or whatever you use for pixels.
-
-If you don't want to read in the whole image at once, you can
-use png_read_rows() instead.  If there is no interlacing (check
-interlace_type == PNG_INTERLACE_NONE), this is simple:
-
-    png_read_rows(png_ptr, row_pointers, NULL,
-       number_of_rows);
-
-where row_pointers is the same as in the png_read_image() call.
-
-If you are doing this just one row at a time, you can do this with
-a single row_pointer instead of an array of row_pointers:
-
-    png_bytep row_pointer = row;
-    png_read_row(png_ptr, row_pointer, NULL);
-
-If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
-get somewhat harder.  The only current (PNG Specification version 1.2)
-interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7)
-is a somewhat complicated 2D interlace scheme, known as Adam7, that
-breaks down an image into seven smaller images of varying size, based
-on an 8x8 grid.
-
-libpng can fill out those images or it can give them to you "as is".
-If you want them filled out, there are two ways to do that.  The one
-mentioned in the PNG specification is to expand each pixel to cover
-those pixels that have not been read yet (the "rectangle" method).
-This results in a blocky image for the first pass, which gradually
-smooths out as more pixels are read.  The other method is the "sparkle"
-method, where pixels are drawn only in their final locations, with the
-rest of the image remaining whatever colors they were initialized to
-before the start of the read.  The first method usually looks better,
-but tends to be slower, as there are more pixels to put in the rows.
-
-If you don't want libpng to handle the interlacing details, just call
-png_read_rows() seven times to read in all seven images.  Each of the
-images is a valid image by itself, or they can all be combined on an
-8x8 grid to form a single image (although if you intend to combine them
-you would be far better off using the libpng interlace handling).
-
-The first pass will return an image 1/8 as wide as the entire image
-(every 8th column starting in column 0) and 1/8 as high as the original
-(every 8th row starting in row 0), the second will be 1/8 as wide
-(starting in column 4) and 1/8 as high (also starting in row 0).  The
-third pass will be 1/4 as wide (every 4th pixel starting in column 0) and
-1/8 as high (every 8th row starting in row 4), and the fourth pass will
-be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
-and every 4th row starting in row 0).  The fifth pass will return an
-image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
-while the sixth pass will be 1/2 as wide and 1/2 as high as the original
-(starting in column 1 and row 0).  The seventh and final pass will be as
-wide as the original, and 1/2 as high, containing all of the odd
-numbered scanlines.  Phew!
-
-If you want libpng to expand the images, call this before calling
-png_start_read_image() or png_read_update_info():
-
-    if (interlace_type == PNG_INTERLACE_ADAM7)
-        number_of_passes
-           = png_set_interlace_handling(png_ptr);
-
-This will return the number of passes needed.  Currently, this
-is seven, but may change if another interlace type is added.
-This function can be called even if the file is not interlaced,
-where it will return one pass.
-
-If you are not going to display the image after each pass, but are
-going to wait until the entire image is read in, use the sparkle
-effect.  This effect is faster and the end result of either method
-is exactly the same.  If you are planning on displaying the image
-after each pass, the "rectangle" effect is generally considered the
-better looking one.
-
-If you only want the "sparkle" effect, just call png_read_rows() as
-normal, with the third parameter NULL.  Make sure you make pass over
-the image number_of_passes times, and you don't change the data in the
-rows between calls.  You can change the locations of the data, just
-not the data.  Each pass only writes the pixels appropriate for that
-pass, and assumes the data from previous passes is still valid.
-
-    png_read_rows(png_ptr, row_pointers, NULL,
-       number_of_rows);
-
-If you only want the first effect (the rectangles), do the same as
-before except pass the row buffer in the third parameter, and leave
-the second parameter NULL.
-
-    png_read_rows(png_ptr, NULL, row_pointers,
-       number_of_rows);
-
-Finishing a sequential read
-
-After you are finished reading the image through the
-low-level interface, you can finish reading the file.  If you are
-interested in comments or time, which may be stored either before or
-after the image data, you should pass the separate png_info struct if
-you want to keep the comments from before and after the image
-separate.  If you are not interested, you can pass NULL.
-
-   png_read_end(png_ptr, end_info);
-
-When you are done, you can free all memory allocated by libpng like this:
-
-   png_destroy_read_struct(&png_ptr, &info_ptr,
-       &end_info);
-
-It is also possible to individually free the info_ptr members that
-point to libpng-allocated storage with the following function:
-
-    png_free_data(png_ptr, info_ptr, mask, seq)
-    mask - identifies data to be freed, a mask
-           containing the bitwise OR of one or
-           more of
-             PNG_FREE_PLTE, PNG_FREE_TRNS,
-             PNG_FREE_HIST, PNG_FREE_ICCP,
-             PNG_FREE_PCAL, PNG_FREE_ROWS,
-             PNG_FREE_SCAL, PNG_FREE_SPLT,
-             PNG_FREE_TEXT, PNG_FREE_UNKN,
-           or simply PNG_FREE_ALL
-    seq  - sequence number of item to be freed
-           (-1 for all items)
-
-This function may be safely called when the relevant storage has
-already been freed, or has not yet been allocated, or was allocated
-by the user and not by libpng,  and will in those
-cases do nothing.  The "seq" parameter is ignored if only one item
-of the selected data type, such as PLTE, is allowed.  If "seq" is not
--1, and multiple items are allowed for the data type identified in
-the mask, such as text or sPLT, only the n'th item in the structure
-is freed, where n is "seq".
-
-The default behavior is only to free data that was allocated internally
-by libpng.  This can be changed, so that libpng will not free the data,
-or so that it will free data that was allocated by the user with png_malloc()
-or png_zalloc() and passed in via a png_set_*() function, with
-
-    png_data_freer(png_ptr, info_ptr, freer, mask)
-    mask   - which data elements are affected
-             same choices as in png_free_data()
-    freer  - one of
-               PNG_DESTROY_WILL_FREE_DATA
-               PNG_SET_WILL_FREE_DATA
-               PNG_USER_WILL_FREE_DATA
-
-This function only affects data that has already been allocated.
-You can call this function after reading the PNG data but before calling
-any png_set_*() functions, to control whether the user or the png_set_*()
-function is responsible for freeing any existing data that might be present,
-and again after the png_set_*() functions to control whether the user
-or png_destroy_*() is supposed to free the data.  When the user assumes
-responsibility for libpng-allocated data, the application must use
-png_free() to free it, and when the user transfers responsibility to libpng
-for data that the user has allocated, the user must have used png_malloc()
-or png_zalloc() to allocate it.
-
-If you allocated your row_pointers in a single block, as suggested above in
-the description of the high level read interface, you must not transfer
-responsibility for freeing it to the png_set_rows or png_read_destroy function,
-because they would also try to free the individual row_pointers[i].
-
-If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
-separately, do not transfer responsibility for freeing text_ptr to libpng,
-because when libpng fills a png_text structure it combines these members with
-the key member, and png_free_data() will free only text_ptr.key.  Similarly,
-if you transfer responsibility for free'ing text_ptr from libpng to your
-application, your application must not separately free those members.
-
-The png_free_data() function will turn off the "valid" flag for anything
-it frees.  If you need to turn the flag off for a chunk that was freed by your
-application instead of by libpng, you can use
-
-    png_set_invalid(png_ptr, info_ptr, mask);
-    mask - identifies the chunks to be made invalid,
-           containing the bitwise OR of one or
-           more of
-             PNG_INFO_gAMA, PNG_INFO_sBIT,
-             PNG_INFO_cHRM, PNG_INFO_PLTE,
-             PNG_INFO_tRNS, PNG_INFO_bKGD,
-             PNG_INFO_hIST, PNG_INFO_pHYs,
-             PNG_INFO_oFFs, PNG_INFO_tIME,
-             PNG_INFO_pCAL, PNG_INFO_sRGB,
-             PNG_INFO_iCCP, PNG_INFO_sPLT,
-             PNG_INFO_sCAL, PNG_INFO_IDAT
-
-For a more compact example of reading a PNG image, see the file example.c.
-
-Reading PNG files progressively
-
-The progressive reader is slightly different then the non-progressive
-reader.  Instead of calling png_read_info(), png_read_rows(), and
-png_read_end(), you make one call to png_process_data(), which calls
-callbacks when it has the info, a row, or the end of the image.  You
-set up these callbacks with png_set_progressive_read_fn().  You don't
-have to worry about the input/output functions of libpng, as you are
-giving the library the data directly in png_process_data().  I will
-assume that you have read the section on reading PNG files above,
-so I will only highlight the differences (although I will show
-all of the code).
-
-png_structp png_ptr;
-png_infop info_ptr;
-
- /*  An example code fragment of how you would
-     initialize the progressive reader in your
-     application. */
- int
- initialize_png_reader()
- {
-    png_ptr = png_create_read_struct
-        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-         user_error_fn, user_warning_fn);
-    if (!png_ptr)
-        return (ERROR);
-    info_ptr = png_create_info_struct(png_ptr);
-    if (!info_ptr)
-    {
-        png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
-           (png_infopp)NULL);
-        return (ERROR);
-    }
-
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-        png_destroy_read_struct(&png_ptr, &info_ptr,
-           (png_infopp)NULL);
-        return (ERROR);
-    }
-
-    /* This one's new.  You can provide functions
-       to be called when the header info is valid,
-       when each row is completed, and when the image
-       is finished.  If you aren't using all functions,
-       you can specify NULL parameters.  Even when all
-       three functions are NULL, you need to call
-       png_set_progressive_read_fn().  You can use
-       any struct as the user_ptr (cast to a void pointer
-       for the function call), and retrieve the pointer
-       from inside the callbacks using the function
-
-          png_get_progressive_ptr(png_ptr);
-
-       which will return a void pointer, which you have
-       to cast appropriately.
-     */
-    png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
-        info_callback, row_callback, end_callback);
-
-    return 0;
- }
-
- /* A code fragment that you call as you receive blocks
-   of data */
- int
- process_data(png_bytep buffer, png_uint_32 length)
- {
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-        png_destroy_read_struct(&png_ptr, &info_ptr,
-           (png_infopp)NULL);
-        return (ERROR);
-    }
-
-    /* This one's new also.  Simply give it a chunk
-       of data from the file stream (in order, of
-       course).  On machines with segmented memory
-       models machines, don't give it any more than
-       64K.  The library seems to run fine with sizes
-       of 4K. Although you can give it much less if
-       necessary (I assume you can give it chunks of
-       1 byte, I haven't tried less then 256 bytes
-       yet).  When this function returns, you may
-       want to display any rows that were generated
-       in the row callback if you don't already do
-       so there.
-     */
-    png_process_data(png_ptr, info_ptr, buffer, length);
-    return 0;
- }
-
- /* This function is called (as set by
-    png_set_progressive_read_fn() above) when enough data
-    has been supplied so all of the header has been
-    read.
- */
- void
- info_callback(png_structp png_ptr, png_infop info)
- {
-    /* Do any setup here, including setting any of
-       the transformations mentioned in the Reading
-       PNG files section.  For now, you _must_ call
-       either png_start_read_image() or
-       png_read_update_info() after all the
-       transformations are set (even if you don't set
-       any).  You may start getting rows before
-       png_process_data() returns, so this is your
-       last chance to prepare for that.
-     */
- }
-
- /* This function is called when each row of image
-    data is complete */
- void
- row_callback(png_structp png_ptr, png_bytep new_row,
-    png_uint_32 row_num, int pass)
- {
-    /* If the image is interlaced, and you turned
-       on the interlace handler, this function will
-       be called for every row in every pass.  Some
-       of these rows will not be changed from the
-       previous pass.  When the row is not changed,
-       the new_row variable will be NULL.  The rows
-       and passes are called in order, so you don't
-       really need the row_num and pass, but I'm
-       supplying them because it may make your life
-       easier.
-
-       For the non-NULL rows of interlaced images,
-       you must call png_progressive_combine_row()
-       passing in the row and the old row.  You can
-       call this function for NULL rows (it will just
-       return) and for non-interlaced images (it just
-       does the memcpy for you) if it will make the
-       code easier.  Thus, you can just do this for
-       all cases:
-     */
-
-        png_progressive_combine_row(png_ptr, old_row,
-          new_row);
-
-    /* where old_row is what was displayed for
-       previously for the row.  Note that the first
-       pass (pass == 0, really) will completely cover
-       the old row, so the rows do not have to be
-       initialized.  After the first pass (and only
-       for interlaced images), you will have to pass
-       the current row, and the function will combine
-       the old row and the new row.
-    */
- }
-
- void
- end_callback(png_structp png_ptr, png_infop info)
- {
-    /* This function is called after the whole image
-       has been read, including any chunks after the
-       image (up to and including the IEND).  You
-       will usually have the same info chunk as you
-       had in the header, although some data may have
-       been added to the comments and time fields.
-
-       Most people won't do much here, perhaps setting
-       a flag that marks the image as finished.
-     */
- }
-
-
-
-IV. Writing
-
-Much of this is very similar to reading.  However, everything of
-importance is repeated here, so you won't have to constantly look
-back up in the reading section to understand writing.
-
-Setup
-
-You will want to do the I/O initialization before you get into libpng,
-so if it doesn't work, you don't have anything to undo. If you are not
-using the standard I/O functions, you will need to replace them with
-custom writing functions.  See the discussion under Customizing libpng.
-
-    FILE *fp = fopen(file_name, "wb");
-    if (!fp)
-    {
-       return (ERROR);
-    }
-
-Next, png_struct and png_info need to be allocated and initialized.
-As these can be both relatively large, you may not want to store these
-on the stack, unless you have stack space to spare.  Of course, you
-will want to check if they return NULL.  If you are also reading,
-you won't want to name your read structure and your write structure
-both "png_ptr"; you can call them anything you like, such as
-"read_ptr" and "write_ptr".  Look at pngtest.c, for example.
-
-    png_structp png_ptr = png_create_write_struct
-       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-        user_error_fn, user_warning_fn);
-    if (!png_ptr)
-       return (ERROR);
-
-    png_infop info_ptr = png_create_info_struct(png_ptr);
-    if (!info_ptr)
-    {
-       png_destroy_write_struct(&png_ptr,
-         (png_infopp)NULL);
-       return (ERROR);
-    }
-
-If you want to use your own memory allocation routines,
-define PNG_USER_MEM_SUPPORTED and use
-png_create_write_struct_2() instead of png_create_write_struct():
-
-    png_structp png_ptr = png_create_write_struct_2
-       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
-        user_error_fn, user_warning_fn, (png_voidp)
-        user_mem_ptr, user_malloc_fn, user_free_fn);
-
-After you have these structures, you will need to set up the
-error handling.  When libpng encounters an error, it expects to
-longjmp() back to your routine.  Therefore, you will need to call
-setjmp() and pass the png_jmpbuf(png_ptr).  If you
-write the file from different routines, you will need to update
-the png_jmpbuf(png_ptr) every time you enter a new routine that will
-call a png_*() function.  See your documentation of setjmp/longjmp
-for your compiler for more information on setjmp/longjmp.  See
-the discussion on libpng error handling in the Customizing Libpng
-section below for more information on the libpng error handling.
-
-    if (setjmp(png_jmpbuf(png_ptr)))
-    {
-       png_destroy_write_struct(&png_ptr, &info_ptr);
-       fclose(fp);
-       return (ERROR);
-    }
-    ...
-    return;
-
-If you would rather avoid the complexity of setjmp/longjmp issues,
-you can compile libpng with PNG_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, num_trans,
-       trans_values);
-    trans          - array of transparent entries for
-                     palette (PNG_INFO_tRNS)
-    trans_values   - 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).
-    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()
-and png_free().  These currently just call the standard C functions.  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 protected by the
-PNG_INTERNAL definition, which is only defined for those routines inside
-libpng itself.  The files in libpng proper only include png.h, which
-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 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.
-
-IX.  (Omitted)
-
-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.
-
-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
-the PNG_INTERNAL section of png.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)
-       x[i] = a(x) + (int)b;
-
-We prefer #ifdef and #ifndef to #if defined() and if !defined()
-when there is only one macro being tested.
-
-Other rules can be inferred by inspecting the libpng
-source.
-
-XIII. Y2K Compliance in libpng
-
-September 10, 2009
-
-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 1.2.40 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