DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Mercurial (5b81998bb7ab)

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Libpng-manual.txt - A description on how to use and modify libpng

 libpng version 1.5.11 - June 14, 2012
 Updated and distributed by Glenn Randers-Pehrson
 <glennrp at users.sourceforge.net>
 Copyright (c) 1998-2011 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 1.5.11 - June 14, 2012
 Updated and distributed by Glenn Randers-Pehrson
 Copyright (c) 1998-2011 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.  Both are internal structures that are no longer exposed
in the libpng interface (as of libpng 1.5.0).

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, and direct access to the png_info fields was
deprecated..

The png_struct structure is the object used by the library to decode a
single image.  As of 1.5.0 this structure is also not exposed.

Almost all libpng APIs require a pointer to a png_struct as the first argument.
Many (in particular the png_set and png_get APIs) also require a pointer
to png_info as the second argument.  Some application visible macros
defined in png.h designed for basic data access (reading and writing
integers in the PNG format) don't take a png_info pointer, but it's almost
always safe to assume that a (png_struct*) has to be passed to call an API
function.

You can have more than one png_info structure associated with an image,
as illustrated in pngtest.c, one for information valid prior to the
IDAT chunks and another (called "end_info" below) for things after them.

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>

and also (as of libpng-1.5.0) the zlib header file, if you need it:

#include <zlib.h>

Types

The png.h header file defines a number of integral types used by the
APIs.  Most of these are fairly obvious; for example types corresponding
to integers of particular sizes and types for passing color values.

One exception is how non-integral numbers are handled.  For application
convenience most APIs that take such numbers have C (double) arguments;
however, internally PNG, and libpng, use 32 bit signed integers and encode
the value by multiplying by 100,000.  As of libpng 1.5.0 a convenience
macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
which is simply (png_int_32).

All APIs that take (double) arguments also have a matching API that
takes the corresponding fixed point integer arguments.  The fixed point
API has the same name as the floating point one with "_fixed" appended.
The actual range of values permitted in the APIs is frequently less than
the full range of (png_fixed_point) (-21474 to +21474).  When APIs require
a non-negative argument the type is recorded as png_uint_32 above.  Consult
the header file and the text below for more information.

Special care must be take with sCAL chunk handling because the chunk itself
uses non-integral values encoded as strings containing decimal floating point
numbers.  See the comments in the header file.

Configuration

The main header file function declarations are frequently protected by C
preprocessing directives of the form:

    #ifdef PNG_feature_SUPPORTED
    declare-function
    #endif
    ...
    #ifdef PNG_feature_SUPPORTED
    use-function
    #endif

The library can be built without support for these APIs, although a
standard build will have all implemented APIs.  Application programs
should check the feature macros before using an API for maximum
portability.  From libpng 1.5.0 the feature macros set during the build
of libpng are recorded in the header file "pnglibconf.h" and this file
is always included by png.h.

If you don't need to change the library configuration from the default, skip to
the next section ("Reading").

Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
of the build project files in the 'projects' directory simply copy
scripts/pnglibconf.h.prebuilt to pnglibconf.h.  This means that these build
systems do not permit easy auto-configuration of the library - they only
support the default configuration.

The easiest way to make minor changes to the libpng configuration when
auto-configuration is supported is to add definitions to the command line
using (typically) CPPFLAGS.  For example:

CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

will change the internal libpng math implementation for gamma correction and
other arithmetic calculations to fixed point, avoiding the need for fast
floating point support.  The result can be seen in the generated pnglibconf.h -
make sure it contains the changed feature macro setting.

If you need to make more extensive configuration changes - more than one or two
feature macro settings - you can either add -DPNG_USER_CONFIG to the build
command line and put a list of feature macro settings in pngusr.h or you can set
DFA_XTRA (a makefile variable) to a file containing the same information in the
form of 'option' settings.

A. Changing pnglibconf.h

A variety of methods exist to build libpng.  Not all of these support
reconfiguration of pnglibconf.h.  To reconfigure pnglibconf.h it must either be
rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.

Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
pnglibconf.h and changing the lines defining the supported features, paying
very close attention to the 'option' information in scripts/pnglibconf.dfa
that describes those features and their requirements.  This is easy to get
wrong.

B. Configuration using DFA_XTRA

Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
variant such as 'nawk' or 'gawk', is available.  The configure build will
automatically find an appropriate awk and build pnglibconf.h.
The scripts/pnglibconf.mak file contains a set of make rules for doing the
same thing if configure is not used, and many of the makefiles in the scripts
directory use this approach.

When rebuilding simply write a new file containing changed options and set
DFA_XTRA to the name of this file.  This causes the build to append the new file
to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file should contain lines
of the following forms:

everything = off

This turns all optional features off.  Include it at the start of pngusr.dfa to
make it easier to build a minimal configuration.  You will need to turn at least
some features on afterward to enable either reading or writing code, or both.

option feature on
option feature off

Enable or disable a single feature.  This will automatically enable other
features required by a feature that is turned on or disable other features that
require a feature which is turned off.  Conflicting settings will cause an error
message to be emitted by awk.

setting feature default value

Changes the default value of setting 'feature' to 'value'.  There are a small
number of settings listed at the top of pnglibconf.h, they are documented in the
source code.  Most of these values have performance implications for the library
but most of them have no visible effect on the API.  Some can also be overridden
from the API.

This method of building a customized pnglibconf.h is illustrated in
contrib/pngminim/*.  See the "$(PNGCONF):" target in the makefile and
pngusr.dfa in these directories.

C. Configuration using PNG_USR_CONFIG

If -DPNG_USR_CONFIG is added to the CFLAGS when pnglibconf.h is built the file
pngusr.h will automatically be included before the options in
scripts/pnglibconf.dfa are processed.  Your pngusr.h file should contain only
macro definitions turning features on or off or setting settings.

Apart from the global setting "everything = off" all the options listed above
can be set using macros in pngusr.h:

#define PNG_feature_SUPPORTED

is equivalent to:

option feature on

#define PNG_NO_feature

is equivalent to:

option feature off

#define PNG_feature value

is equivalent to:

setting feature default value

Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
pngusr file you supply override the contents of scripts/pnglibconf.dfa

If confusing or incomprehensible behavior results it is possible to
examine the intermediate file pnglibconf.dfn to find the full set of
dependency information for each setting and option.  Simply locate the
feature in the file and read the C comments that precede it.

This method is also illustrated in the contrib/pngminim/* makefiles and
pngusr.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);
    }

If you want to use your own memory allocation routines,
use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, 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 longjmp buffer 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);
    }

Pass (png_infopp)NULL instead of &end_info if you didn't create
an end_info structure.

If you would rather avoid the complexity of setjmp/longjmp issues,
you can compile libpng with PNG_NO_SETJMP, in which case
errors will result in a call to PNG_ABORT() which defaults to abort().

You can #define PNG_ABORT() to a function that does something
more useful than abort(), as long as your function does not
return.

Now you need to set up the input code.  The default for libpng is to
use the C function fread().  If you use this, you will need to pass a
valid FILE * in the function png_init_io().  Be sure that the file is
opened in binary mode.  If you wish to handle reading data in another
way, you need not call the png_init_io() function, but you must then
implement the libpng I/O methods discussed in the Customizing Libpng
section below.

    png_init_io(png_ptr, fp);

If you had previously opened the file and read any of the signature from
the beginning in order to see if this was a PNG file, you need to let
libpng know that there are some bytes missing from the start of the file.

    png_set_sig_bytes(png_ptr, number);

You can change the zlib compression buffer size to be used while
reading compressed data with

    png_set_compression_buffer_size(png_ptr, buffer_size);

where the default size is 8192 bytes.  Note that the buffer size
is changed immediately and the buffer is reallocated immediately,
instead of setting a flag to be acted upon later.

If you want CRC errors to be handled in a different manner than
the default, use

    png_set_crc_action(png_ptr, crit_action, ancil_action);

The values for png_set_crc_action() say how libpng is to handle CRC errors in
ancillary and critical chunks, and whether to use the data contained
therein.  Note that it is impossible to "discard" data in a critical
chunk.

Choices for (int) crit_action are
   PNG_CRC_DEFAULT      0  error/quit
   PNG_CRC_ERROR_QUIT   1  error/quit
   PNG_CRC_WARN_USE     3  warn/use data
   PNG_CRC_QUIET_USE    4  quiet/use data
   PNG_CRC_NO_CHANGE    5  use the current value

Choices for (int) ancil_action are
   PNG_CRC_DEFAULT      0  error/quit
   PNG_CRC_ERROR_QUIT   1  error/quit
   PNG_CRC_WARN_DISCARD 2  warn/discard data
   PNG_CRC_WARN_USE     3  warn/use data
   PNG_CRC_QUIET_USE    4  quiet/use data
   PNG_CRC_NO_CHANGE    5  use the current value

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_structp png_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_structp png_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);

When this function is called the row has already been completely processed and
the 'row' and 'pass' refer to the next row to be handled.  For the
non-interlaced case the row that was just handled is simply one less than the
passed in row number, and pass will always be 0.  For the interlaced case the
same applies unless the row value is 0, in which case the row just handled was
the last one from one of the preceding passes.  Because interlacing may skip a
pass you cannot be sure that the preceding pass is just 'pass-1', if you really
need to know what the last pass is record (row,pass) from the callback and use
the last recorded value each time.

As with the user transform you can find the output row using the
PNG_ROW_FROM_PASS_ROW macro.

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.
If you know that your application will never make use of some particular
chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.

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 change 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().

When writing a PNG datastream, put this statement before calling
png_write_info() or png_write_png().

If you need to retrieve the limits that are being applied, use

   width_max = png_get_user_width_max(png_ptr);
   height_max = png_get_user_height_max(png_ptr);

The PNG specification sets no limit on the number of ancillary chunks
allowed in a PNG datastream.  You can impose a limit on the total number
of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with

   png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

where 0x7fffffffL means unlimited.  You can retrieve this limit with

   chunk_cache_max = png_get_chunk_cache_max(png_ptr);

This limit also applies to the number of buffers that can be allocated
by png_decompress_chunk() while decompressing iTXt, zTXt, and iCCP chunks.

You can also set a limit on the amount of memory that a compressed chunk
other than IDAT can occupy, with

   png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

and you can retrieve the limit with

   chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

Any chunks that would cause either of these limits to be exceeded will
be ignored.

Information about your system

If you intend to display the PNG or to incorporate it in other image data you
need to tell libpng information about your display or drawing surface so that
libpng can convert the values in the image to match the display.

From libpng-1.5.4 this information can be set before reading the PNG file
header.  In earlier versions png_set_gamma() existed but behaved incorrectly if
called before the PNG file header had been read and png_set_alpha_mode() did not
exist.

If you need to support versions prior to libpng-1.5.4 test the version number
as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
described in the appropriate manual page.

You give libpng the encoding expected by your system expressed as a 'gamma'
value.  You can also specify a default encoding for the PNG file in
case the required information is missing from the file.  By default libpng
assumes that the PNG data matches your system, to keep this default call:

   png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/);

or you can use the fixed point equivalent:

   png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, PNG_FP_1/screen_gamma);

If you don't know the gamma for your system it is probably 2.2 - a good
approximation to the IEC standard for display systems (sRGB).  If images are
too contrasty or washed out you got the value wrong - check your system
documentation!

Many systems permit the system gamma to be changed via a lookup table in the
display driver, a few systems, including older Macs, change the response by
default.  As of 1.5.4 three special values are available to handle common
situations:

   PNG_DEFAULT_sRGB: Indicates that the system conforms to the IEC 61966-2-1
                     standard.  This matches almost all systems.
   PNG_GAMMA_MAC_18: Indicates that the system is an older (pre Mac OS 10.6)
                     Apple Macintosh system with the default settings.
   PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates that the
                     system expects data with no gamma encoding.

You would use the linear (unencoded) value if you need to process the pixel
values further because this avoids the need to decode and reencode each
component value whenever arithmetic is performed.  A lot of graphics software
uses linear values for this reason, often with higher precision component values
to preserve overall accuracy.

The second thing you may need to tell libpng about is how your system handles
alpha channel information.  Some, but not all, PNG files contain an alpha
channel.  To display these files correctly you need to compose the data onto a
suitable background, as described in the PNG specification.

Libpng only supports composing onto a single color (using png_set_background;
see below).  Otherwise you must do the composition yourself and, in this case,
you may need to call png_set_alpha_mode:

#if PNG_LIBPNG_VER >= 10504
   png_set_alpha_mode(png_ptr, mode, screen_gamma);
#else
   png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
#endif

The screen_gamma value is the same as the argument to png_set_gamma; however,
how it affects the output depends on the mode.  png_set_alpha_mode() sets the
file gamma default to 1/screen_gamma, so normally you don't need to call
png_set_gamma.  If you need different defaults call png_set_gamma() before
png_set_alpha_mode() - if you call it after it will override the settings made
by png_set_alpha_mode().

The mode is as follows:

    PNG_ALPHA_PNG: The data is encoded according to the PNG specification.  Red,
green and blue, or gray, components are gamma encoded color
values and are not premultiplied by the alpha value.  The
alpha value is a linear measure of the contribution of the
pixel to the corresponding final output pixel.

You should normally use this format if you intend to perform
color correction on the color values; most, maybe all, color
correction software has no handling for the alpha channel and,
anyway, the math to handle pre-multiplied component values is
unnecessarily complex.

Before you do any arithmetic on the component values you need
to remove the gamma encoding and multiply out the alpha
channel.  See the PNG specification for more detail.  It is
important to note that when an image with an alpha channel is
scaled, linear encoded, pre-multiplied component values must
be used!

The remaining modes assume you don't need to do any further color correction or
that if you do, your color correction software knows all about alpha (it
probably doesn't!)

    PNG_ALPHA_STANDARD:  The data libpng produces
is encoded in the standard way
assumed by most correctly written graphics software.
The gamma encoding will be removed by libpng and the
linear component values will be pre-multiplied by the
alpha channel.

With this format the final image must be re-encoded to
match the display gamma before the image is displayed.
If your system doesn't do that, yet still seems to
perform arithmetic on the pixels without decoding them,
it is broken - check out the modes below.

With PNG_ALPHA_STANDARD libpng always produces linear
component values, whatever screen_gamma you supply.  The
screen_gamma value is, however, used as a default for
the file gamma if the PNG file has no gamma information.

If you call png_set_gamma() after png_set_alpha_mode() you
will override the linear encoding.  Instead the
pre-multiplied pixel values will be gamma encoded but
the alpha channel will still be linear.  This may
actually match the requirements of some broken software,
but it is unlikely.

While linear 8-bit data is often used it has
insufficient precision for any image with a reasonable
dynamic range.  To avoid problems, and if your software
supports it, use png_set_expand_16() to force all
components to 16 bits.

    PNG_ALPHA_OPTIMIZED: This mode is the same
as PNG_ALPHA_STANDARD except that
completely opaque pixels are gamma encoded according to
the screen_gamma value.  Pixels with alpha less than 1.0
will still have linear components.

Use this format if you have control over your
compositing software and so don't do other arithmetic
(such as scaling) on the data you get from libpng.  Your
compositing software can simply copy opaque pixels to
the output but still has linear values for the
non-opaque pixels.

In normal compositing, where the alpha channel encodes
partial pixel coverage (as opposed to broad area
translucency), the inaccuracies of the 8-bit
representation of non-opaque pixels are irrelevant.

You can also try this format if your software is broken;
it might look better.

    PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD;
however, all component values,
including the alpha channel are gamma encoded.  This is
an appropriate format to try if your software, or more
likely hardware, is totally broken, i.e., if it performs
linear arithmetic directly on gamma encoded values.

In most cases of broken software or hardware the bug in the final display
manifests as a subtle halo around composited parts of the image.  You may not
even perceive this as a halo; the composited part of the image may simply appear
separate from the background, as though it had been cut out of paper and pasted
on afterward.

If you don't have to deal with bugs in software or hardware, or if you can fix
them, there are three recommended ways of using png_set_alpha_mode():

   png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
       screen_gamma);

You can do color correction on the result (libpng does not currently
support color correction internally).  When you handle the alpha channel
you need to undo the gamma encoding and multiply out the alpha.

   png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
       screen_gamma);
   png_set_expand_16(png_ptr);

If you are using the high level interface, don't call png_set_expand_16();
instead pass PNG_TRANSFORM_EXPAND_16 to the interface.

With this mode you can't do color correction, but you can do arithmetic,
including composition and scaling, on the data without further processing.

   png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
       screen_gamma);

You can avoid the expansion to 16-bit components with this mode, but you
lose the ability to scale the image or perform other linear arithmetic.
All you can do is compose the result onto a matching output.  Since this
mode is libpng-specific you also need to write your own composition
software.

If you don't need, or can't handle, the alpha channel you can call
png_set_background() to remove it by compositing against a fixed color.  Don't
call png_set_strip_alpha() to do this - it will leave spurious pixel values in
transparent parts of this image.

   png_set_background(png_ptr, &background_color,
       PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

The background_color is an RGB or grayscale value according to the data format
libpng will produce for you.  Because you don't yet know the format of the PNG
file, if you call png_set_background at this point you must arrange for the
format produced by libpng to always have 8-bit or 16-bit components and then
store the color as an 8-bit or 16-bit color as appropriate.  The color contains
separate gray and RGB component values, so you can let libpng produce gray or
RGB output according to the input format, but low bit depth grayscale images
must always be converted to at least 8-bit format.  (Even though low bit depth
grayscale images can't have an alpha channel they can have a transparent
color!)

You set the transforms you need later, either as flags to the high level
interface or libpng API calls for the low level interface.  For reference the
settings and API calls required are:

8-bit values:
   PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
   png_set_expand(png_ptr); png_set_scale_16(png_ptr);

   If you must get exactly the same inaccurate results
   produced by default in versions prior to libpng-1.5.4,
   use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
   instead.

16-bit values:
   PNG_TRANSFORM_EXPAND_16
   png_set_expand_16(png_ptr);

In either case palette image data will be expanded to RGB.  If you just want
color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
to the list.

Calling png_set_background before the PNG file header is read will not work
prior to libpng-1.5.4.  Because the failure may result in unexpected warnings or
errors it is therefore much safer to call png_set_background after the head has
been read.  Unfortunately this means that prior to libpng-1.5.4 it cannot be
used with the high level interface.

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_SCALE_16      Strip 16-bit samples to
                                8-bit accurately
    PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
                                8-bit less accurately
    PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
    PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                samples to bytes
    PNG_TRANSFORM_PACKSWAP      Change order of packed
                                pixels to LSB first
    PNG_TRANSFORM_EXPAND        Perform set_expand()
    PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
    PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                sBIT depth
    PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                to BGRA
    PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                to AG
    PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                to transparency
    PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
    PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                to RGB (or GA to RGBA)
    PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits

(This excludes setting a background color, doing gamma transformation,
quantizing, and setting filler.)  If this is the case, simply do this:

    png_read_png(png_ptr, info_ptr, png_transforms, NULL)

where png_transforms is an integer containing the bitwise OR of some
set of transformation flags.  This call is equivalent to png_read_info(),
followed the set of transformations indicated by the transform mask,
then png_read_image(), and finally png_read_end().

(The final parameter of this call is not yet used.  Someday it might point
to transformation parameters required by some future input transform.)

You must use png_transforms and not call any png_set_transform() functions
when you use png_read_png().

After you have called png_read_png(), you can retrieve the image data
with

   row_pointers = png_get_rows(png_ptr, info_ptr);

where row_pointers is an array of pointers to the pixel data for each row:

   png_bytep row_pointers[height];

If you know your image size and pixel size ahead of time, you can allocate
row_pointers prior to calling png_read_png() with

   if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
      png_error (png_ptr,
          "Image is too tall to process in memory");

   if (width > PNG_UINT_32_MAX/pixel_size)
      png_error (png_ptr,
          "Image is too wide to process in memory");

   row_pointers = png_malloc(png_ptr,
       height*png_sizeof(png_bytep));

   for (int i=0; i<height, i++)
      row_pointers[i]=NULL;  /* security precaution */

   for (int i=0; i<height, i++)
      row_pointers[i]=png_malloc(png_ptr,
          width*pixel_size);

   png_set_rows(png_ptr, info_ptr, &row_pointers);

Alternatively you could allocate your image in one big block and define
row_pointers[i] to point into the proper places in your block.

If you use png_set_rows(), the application is responsible for freeing
row_pointers (and row_pointers[i], if they were separately allocated).

If you don't allocate row_pointers ahead of time, png_read_png() will
do it, and it'll be free'ed by libpng 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.

This also copies some of the data from the PNG file into the decode structure
for use in later transformations.  Important information copied in is:

1) The PNG file gamma from the gAMA chunk.  This overwrites the default value
provided by an earlier call to png_set_gamma or png_set_alpha_mode.

2) Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
damages the information provided by an earlier call to png_set_background
resulting in unexpected behavior.  Libpng-1.5.4 no longer does this.

3) The number of significant bits in each component value.  Libpng uses this to
optimize gamma handling by reducing the internal lookup table sizes.

4) The transparent color information from a tRNS chunk.  This can be modified by
a later call to png_set_tRNS.

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

    interlace_type - (PNG_INTERLACE_NONE or
                     PNG_INTERLACE_ADAM7)

    compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                     for PNG 1.0)

    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)

    Any or all of interlace_type, compression_type, or
    filter_method can be NULL if you are
    not interested in their values.

    Note that png_get_IHDR() returns 32-bit data into
    the application's width and height variables.
    This is an unsafe situation if these are 16-bit
    variables.  In such situations, the
    png_get_image_width() and png_get_image_height()
    functions described below are safer.

    width            = png_get_image_width(png_ptr,
                         info_ptr);

    height           = png_get_image_height(png_ptr,
                         info_ptr);

    bit_depth        = png_get_bit_depth(png_ptr,
                         info_ptr);

    color_type       = png_get_color_type(png_ptr,
                         info_ptr);

    interlace_type   = png_get_interlace_type(png_ptr,
                         info_ptr);

    compression_type = png_get_compression_type(png_ptr,
                         info_ptr);

    filter_method    = png_get_filter_type(png_ptr,
                         info_ptr);

    channels = png_get_channels(png_ptr, info_ptr);

    channels       - number of channels of info for the
                     color type (valid values are 1 (GRAY,
                     PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                     4 (RGB_ALPHA or RGB + filler byte))

    rowbytes = png_get_rowbytes(png_ptr, info_ptr);

    rowbytes       - number of bytes needed to hold a row

    signature = png_get_signature(png_ptr, info_ptr);

    signature      - holds the signature read from the
                     file (if any).  The data is kept in
                     the same offset it would be if the
                     whole signature were read (i.e. if an
                     application had already read in 4
                     bytes of signature before starting
                     libpng, the remaining 4 bytes would
                     be in signature[4] through signature[7]
                     (see png_set_sig_bytes())).

These are also important, but their validity depends on whether the chunk
has been read.  The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
data has been read, or zero if it is missing.  The parameters to the
png_get_<chunk> are set directly if they are simple data types, or a
pointer into the info_ptr is returned for any complex types.

The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks
is simply returned to give the application information about how the
image was encoded.  Libpng itself only does transformations using the file
gamma when combining semitransparent pixels with the background color.

    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, &file_gamma);
    png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);

    file_gamma     - the gamma at which the file was
                     written (PNG_INFO_gAMA)

    int_file_gamma - 100,000 times the gamma at which the
                     file is written

    png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x, &red_y,
                     &green_x, &green_y, &blue_x, &blue_y)
    png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, &green_X,
                     &green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z)
    png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y,
                     &int_red_x, &int_red_y, &int_green_x, &int_green_y,
                     &int_blue_x, &int_blue_y)
    png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
                     &int_red_Z, &int_green_X, &int_green_Y, &int_green_Z,
                     &int_blue_X, &int_blue_Y, &int_blue_Z)

    {white,red,green,blue}_{x,y}
                     A color space encoding specified using the chromaticities
                     of the end points and the white point. (PNG_INFO_cHRM)

    {red,green,blue}_{X,Y,Z}
                     A color space encoding specified using the encoding end
                     points - the CIE tristimulus specification of the intended
                     color of the red, green and blue channels in the PNG RGB
                     data.  The white point is simply the sum of the three end
                     points. (PNG_INFO_cHRM)

    png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

    file_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_type - The compression type; always
                       PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                       You may give NULL to this argument to
                       ignore it.

    profile          - International Color Consortium color
                       profile data. May contain NULs.

    proflen          - length of profile data in bytes.

    png_get_sBIT(png_ptr, info_ptr, &sig_bit);

    sig_bit        - the number of significant bits for
                     (PNG_INFO_sBIT) each of the gray,
                     red, green, and blue channels,
                     whichever are appropriate for the
                     given color type (png_color_16)

    png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                     &num_trans, &trans_color);

    trans_alpha    - array of alpha (transparency)
                     entries for palette (PNG_INFO_tRNS)

    num_trans      - number of transparent entries
                     (PNG_INFO_tRNS)

    trans_color    - graylevel or color sample values of
                     the single transparent color for
                     non-paletted images (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 (of type
                     png_color_16p) (PNG_VALID_bKGD)
                     valid 16-bit red, green and blue
                     values, regardless of color_type

    num_comments   = png_get_text(png_ptr, info_ptr,
                     &text_ptr, &num_text);

    num_comments   - number of comments

    text_ptr       - array of png_text holding image
                     comments

    text_ptr[i].compression - type of compression used
                 on "text" PNG_TEXT_COMPRESSION_NONE
                           PNG_TEXT_COMPRESSION_zTXt
                           PNG_ITXT_COMPRESSION_NONE
                           PNG_ITXT_COMPRESSION_zTXt

    text_ptr[i].key   - keyword for comment.  Must contain
                         1-79 characters.

    text_ptr[i].text  - text comments for current
                         keyword.  Can be empty.

    text_ptr[i].text_length - length of text string,
                 after decompression, 0 for iTXt

    text_ptr[i].itxt_length - length of itxt string,
                 after decompression, 0 for tEXt/zTXt

    text_ptr[i].lang  - language of comment (empty
                         string for unknown).

    text_ptr[i].lang_key  - keyword in UTF-8
                         (empty string for unknown).

    Note that the itxt_length, lang, and lang_key
    members of the text_ptr structure only exist when the
    library is built with iTXt chunk support.  Prior to
    libpng-1.4.0 the library was built by default without
    iTXt support. Also note that when iTXt is supported,
    they contain NULL pointers when the "compression"
    field contains PNG_TEXT_COMPRESSION_NONE or
    PNG_TEXT_COMPRESSION_zTXt.

    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);

    num_spalettes  - number of sPLT chunks read.

    palette_ptr    - array of palette structures holding
                     contents of one or more 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 (can be negative)

    offset_y       - positive offset from the top edge
                     of the screen (can be negative)

    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
                  (expressed as a string)

    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 value of "location" is a bitwise "or" of

         PNG_HAVE_IHDR  (0x01)
         PNG_HAVE_PLTE  (0x02)
         PNG_AFTER_IDAT (0x08)

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

    Note that because of the way the resolutions are
       stored internally, the inch conversions won't
       come out to exactly even number.  For example,
       72 dpi is stored as 0.28346 pixels/meter, and
       when this is retrieved it is 71.9988 dpi, so
       be sure to round the returned value appropriately
       if you want to display a reasonable-looking result.

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.  The
       remark about inexact inch conversions applies here
       as well, because a value in inches can't always be
       converted to microns and back without some loss
       of precision.

For more information, see 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.

Transformations you request are ignored if they don't have any meaning for a
particular input data format.  However some transformations can have an effect
as a result of a previous transformation.  If you specify a contradictory set of
transformations, for example both adding and removing the alpha channel, you
cannot predict the final result.

The color used for the transparency values should be supplied in the same
format/depth as the current image data.  It is stored in the same format/depth
as the image data in a tRNS chunk, so this is what libpng expects for this data.

The color used for the background value depends on the need_expand argument as
described 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_scale_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(), png_set_strip_16(),
or png_set_scale_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 (png_get_valid(png_ptr, info_ptr,
        PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

    if (color_type == PNG_COLOR_TYPE_GRAY &&
        bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

The first two functions are actually aliases for png_set_expand(), added
in libpng version 1.0.4, with the function names expanded to improve code
readability.  In some future version they may actually do different
things.

As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
added.  It expands the sample depth without changing tRNS to alpha.

As of libpng version 1.5.2, png_set_expand_16() was added.  It behaves as
png_set_expand(); however, the resultant channels have 16 bits rather than 8.
Use this when the output color or gray channels are made linear to avoid fairly
severe accuracy loss.

   if (bit_depth < 16)
      png_set_expand_16(png_ptr);

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)
#if PNG_LIBPNG_VER >= 10504
       png_set_scale_16(png_ptr);
#else
       png_set_strip_16(png_ptr);
#endif

(The more accurate "png_set_scale_16()" API became available in libpng version
1.5.4).

If you need to process the alpha channel on the image separately from the image
data (for example if you convert it to a bitmap mask) it is possible to have
libpng strip the channel leaving just RGB or gray data:

    if (color_type & PNG_COLOR_MASK_ALPHA)
       png_set_strip_alpha(png_ptr);

If you strip the alpha channel you need to find some other way of dealing with
the information.  If, instead, you want to convert the image to an opaque
version with no alpha channel use png_set_background; see below.

As of libpng version 1.5.2, almost all useful expansions are supported, the
major ommissions are conversion of grayscale to indexed images (which can be
done trivially in the application) and conversion of indexed to grayscale (which
can be done by a trivial manipulation of the palette.)

In the following table, the 01 means grayscale with depth<8, 31 means
indexed with depth<8, other numerals represent the color type, "T" means
the tRNS chunk is present, A means an alpha channel is present, and O
means tRNS or alpha is present but all pixels in the image are opaque.

  FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
   TO
   01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
   31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
    0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
   0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
   0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
    2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
   2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
   2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
    3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
   3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
   3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
   4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
   4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
   6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
   6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +

Within the matrix,
     "+" identifies entries where 'from' and 'to' are the same.
     "-" means the transformation is not supported.
     "." means nothing is necessary (a tRNS chunk can just be ignored).
     "t" means the transformation is obtained by png_set_tRNS.
     "A" means the transformation is obtained by png_set_add_alpha().
     "X" means the transformation is obtained by png_set_expand().
     "1" means the transformation is obtained by
         png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() if there
         is no transparency in the original or the final format).
     "C" means the transformation is obtained by png_set_gray_to_rgb().
     "G" means the transformation is obtained by png_set_rgb_to_gray().
     "P" means the transformation is obtained by
         png_set_expand_palette_to_rgb().
     "p" means the transformation is obtained by png_set_packing().
     "Q" means the transformation is obtained by png_set_quantize().
     "T" means the transformation is obtained by png_set_tRNS_to_alpha().
     "B" means the transformation is obtained by png_set_background(), or
         png_strip_alpha().

When an entry has multiple transforms listed all are required to cause the
right overall transformation.  When two transforms are separated by a comma
either will do the job.  When transforms are enclosed in [] the transform should
do the job but this is currently unimplemented - a different format will result
if the suggested transformations are used.

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(png_ptr, error_action, double red_weight,
          double 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

    green_weight:     weight of green component
                      If either weight is negative, default
                      weights are used.

In the corresponding fixed point API the red_weight and green_weight values are
simply scaled by 100,000:

    png_set_rgb_to_gray(png_ptr, error_action, png_fixed_point red_weight,
       png_fixed_point green_weight);

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.  Background and sBIT data
will be silently converted to grayscale, using the green channel
data for sBIT, regardless of the error_action setting.

The default values come from the PNG file cHRM chunk if present; otherwise, the
defaults correspond to the ITU-R recommendation 709, and also the sRGB color
space, as recommended in the Charles Poynton's Colour FAQ,
<http://www.poynton.com/>, in section 9:

   <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

    Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

Previous versions of this document, 1998 through 2002, recommended a slightly
different formula:

    Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

Libpng uses an integer approximation:

    Y = (6968 * R + 23434 * G + 2366 * B)/32768

The calculation is done in a linear colorspace, if the image gamma
can be determined.

The png_set_background() function has been described already; it tells libpng to
composite images with alpha or simple transparency against the supplied
background color.  For compatibility with versions of libpng earlier than
libpng-1.5.4 it is recommended that you call the function after reading the file
header, even if you don't want to use the color in a bKGD chunk, if one exists.

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 how the color is represented, both the format of the
component values in the color (the number of bits) and the gamma encoding of the
color.  The function takes two arguments, background_gamma_mode and need_expand
to convey this information; however, only two combinations are likely to be
useful:

    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/*needs to be expanded*/, 1);
    else
       png_set_background(png_ptr, &my_background,
           PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

The second call was described above - my_background is in the format of the
final, display, output produced by libpng.  Because you now know the format of
the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
output and to retain palette images (the palette colors will be modified
appropriately and the tRNS chunk removed.)  However, if you are doing this,
take great care not to ask for transformations without checking first that
they apply!

In the first call the background color has the original bit depth and color type
of the PNG file.  So, for palette images the color is supplied as a palette
index and for low bit greyscale images the color is a reduced bit value in
image_background->gray.

If you didn't call png_set_gamma() before reading the file header, for example
if you need your code to remain compatible with older versions of libpng prior
to libpng-1.5.4, this is the place to call it.

Do not call it if you called png_set_alpha_mode(); doing so will damage the
settings put in place by png_set_alpha_mode().  (If png_set_alpha_mode() is
supported then you can certainly do png_set_gamma() before reading the PNG
header.)

This API unconditionally sets the screen and file gamma values, so it will
override the value in the PNG file unless it is called before the PNG file
reading starts.  For this reason you must always call it with the PNG file
value when you call it in this position:

   if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
      png_set_gamma(png_ptr, screen_gamma, file_gamma);

   else
      png_set_gamma(png_ptr, screen_gamma, 0.45455);

If you need to reduce an RGB file to a paletted file, or if a paletted
file has more entries then will fit on your screen, png_set_quantize()
will do that.  Note that this is a simple match quantization that merely
finds the closest color available.  This should work fairly well with
optimized palettes, but fairly badly with linear color cubes.  If you
pass a palette that is larger than 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, libpng will use it to make
more intelligent choices when reducing the palette.  If there is no
histogram, it may not do as good a job.

   if (color_type & PNG_COLOR_MASK_COLOR)
   {
      if (png_get_valid(png_ptr, info_ptr,
          PNG_INFO_PLTE))
      {
         png_uint_16p histogram = NULL;

         png_get_hIST(png_ptr, info_ptr,
             &histogram);
         png_set_quantize(png_ptr, palette, num_palette,
            max_screen_colors, histogram, 1);
      }

      else
      {
         png_color std_color_cube[MAX_SCREEN_COLORS] =
            { ... colors ... };

         png_set_quantize(png_ptr, std_color_cube,
            MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
            NULL,0);
      }
   }

PNG files describe monochrome as black being zero and white being one.
The following code will reverse this (make black be one and white be
zero):

   if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
      png_set_invert_mono(png_ptr);

This function can also be used to invert grayscale and gray-alpha images:

   if (color_type == PNG_COLOR_TYPE_GRAY ||
       color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
      png_set_invert_mono(png_ptr);

PNG files store 16-bit pixels in network byte order (big-endian,
ie. most significant bits first).  This code changes the storage to the
other way (little-endian, i.e. least significant bits first, the
way PCs store them):

    if (bit_depth == 16)
       png_set_swap(png_ptr);

If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
need to change the order the pixels are packed into bytes, you can use:

    if (bit_depth < 8)
       png_set_packswap(png_ptr);

Finally, you can write your own transformation function if none of
the existing ones meets your needs.  This is done by setting a callback
with

    png_set_read_user_transform_fn(png_ptr,
        read_transform_fn);

You must supply the function

    void read_transform_fn(png_structp png_ptr, png_row_infop
        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.  Take care with
interlaced images if you do the interlace yourself - the width of the row is the
width in 'row_info', not the overall image width.

If supported, libpng provides two information routines that you can use to find
where you are in processing the image:

   png_get_current_pass_number(png_structp png_ptr);
   png_get_current_row_number(png_structp png_ptr);

Don't try using these outside a transform callback - firstly they are only
supported if user transforms are supported, secondly they may well return
unexpected results unless the row is actually being processed at the moment they
are called.

With interlaced
images the value returned is the row in the input sub-image image.  Use
PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).

The discussion of interlace handling above contains more information on how to
use these values.

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.

    png_read_update_info(png_ptr, info_ptr);

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.  You may
only call png_read_update_info() once with a particular 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.

Remember: Before you call png_read_update_info(), the png_get_*()
functions return the values corresponding to the original PNG image.
After you call png_read_update_info the values refer to the image
that libpng will output.  Consequently you must call all the png_set_
functions before you call png_read_update_info().  This is particularly
important for png_set_interlace_handling() - if you are going to call
png_read_update_info() you must call png_set_interlace_handling() before
it unless you want to receive interlaced output.

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() (unless you call
png_read_update_info()) 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);
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.  This number is defined (from libpng 1.5) as
PNG_INTERLACE_ADAM7_PASSES in png.h

libpng can fill out those images or it can give them to you "as is".
It is almost always better to have libpng handle the interlacing for you.
If you want the images 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, as is likely, 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.
You then need to read the whole image 'number_of_passes' times.  Each time
will distribute the pixels from the current pass to the correct place in
the output image, so you need to supply the same rows to png_read_rows in
each 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);

If you don't want libpng to handle the interlacing details, just call
png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
Each of the images is a valid image by itself; however, you will almost
certainly need to distribute the pixels from each sub-image to the
correct place.  This is where everything gets very tricky.

If you want to retrieve the separate images you must pass the correct
number of rows to each successive call of png_read_rows().  The calculation
gets pretty complicated for small images, where some sub-images may
not even exist because either their width or height ends up zero.
libpng provides two macros to help you in 1.5 and later versions:

   png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
   png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

Respectively these tell you the width and height of the sub-image
corresponding to the numbered pass.  'pass' is in in the range 0 to 6 -
this can be confusing because the specification refers to the same passes
as 1 to 7!  Be careful, you must check both the width and height before
calling png_read_rows() and not call it for that pass if either is zero.

You can, of course, read each sub-image row by row.  If you want to
produce optimal code to make a pixel-by-pixel transformation of an
interlaced image this is the best approach; read each row of each pass,
transform it, and write it out to a new interlaced image.

If you want to de-interlace the image yourself libpng provides further
macros to help that tell you where to place the pixels in the output image.
Because the interlacing scheme is rectangular - sub-image pixels are always
arranged on a rectangular grid - all you need to know for each pass is the
starting column and row in the output image of the first pixel plus the
spacing between each pixel.  As of libpng 1.5 there are four macros to
retrieve this information:

   png_uint_32 x = PNG_PASS_START_COL(pass);
   png_uint_32 y = PNG_PASS_START_ROW(pass);
   png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
   png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

These allow you to write the obvious loop:

   png_uint_32 input_y = 0;
   png_uint_32 output_y = PNG_PASS_START_ROW(pass);

   while (output_y < output_image_height)
   {
      png_uint_32 input_x = 0;
      png_uint_32 output_x = PNG_PASS_START_COL(pass);

      while (output_x < output_image_width)
      {
         image[output_y][output_x] =
             subimage[pass][input_y][input_x++];

         output_x += xStep;
      }

      ++input_y;
      output_y += yStep;
   }

Notice that the steps between successive output rows and columns are
returned as shifts.  This is possible because the pixels in the subimages
are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
image.  In practice you may need to directly calculate the output coordinate
given an input coordinate.  libpng provides two further macros for this
purpose:

   png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
   png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

Finally a pair of macros are provided to tell you if a particular image
row or column appears in a given pass:

   int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
   int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

Bear in mind that you will probably also need to check the width and height
of the pass in addition to the above to be sure the pass even exists!

With any luck you are convinced by now that you don't want to do your own
interlace handling.  In reality normally the only good reason for doing this
is if you are processing PNG files on a pixel-by-pixel basis and don't want
to load the whole file into memory when it is interlaced.

libpng includes a test program, pngvalid, that illustrates reading and
writing of interlaced images.  If you can't get interlacing to work in your
code and don't want to leave it to libpng (the recommended approach), see
how pngvalid.c does it.

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.

    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);
    }

   png_read_end(png_ptr, end_info);

If you are not interested, you should still call png_read_end()
but you can pass NULL, avoiding the need to create an end_info structure.

   png_read_end(png_ptr, (png_infop)NULL);

If you don't call png_read_end(), then your file pointer will be
left pointing to the first chunk after the last IDAT, which is probably
not what you want if you expect to read something beyond the end of
the PNG datastream.

When you are done, you can free all memory allocated by libpng like this:

   png_destroy_read_struct(&png_ptr, &info_ptr,
       &end_info);

or, if you didn't create an end_info structure,

   png_destroy_read_struct(&png_ptr, &info_ptr,
       (png_infopp)NULL);

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)

    freer  - one of
               PNG_DESTROY_WILL_FREE_DATA
               PNG_SET_WILL_FREE_DATA
               PNG_USER_WILL_FREE_DATA

    mask   - which data elements are affected
             same choices as in png_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);

    /* At this point you can call png_process_data_skip if
       you want to handle data the library will skip yourself;
       it simply returns the number of bytes to skip (and stops
       libpng skipping that number of bytes on the next
       png_process_data call).
    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 is where you turn on interlace handling,
       assuming you don't want to do it yourself.

       If you need to you can stop the processing of
       your original input data at this point by calling
       png_process_data_pause.  This returns the number
       of unprocessed bytes from the last png_process_data
       call - it is up to you to ensure that the next call
       sees these bytes again.  If you don't want to bother
       with this you can get libpng to cache the unread
       bytes by setting the 'save' parameter (see png.h) but
       then libpng will have to copy the data internally.
     */
 }

 /* 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.

       If you did not turn on interlace handling then
       the callback is called for each row of each
       sub-image when the image is interlaced.  In this
       case 'row_num' is the row in the sub-image, not
       the row in the output image as it is in all other
       cases.

       For the non-NULL rows of interlaced images when
       you have switched on libpng interlace handling,
       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 if you switch on interlace handling;
     */

        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.

       You can also call png_process_data_pause in this
       callback - see above.
    */
 }

 void
 end_callback(png_structp png_ptr, png_infop info)
 {
    /* This function is called after the whole image
       has been read, including any chunks after the
       image (up to and including the IEND).  You
       will usually have the same info chunk as you
       had in the header, although some data may have
       been added to the comments and time fields.

       Most people won't do much here, perhaps setting
       a flag that marks the image as finished.
     */
 }



IV. Writing

Much of this is very similar to reading.  However, everything of
importance is repeated here, so you won't have to constantly look
back up in the reading section to understand writing.

Setup

You will want to do the I/O initialization before you get into libpng,
so if it doesn't work, you don't have anything to undo. If you are not
using the standard I/O functions, you will need to replace them with
custom writing functions.  See the discussion under Customizing libpng.

    FILE *fp = fopen(file_name, "wb");

    if (!fp)
       return (ERROR);

Next, png_struct and png_info need to be allocated and initialized.
As these can be both relatively large, you may not want to store these
on the stack, unless you have stack space to spare.  Of course, you
will want to check if they return NULL.  If you are also reading,
you won't want to name your read structure and your write structure
both "png_ptr"; you can call them anything you like, such as
"read_ptr" and "write_ptr".  Look at pngtest.c, for example.

    png_structp png_ptr = png_create_write_struct
       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
        user_error_fn, user_warning_fn);

    if (!png_ptr)
       return (ERROR);

    png_infop info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr)
    {
       png_destroy_write_struct(&png_ptr,
           (png_infopp)NULL);
       return (ERROR);
    }

If you want to use your own memory allocation routines,
define PNG_USER_MEM_SUPPORTED and use
png_create_write_struct_2() instead of png_create_write_struct():

    png_structp png_ptr = png_create_write_struct_2
       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
        user_error_fn, user_warning_fn, (png_voidp)
        user_mem_ptr, user_malloc_fn, user_free_fn);

After you have these structures, you will need to set up the
error handling.  When libpng encounters an error, it expects to
longjmp() back to your routine.  Therefore, you will need to call
setjmp() and pass the png_jmpbuf(png_ptr).  If you
write the file from different routines, you will need to update
the png_jmpbuf(png_ptr) every time you enter a new routine that will
call a png_*() function.  See your documentation of setjmp/longjmp
for your compiler for more information on setjmp/longjmp.  See
the discussion on libpng error handling in the Customizing Libpng
section below for more information on the libpng error handling.

    if (setjmp(png_jmpbuf(png_ptr)))
    {
    png_destroy_write_struct(&png_ptr, &info_ptr);
       fclose(fp);
       return (ERROR);
    }
    ...
    return;

If you would rather avoid the complexity of setjmp/longjmp issues,
you can compile libpng with PNG_NO_SETJMP, in which case
errors will result in a call to PNG_ABORT() which defaults to abort().

You can #define PNG_ABORT() to a function that does something
more useful than abort(), as long as your function does not
return.

Now you need to set up the output code.  The default for libpng is to
use the C function fwrite().  If you use this, you will need to pass a
valid FILE * in the function png_init_io().  Be sure that the file is
opened in binary mode.  Again, if you wish to handle writing data in
another way, see the discussion on libpng I/O handling in the Customizing
Libpng section below.

    png_init_io(png_ptr, fp);

If you are embedding your PNG into a datastream such as MNG, and don't
want libpng to write the 8-byte signature, or if you have already
written the signature in your application, use

    png_set_sig_bytes(png_ptr, 8);

to inform libpng that it should not write a signature.

Write callbacks

At this point, you can set up a callback function that will be
called after each row has been written, which you can use to control
a progress meter or the like.  It's demonstrated in pngtest.c.
You must supply a function

    void write_row_callback(png_structp 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);

When this function is called the row has already been completely processed and
it has also been written out.  The 'row' and 'pass' refer to the next row to be
handled.  For the
non-interlaced case the row that was just handled is simply one less than the
passed in row number, and pass will always be 0.  For the interlaced case the
same applies unless the row value is 0, in which case the row just handled was
the last one from one of the preceding passes.  Because interlacing may skip a
pass you cannot be sure that the preceding pass is just 'pass-1', if you really
need to know what the last pass is record (row,pass) from the callback and use
the last recorded value each time.

As with the user transform you can find the output row using the
PNG_ROW_FROM_PASS_ROW macro.

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.

    #include zlib.h

    /* Set the zlib compression level */
    png_set_compression_level(png_ptr,
        Z_BEST_COMPRESSION);

    /* Set other zlib parameters for compressing IDAT */
    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)

    /* Set zlib parameters for text compression
     * If you don't call these, the parameters
     * fall back on those defined for IDAT chunks
     */
    png_set_text_compression_mem_level(png_ptr, 8);
    png_set_text_compression_strategy(png_ptr,
        Z_DEFAULT_STRATEGY);
    png_set_text_compression_window_bits(png_ptr, 15);
    png_set_text_compression_method(png_ptr, 8);

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.