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

 libpng version 1.2.40 - September 10, 2009

 Updated and distributed by Glenn Randers-Pehrson

 <glennrp at users.sourceforge.net>

 Copyright (c) 1998-2009 Glenn Randers-Pehrson

 This document is released under the libpng license.

 For conditions of distribution and use, see the disclaimer

 and license in png.h

 Based on:

 libpng versions 0.97, January 1998, through 1.2.40 - September 10, 2009

 Updated and distributed by Glenn Randers-Pehrson

 Copyright (c) 1998-2009 Glenn Randers-Pehrson

 libpng 1.0 beta 6  version 0.96 May 28, 1997

 Updated and distributed by Andreas Dilger

 Copyright (c) 1996, 1997 Andreas Dilger

 libpng 1.0 beta 2 - version 0.88  January 26, 1996

 For conditions of distribution and use, see copyright

 notice in png.h. Copyright (c) 1995, 1996 Guy Eric

 Schalnat, Group 42, Inc.

 Updated/rewritten per request in the libpng FAQ

 Copyright (c) 1995, 1996 Frank J. T. Wojcik

 December 18, 1995 & January 20, 1996

I. Introduction

This file describes how to use and modify the PNG reference library

(known as libpng) for your own use.  There are five sections to this

file: introduction, structures, reading, writing, and modification and

configuration notes for various special platforms.  In addition to this

file, example.c is a good starting point for using the library, as

it is heavily commented and should include everything most people

will need.  We assume that libpng is already installed; see the

INSTALL file for instructions on how to install libpng.

For examples of libpng usage, see the files "example.c", "pngtest.c",

and the files in the "contrib" directory, all of which are included in the

libpng distribution.

Libpng was written as a companion to the PNG specification, as a way

of reducing the amount of time and effort it takes to support the PNG

file format in application programs.

The PNG specification (second edition), November 2003, is available as

a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at

<http://www.w3.org/TR/2003/REC-PNG-20031110/

The W3C and ISO documents have identical technical content.

The PNG-1.2 specification is available at

<http://www.libpng.org/pub/png/documents/>.  It is technically equivalent

to the PNG specification (second edition) but has some additional material.

The PNG-1.0 specification is available

as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a

W3C Recommendation <http://www.w3.org/TR/REC.png.html>.

Some additional chunks are described in the special-purpose public chunks

documents at <http://www.libpng.org/pub/png/documents/>.

Other information

about PNG, and the latest version of libpng, can be found at the PNG home

page, <http://www.libpng.org/pub/png/>.

Most users will not have to modify the library significantly; advanced

users may want to modify it more.  All attempts were made to make it as

complete as possible, while keeping the code easy to understand.

Currently, this library only supports C.  Support for other languages

is being considered.

Libpng has been designed to handle multiple sessions at one time,

to be easily modifiable, to be portable to the vast majority of

machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy

to use.  The ultimate goal of libpng is to promote the acceptance of

the PNG file format in whatever way possible.  While there is still

work to be done (see the TODO file), libpng should cover the

majority of the needs of its users.

Libpng uses zlib for its compression and decompression of PNG files.

Further information about zlib, and the latest version of zlib, can

be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.

The zlib compression utility is a general purpose utility that is

useful for more than PNG files, and can be used without libpng.

See the documentation delivered with zlib for more details.

You can usually find the source files for the zlib utility wherever you

find the libpng source files.

Libpng is thread safe, provided the threads are using different

instances of the structures.  Each thread should have its own

png_struct and png_info instances, and thus its own image.

Libpng does not protect itself against two threads using the

same instance of a structure.

II. Structures

There are two main structures that are important to libpng, png_struct

and png_info.  The first, png_struct, is an internal structure that

will not, for the most part, be used by a user except as the first

variable passed to every libpng function call.

The png_info structure is designed to provide information about the

PNG file.  At one time, the fields of png_info were intended to be

directly accessible to the user.  However, this tended to cause problems

with applications using dynamically loaded libraries, and as a result

a set of interface functions for png_info (the png_get_*() and png_set_*()

functions) was developed.  The fields of png_info are still available for

older applications, but it is suggested that applications use the new

interfaces if at all possible.

Applications that do make direct access to the members of png_struct (except

for png_ptr->jmpbuf) must be recompiled whenever the library is updated,

and applications that make direct access to the members of png_info must

be recompiled if they were compiled or loaded with libpng version 1.0.6,

in which the members were in a different order.  In version 1.0.7, the

members of the png_info structure reverted to the old order, as they were

in versions 0.97c through 1.0.5.  Starting with version 2.0.0, both

structures are going to be hidden, and the contents of the structures will

only be accessible through the png_get/png_set functions.

The png.h header file is an invaluable reference for programming with libpng.

And while I'm on the topic, make sure you include the libpng header file:

#include <png.h>

III. Reading

We'll now walk you through the possible functions to call when reading

in a PNG file sequentially, briefly explaining the syntax and purpose

of each one.  See example.c and png.h for more detail.  While

progressive reading is covered in the next section, you will still

need some of the functions discussed in this section to read a PNG

file.

Setup

You will want to do the I/O initialization(*) before you get into libpng,

so if it doesn't work, you don't have much to undo.  Of course, you

will also want to insure that you are, in fact, dealing with a PNG

file.  Libpng provides a simple check to see if a file is a PNG file.

To use it, pass in the first 1 to 8 bytes of the file to the function

png_sig_cmp(), and it will return 0 (false) if the bytes match the

corresponding bytes of the PNG signature, or nonzero (true) otherwise.

Of course, the more bytes you pass in, the greater the accuracy of the

prediction.

If you are intending to keep the file pointer open for use in libpng,

you must ensure you don't read more than 8 bytes from the beginning

of the file, and you also have to make a call to png_set_sig_bytes_read()

with the number of bytes you read from the beginning.  Libpng will

then only check the bytes (if any) that your program didn't read.

(*): If you are not using the standard I/O functions, you will need

to replace them with custom functions.  See the discussion under

Customizing libpng.

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

    if (!fp)

    {

        return (ERROR);

    }

    fread(header, 1, number, fp);

    is_png = !png_sig_cmp(header, 0, number);

    if (!is_png)

    {

        return (NOT_PNG);

    }

Next, png_struct and png_info need to be allocated and initialized.  In

order to ensure that the size of these structures is correct even with a

dynamically linked libpng, there are functions to initialize and

allocate the structures.  We also pass the library version, optional

pointers to error handling functions, and a pointer to a data struct for

use by the error functions, if necessary (the pointer and functions can

be NULL if the default error handlers are to be used).  See the section

on Changes to Libpng below regarding the old initialization functions.

The structure allocation functions quietly return NULL if they fail to

create the structure, so your application should check for that.

    png_structp png_ptr = png_create_read_struct

       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,

        user_error_fn, user_warning_fn);

    if (!png_ptr)

        return (ERROR);

    png_infop info_ptr = png_create_info_struct(png_ptr);

    if (!info_ptr)

    {

        png_destroy_read_struct(&png_ptr,

           (png_infopp)NULL, (png_infopp)NULL);

        return (ERROR);

    }

    png_infop end_info = png_create_info_struct(png_ptr);

    if (!end_info)

    {

        png_destroy_read_struct(&png_ptr, &info_ptr,

          (png_infopp)NULL);

        return (ERROR);

    }

If you want to use your own memory allocation routines,

define PNG_USER_MEM_SUPPORTED and use

png_create_read_struct_2() instead of png_create_read_struct():

    png_structp png_ptr = png_create_read_struct_2

       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,

        user_error_fn, user_warning_fn, (png_voidp)

        user_mem_ptr, user_malloc_fn, user_free_fn);

The error handling routines passed to png_create_read_struct()

and the memory alloc/free routines passed to png_create_struct_2()

are only necessary if you are not using the libpng supplied error

handling and memory alloc/free functions.

When libpng encounters an error, it expects to longjmp back

to your routine.  Therefore, you will need to call setjmp and pass

your png_jmpbuf(png_ptr).  If you read the file from different

routines, you will need to update the jmpbuf field every time you enter

a new routine that will call a png_*() function.

See your documentation of setjmp/longjmp for your compiler for more

information on setjmp/longjmp.  See the discussion on libpng error

handling in the Customizing Libpng section below for more information

on the libpng error handling.  If an error occurs, and libpng longjmp's

back to your setjmp, you will want to call png_destroy_read_struct() to

free any memory.

    if (setjmp(png_jmpbuf(png_ptr)))

    {

        png_destroy_read_struct(&png_ptr, &info_ptr,

           &end_info);

        fclose(fp);

        return (ERROR);

    }

If you would rather avoid the complexity of setjmp/longjmp issues,

you can compile libpng with PNG_SETJMP_NOT_SUPPORTED, in which case

errors will result in a call to PNG_ABORT() which defaults to abort().

Now you need to set up the input code.  The default for libpng is to

use the C function fread().  If you use this, you will need to pass a

valid FILE * in the function png_init_io().  Be sure that the file is

opened in binary mode.  If you wish to handle reading data in another

way, you need not call the png_init_io() function, but you must then

implement the libpng I/O methods discussed in the Customizing Libpng

section below.

    png_init_io(png_ptr, fp);

If you had previously opened the file and read any of the signature from

the beginning in order to see if this was a PNG file, you need to let

libpng know that there are some bytes missing from the start of the file.

    png_set_sig_bytes(png_ptr, number);

Setting up callback code

You can set up a callback function to handle any unknown chunks in the

input stream. You must supply the function

    read_chunk_callback(png_ptr ptr,

         png_unknown_chunkp chunk);

    {

       /* The unknown chunk structure contains your

          chunk data, along with similar data for any other

          unknown chunks: */

           png_byte name[5];

           png_byte *data;

           png_size_t size;

       /* Note that libpng has already taken care of

          the CRC handling */

       /* put your code here.  Search for your chunk in the

          unknown chunk structure, process it, and return one

          of the following: */

       return (-n); /* chunk had an error */

       return (0); /* did not recognize */

       return (n); /* success */

    }

(You can give your function another name that you like instead of

"read_chunk_callback")

To inform libpng about your function, use

    png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,

        read_chunk_callback);

This names not only the callback function, but also a user pointer that

you can retrieve with

    png_get_user_chunk_ptr(png_ptr);

If you call the png_set_read_user_chunk_fn() function, then all unknown

chunks will be saved when read, in case your callback function will need

one or more of them.  This behavior can be changed with the

png_set_keep_unknown_chunks() function, described below.

At this point, you can set up a callback function that will be

called after each row has been read, which you can use to control

a progress meter or the like.  It's demonstrated in pngtest.c.

You must supply a function

    void read_row_callback(png_ptr ptr, png_uint_32 row,

       int pass);

    {

      /* put your code here */

    }

(You can give it another name that you like instead of "read_row_callback")

To inform libpng about your function, use

    png_set_read_status_fn(png_ptr, read_row_callback);

Unknown-chunk handling

Now you get to set the way the library processes unknown chunks in the

input PNG stream. Both known and unknown chunks will be read.  Normal

behavior is that known chunks will be parsed into information in

various info_ptr members while unknown chunks will be discarded. This

behavior can be wasteful if your application will never use some known

chunk types. To change this, you can call:

    png_set_keep_unknown_chunks(png_ptr, keep,

        chunk_list, num_chunks);

    keep       - 0: default unknown chunk handling

                 1: ignore; do not keep

                 2: keep only if safe-to-copy

                 3: keep even if unsafe-to-copy

               You can use these definitions:

                 PNG_HANDLE_CHUNK_AS_DEFAULT   0

                 PNG_HANDLE_CHUNK_NEVER        1

                 PNG_HANDLE_CHUNK_IF_SAFE      2

                 PNG_HANDLE_CHUNK_ALWAYS       3

    chunk_list - list of chunks affected (a byte string,

                 five bytes per chunk, NULL or '\0' if

                 num_chunks is 0)

    num_chunks - number of chunks affected; if 0, all

                 unknown chunks are affected.  If nonzero,

                 only the chunks in the list are affected

Unknown chunks declared in this way will be saved as raw data onto a

list of png_unknown_chunk structures.  If a chunk that is normally

known to libpng is named in the list, it will be handled as unknown,

according to the "keep" directive.  If a chunk is named in successive

instances of png_set_keep_unknown_chunks(), the final instance will

take precedence.  The IHDR and IEND chunks should not be named in

chunk_list; if they are, libpng will process them normally anyway.

Here is an example of the usage of png_set_keep_unknown_chunks(),

where the private "vpAg" chunk will later be processed by a user chunk

callback function:

    png_byte vpAg[5]={118, 112,  65, 103, (png_byte) '\0'};

    #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)

      png_byte unused_chunks[]=

      {

        104,  73,  83,  84, (png_byte) '\0',   /* hIST */

        105,  84,  88, 116, (png_byte) '\0',   /* iTXt */

        112,  67,  65,  76, (png_byte) '\0',   /* pCAL */

        115,  67,  65,  76, (png_byte) '\0',   /* sCAL */

        115,  80,  76,  84, (png_byte) '\0',   /* sPLT */

        116,  73,  77,  69, (png_byte) '\0',   /* tIME */

      };

    #endif

    ...

    #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)

      /* ignore all unknown chunks: */

      png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);

      /* except for vpAg: */

      png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

      /* also ignore unused known chunks: */

      png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,

         (int)sizeof(unused_chunks)/5);

    #endif

User limits

The PNG specification allows the width and height of an image to be as

large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.

Since very few applications really need to process such large images,

we have imposed an arbitrary 1-million limit on rows and columns.

Larger images will be rejected immediately with a png_error() call. If

you wish to override this limit, you can use

   png_set_user_limits(png_ptr, width_max, height_max);

to set your own limits, or use width_max = height_max = 0x7fffffffL

to allow all valid dimensions (libpng may reject some very large images

anyway because of potential buffer overflow conditions).

You should put this statement after you create the PNG structure and

before calling png_read_info(), png_read_png(), or png_process_data().

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

   width_max = png_get_user_width_max(png_ptr);

   height_max = png_get_user_height_max(png_ptr);

The high-level read interface

At this point there are two ways to proceed; through the high-level

read interface, or through a sequence of low-level read operations.

You can use the high-level interface if (a) you are willing to read

the entire image into memory, and (b) the input transformations

you want to do are limited to the following set:

    PNG_TRANSFORM_IDENTITY      No transformation

    PNG_TRANSFORM_STRIP_16      Strip 16-bit samples to

                                8 bits

    PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel

    PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit

                                samples to bytes

    PNG_TRANSFORM_PACKSWAP      Change order of packed

                                pixels to LSB first

    PNG_TRANSFORM_EXPAND        Perform set_expand()

    PNG_TRANSFORM_INVERT_MONO   Invert monochrome images

    PNG_TRANSFORM_SHIFT         Normalize pixels to the

                                sBIT depth

    PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA

                                to BGRA

    PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA

                                to AG

    PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity

                                to transparency

    PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples

(This excludes setting a background color, doing gamma transformation,

dithering, and setting filler.)  If this is the case, simply do this:

    png_read_png(png_ptr, info_ptr, png_transforms, NULL)

where png_transforms is an integer containing the bitwise OR of

some set of transformation flags.  This call is equivalent to png_read_info(),

followed the set of transformations indicated by the transform mask,

then png_read_image(), and finally png_read_end().

(The final parameter of this call is not yet used.  Someday it might point

to transformation parameters required by some future input transform.)

You must use png_transforms and not call any png_set_transform() functions

when you use png_read_png().

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

with

   row_pointers = png_get_rows(png_ptr, info_ptr);

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

   png_bytep row_pointers[height];

If you know your image size and pixel size ahead of time, you can allocate

row_pointers prior to calling png_read_png() with

   if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))

      png_error (png_ptr,

         "Image is too tall to process in memory");

   if (width > PNG_UINT_32_MAX/pixel_size)

      png_error (png_ptr,

         "Image is too wide to process in memory");

   row_pointers = png_malloc(png_ptr,

      height*png_sizeof(png_bytep));

   for (int i=0; i<height, i++)

      row_pointers[i]=NULL;  /* security precaution */

   for (int i=0; i<height, i++)

      row_pointers[i]=png_malloc(png_ptr,

         width*pixel_size);

   png_set_rows(png_ptr, info_ptr, &row_pointers);

Alternatively you could allocate your image in one big block and define

row_pointers[i] to point into the proper places in your block.

If you use png_set_rows(), the application is responsible for freeing

row_pointers (and row_pointers[i], if they were separately allocated).

If you don't allocate row_pointers ahead of time, png_read_png() will

do it, and it'll be free'ed when you call png_destroy_*().

The low-level read interface

If you are going the low-level route, you are now ready to read all

the file information up to the actual image data.  You do this with a

call to png_read_info().

    png_read_info(png_ptr, info_ptr);

This will process all chunks up to but not including the image data.

Querying the info structure

Functions are used to get the information from the info_ptr once it

has been read.  Note that these fields may not be completely filled

in until png_read_end() has read the chunk data following the image.

    png_get_IHDR(png_ptr, info_ptr, &width, &height,

       &bit_depth, &color_type, &interlace_type,

       &compression_type, &filter_method);

    width          - holds the width of the image

                     in pixels (up to 2^31).

    height         - holds the height of the image

                     in pixels (up to 2^31).

    bit_depth      - holds the bit depth of one of the

                     image channels.  (valid values are

                     1, 2, 4, 8, 16 and depend also on

                     the color_type.  See also

                     significant bits (sBIT) below).

    color_type     - describes which color/alpha channels

                         are present.

                     PNG_COLOR_TYPE_GRAY

                        (bit depths 1, 2, 4, 8, 16)

                     PNG_COLOR_TYPE_GRAY_ALPHA

                        (bit depths 8, 16)

                     PNG_COLOR_TYPE_PALETTE