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



 libpng version 1.2.16 - January 31, 2007

 Updated and distributed by Glenn Randers-Pehrson

 <glennrp at users.sourceforge.net>

 Copyright (c) 1998-2005 Glenn Randers-Pehrson

 For conditions of distribution and use, see copyright

 notice in png.h.



 based on:



 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.



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/>



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.  Note: thread safety may be defeated

by use of some of the MMX assembler code in pnggccrd.c, which is only

compiled when the user defines PNG_THREAD_UNSAFE_OK.



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 if the bytes match the corresponding

bytes of the PNG signature, or nonzero 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: */

           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.  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);



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);



Width and height 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);



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; unknown chunks will be discarded. To change

this, you can call:



    png_set_keep_unknown_chunks(png_ptr, keep,

        chunk_list, num_chunks);

    keep       - 0: do not handle as unknown

                 1: 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.



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]=png_malloc(png_ptr,

         width*pixel_size);

   png_set_rows(png_ptr, info_ptr, &row_pointers);



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

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



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

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



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

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



The low-level read interface



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

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

call to png_read_info().



    png_read_info(png_ptr, info_ptr);



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



Querying the info structure



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

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

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



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

       &bit_depth, &color_type, &interlace_type,

       &compression_type, &filter_method);



    width          - holds the width of the image

                     in pixels (up to 2^31).

    height         - holds the height of the image

                     in pixels (up to 2^31).

    bit_depth      - holds the bit depth of one of the

                     image channels.  (valid values are

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

                     the color_type.  See also

                     significant bits (sBIT) below).

    color_type     - describes which color/alpha channels

                         are present.

                     PNG_COLOR_TYPE_GRAY

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

                     PNG_COLOR_TYPE_GRAY_ALPHA

                        (bit depths 8, 16)

                     PNG_COLOR_TYPE_PALETTE

                        (bit depths 1, 2, 4, 8)

                     PNG_COLOR_TYPE_RGB

                        (bit_depths 8, 16)

                     PNG_COLOR_TYPE_RGB_ALPHA

                        (bit_depths 8, 16)



                     PNG_COLOR_MASK_PALETTE

                     PNG_COLOR_MASK_COLOR

                     PNG_COLOR_MASK_ALPHA



    filter_method  - (must be PNG_FILTER_TYPE_BASE

                     for PNG 1.0, and can also be

                     PNG_INTRAPIXEL_DIFFERENCING if

                     the PNG datastream is embedded in

                     a MNG-1.0 datastream)

    compression_type - (must be PNG_COMPRESSION_TYPE_BASE

                     for PNG 1.0)

    interlace_type - (PNG_INTERLACE_NONE or

                     PNG_INTERLACE_ADAM7)

    Any or all of interlace_type, compression_type, of

    filter_method can be NULL if you are

    not interested in their values.



    channels = png_get_channels(png_ptr, info_ptr);

    channels       - number of channels of info for the

                     color type (valid values are 1 (GRAY,

                     PALETTE), 2 (GRAY_ALPHA), 3 (RGB),

                     4 (RGB_ALPHA or RGB + filler byte))

    rowbytes = png_get_rowbytes(png_ptr, info_ptr);

    rowbytes       - number of bytes needed to hold a row



    signature = png_get_signature(png_ptr, info_ptr);

    signature      - holds the signature read from the

                     file (if any).  The data is kept in

                     the same offset it would be if the

                     whole signature were read (i.e. if an

                     application had already read in 4

                     bytes of signature before starting

                     libpng, the remaining 4 bytes would

                     be in signature[4] through signature[7]

                     (see png_set_sig_bytes())).





    width            = png_get_image_width(png_ptr,

                         info_ptr);

    height           = png_get_image_height(png_ptr,

                         info_ptr);

    bit_depth        = png_get_bit_depth(png_ptr,

                         info_ptr);

    color_type       = png_get_color_type(png_ptr,

                         info_ptr);

    filter_method    = png_get_filter_type(png_ptr,

                         info_ptr);

    compression_type = png_get_compression_type(png_ptr,

                         info_ptr);

    interlace_type   = png_get_interlace_type(png_ptr,

                         info_ptr);





These are also important, but their validity depends on whether the chunk

has been read.  The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and

png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the

data has been read, or zero if it is missing.  The parameters to the

png_get_<chunk> are set directly if they are simple data types, or a pointer

into the info_ptr is returned for any complex types.



    png_get_PLTE(png_ptr, info_ptr, &palette,

                     &num_palette);

    palette        - the palette for the file

                     (array of png_color)

    num_palette    - number of entries in the palette



    png_get_gAMA(png_ptr, info_ptr, &gamma);

    gamma          - the gamma the file is written

                     at (PNG_INFO_gAMA)



    png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

    srgb_intent    - the rendering intent (PNG_INFO_sRGB)

                     The presence of the sRGB chunk

                     means that the pixel data is in the

                     sRGB color space.  This chunk also

                     implies specific values of gAMA and

                     cHRM.



    png_get_iCCP(png_ptr, info_ptr, &name,

       &compression_type, &profile, &proflen);

    name            - The profile name.

    compression     - The compression type; always

                      PNG_COMPRESSION_TYPE_BASE for PNG 1.0.

                      You may give NULL to this argument to

                      ignore it.

    profile         - International Color Consortium color

                      profile data. May contain NULs.

    proflen         - length of profile data in bytes.



    png_get_sBIT(png_ptr, info_ptr, &sig_bit);

    sig_bit        - the number of significant bits for

                     (PNG_INFO_sBIT) each of the gray,

                     red, green, and blue channels,

                     whichever are appropriate for the

                     given color type (png_color_16)



    png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans,

                     &trans_values);

    trans          - array of transparent entries for

                     palette (PNG_INFO_tRNS)

    trans_values   - graylevel or color sample values of

                     the single transparent color for

                     non-paletted images (PNG_INFO_tRNS)

    num_trans      - number of transparent entries

                     (PNG_INFO_tRNS)



    png_get_hIST(png_ptr, info_ptr, &hist);

                     (PNG_INFO_hIST)

    hist           - histogram of palette (array of

                     png_uint_16)



    png_get_tIME(png_ptr, info_ptr, &mod_time);

    mod_time       - time image was last modified

                    (PNG_VALID_tIME)



    png_get_bKGD(png_ptr, info_ptr, &background);

    background     - background color (PNG_VALID_bKGD)

                     valid 16-bit red, green and blue

                     values, regardless of color_type



    num_comments   = png_get_text(png_ptr, info_ptr,

                     &text_ptr, &num_text);

    num_comments   - number of comments

    text_ptr       - array of png_text holding image

                     comments

    text_ptr[i].compression - type of compression used

                 on "text" PNG_TEXT_COMPRESSION_NONE

                           PNG_TEXT_COMPRESSION_zTXt

                           PNG_ITXT_COMPRESSION_NONE

                           PNG_ITXT_COMPRESSION_zTXt

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

                         1-79 characters.

    text_ptr[i].text  - text comments for current

                         keyword.  Can be empty.

    text_ptr[i].text_length - length of text string,

                 after decompression, 0 for iTXt

    text_ptr[i].itxt_length - length of itxt string,

                 after decompression, 0 for tEXt/zTXt

    text_ptr[i].lang  - language of comment (empty

                         string for unknown).

    text_ptr[i].lang_key  - keyword in UTF-8

                         (empty string for unknown).

    num_text       - number of comments (same as

                     num_comments; you can put NULL here

                     to avoid the duplication)

    Note while png_set_text() will accept text, language,

    and translated keywords that can be NULL pointers, the

    structure returned by png_get_text will always contain

    regular zero-terminated C strings.  They might be

    empty strings but they will never be NULL pointers.



    num_spalettes = png_get_sPLT(png_ptr, info_ptr,

       &palette_ptr);

    palette_ptr    - array of palette structures holding

                     contents of one or more sPLT chunks

                     read.

    num_spalettes  - number of sPLT chunks read.



    png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,

       &unit_type);

    offset_x       - positive offset from the left edge

                     of the screen

    offset_y       - positive offset from the top edge

                     of the screen

    unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER



    png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,

       &unit_type);

    res_x          - pixels/unit physical resolution in

                     x direction

    res_y          - pixels/unit physical resolution in

                     x direction

    unit_type      - PNG_RESOLUTION_UNKNOWN,

                     PNG_RESOLUTION_METER



    png_get_sCAL(png_ptr, info_ptr, &unit, &width,

       &height)

    unit        - physical scale units (an integer)

    width       - width of a pixel in physical scale units

    height      - height of a pixel in physical scale units

                 (width and height are doubles)



    png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,

       &height)

    unit        - physical scale units (an integer)

    width       - width of a pixel in physical scale units

    height      - height of a pixel in physical scale units

                 (width and height are strings like "2.54")



    num_unknown_chunks = png_get_unknown_chunks(png_ptr,

       info_ptr, &unknowns)

    unknowns          - array of png_unknown_chunk

                        structures holding unknown chunks

    unknowns[i].name  - name of unknown chunk

    unknowns[i].data  - data of unknown chunk

    unknowns[i].size  - size of unknown chunk's data

    unknowns[i].location - position of chunk in file



    The value of "i" corresponds to the order in which the

    chunks were read from the PNG file or inserted with the

    png_set_unknown_chunks() function.



The data from the pHYs chunk can be retrieved in several convenient

forms:



    res_x = png_get_x_pixels_per_meter(png_ptr,

       info_ptr)

    res_y = png_get_y_pixels_per_meter(png_ptr,

       info_ptr)

    res_x_and_y = png_get_pixels_per_meter(png_ptr,

       info_ptr)

    res_x = png_get_x_pixels_per_inch(png_ptr,

       info_ptr)

    res_y = png_get_y_pixels_per_inch(png_ptr,

       info_ptr)

    res_x_and_y = png_get_pixels_per_inch(png_ptr,

       info_ptr)

    aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,

       info_ptr)



   (Each of these returns 0 [signifying "unknown"] if

       the data is not present or if res_x is 0;

       res_x_and_y is 0 if res_x != res_y)



The data from the oFFs chunk can be retrieved in several convenient

forms:



    x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

    y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

    x_offset = png_get_x_offset_inches(png_ptr, info_ptr);

    y_offset = png_get_y_offset_inches(png_ptr, info_ptr);



   (Each of these returns 0 [signifying "unknown" if both

       x and y are 0] if the data is not present or if the

       chunk is present but the unit is the pixel)



For more information, see the png_info definition in png.h and the

PNG specification for chunk contents.  Be careful with trusting

rowbytes, as some of the transformations could increase the space

needed to hold a row (expand, filler, gray_to_rgb, etc.).

See png_read_update_info(), below.



A quick word about text_ptr and num_text.  PNG stores comments in

keyword/text pairs, one pair per chunk, with no limit on the number

of text chunks, and a 2^31 byte limit on their size.  While there are

suggested keywords, there is no requirement to restrict the use to these

strings.  It is strongly suggested that keywords and text be sensible

to humans (that's the point), so don't use abbreviations.  Non-printing

symbols are not allowed.  See the PNG specification for more details.

There is also no requirement to have text after the keyword.



Keywords should be limited to 79 Latin-1 characters without leading or

trailing spaces, but non-consecutive spaces are allowed within the

keyword.  It is possible to have the same keyword any number of times.

The text_ptr is an array of png_text structures, each holding a

pointer to a language string, a pointer to a keyword and a pointer to

a text string.  The text string, language code, and translated

keyword may be empty or NULL pointers.  The keyword/text

pairs are put into the array in the order that they are received.

However, some or all of the text chunks may be after the image, so, to

make sure you have read all the text chunks, don't mess with these

until after you read the stuff after the image.  This will be

mentioned again below in the discussion that goes with png_read_end().



Input transformations



After you've read the header information, you can set up the library

to handle any special transformations of the image data.  The various

ways to transform the data will be described in the order that they

should occur.  This is important, as some of these change the color

type and/or bit depth of the data, and some others only work on

certain color types and bit depths.  Even though each transformation

checks to see if it has data that it can do something with, you should

make sure to only enable a transformation if it will be valid for the

data.  For example, don't swap red and blue on grayscale data.



The colors used for the background and transparency values should be

supplied in the same format/depth as the current image data.  They

are stored in the same format/depth as the image data in a bKGD or tRNS

chunk, so this is what libpng expects for this data.  The colors are

transformed to keep in sync with the image data when an application

calls the png_read_update_info() routine (see below).



Data will be decoded into the supplied row buffers packed into bytes

unless the library has been told to transform it into another format.

For example, 4 bit/pixel paletted or grayscale data will be returned

2 pixels/byte with the leftmost pixel in the high-order bits of the

byte, unless png_set_packing() is called.  8-bit RGB data will be stored

in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()

is called to insert filler bytes, either before or after each RGB triplet.

16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant

byte of the color value first, unless png_set_strip_16() is called to

transform it to regular RGB RGB triplets, or png_set_filler() or

png_set_add alpha() is called to insert filler bytes, either before or

after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can

be modified with

png_set_filler(), png_set_add_alpha(), or png_set_strip_16().



The following code transforms grayscale images of less than 8 to 8 bits,

changes paletted images to RGB, and adds a full alpha channel if there is

transparency information in a tRNS chunk.  This is most useful on

grayscale images with bit depths of 2 or 4 or if there is a multiple-image

viewing application that wishes to treat all images in the same way.



    if (color_type == PNG_COLOR_TYPE_PALETTE)

        png_set_palette_to_rgb(png_ptr);



    if (color_type == PNG_COLOR_TYPE_GRAY &&

        bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);



    if (png_get_valid(png_ptr, info_ptr,

        PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);



These three functions are actually aliases for png_set_expand(), added

in libpng version 1.0.4, with the function names expanded to improve code

readability.  In some future version they may actually do different

things.



As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was

added.  It expands the sample depth without changing tRNS to alpha.

At the same time, png_set_gray_1_2_4_to_8() was deprecated, and it

will be removed from a future version.



PNG can have files with 16 bits per channel.  If you only can handle

8 bits per channel, this will strip the pixels down to 8 bit.



    if (bit_depth == 16)

        png_set_strip_16(png_ptr);



If, for some reason, you don't need the alpha channel on an image,

and you want to remove it rather than combining it with the background

(but the image author certainly had in mind that you *would* combine

it with the background, so that's what you should probably do):



    if (color_type & PNG_COLOR_MASK_ALPHA)

        png_set_strip_alpha(png_ptr);



In PNG files, the alpha channel in an image

is the level of opacity.  If you need the alpha channel in an image to

be the level of transparency instead of opacity, you can invert the

alpha channel (or the tRNS chunk data) after it's read, so that 0 is

fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit

images) is fully transparent, with



    png_set_invert_alpha(png_ptr);



PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as

they can, resulting in, for example, 8 pixels per byte for 1 bit

files.  This code expands to 1 pixel per byte without changing the

values of the pixels:



    if (bit_depth < 8)

        png_set_packing(png_ptr);



PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels

stored in a PNG image have been "scaled" or "shifted" up to the next

higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] to

8 bits/sample in the range [0, 255]).  However, it is also possible to

convert the PNG pixel data back to the original bit depth of the image.

This call reduces the pixels back down to the original bit depth:



    png_color_8p sig_bit;



    if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))

        png_set_shift(png_ptr, sig_bit);



PNG files store 3-color pixels in red, green, blue order.  This code

changes the storage of the pixels to blue, green, red:



    if (color_type == PNG_COLOR_TYPE_RGB ||

        color_type == PNG_COLOR_TYPE_RGB_ALPHA)

        png_set_bgr(png_ptr);



PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them

into 4 or 8 bytes for windowing systems that need them in this format:



    if (color_type == PNG_COLOR_TYPE_RGB)

        png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);



where "filler" is the 8 or 16-bit number to fill with, and the location is

either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether

you want the filler before the RGB or after.  This transformation

does not affect images that already have full alpha channels.  To add an

opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which

will generate RGBA pixels.



Note that png_set_filler() does not change the color type.  If you want

to do that, you can add a true alpha channel with



    if (color_type == PNG_COLOR_TYPE_RGB ||

           color_type == PNG_COLOR_TYPE_GRAY)

    png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);



where "filler" contains the alpha value to assign to each pixel.

This function was added in libpng-1.2.7.



If you are reading an image with an alpha channel, and you need the

data as ARGB instead of the normal PNG format RGBA:



    if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)

        png_set_swap_alpha(png_ptr);



For some uses, you may want a grayscale image to be represented as

RGB.  This code will do that conversion:



    if (color_type == PNG_COLOR_TYPE_GRAY ||

        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)

          png_set_gray_to_rgb(png_ptr);



Conversely, you can convert an RGB or RGBA image to grayscale or grayscale

with alpha.



    if (color_type == PNG_COLOR_TYPE_RGB ||

        color_type == PNG_COLOR_TYPE_RGB_ALPHA)

          png_set_rgb_to_gray_fixed(png_ptr, error_action,

             int red_weight, int green_weight);



    error_action = 1: silently do the conversion

    error_action = 2: issue a warning if the original

                      image has any pixel where

                      red != green or red != blue

    error_action = 3: issue an error and abort the

                      conversion if the original

                      image has any pixel where

                      red != green or red != blue



    red_weight:       weight of red component times 100000

    green_weight:     weight of green component times 100000

                      If either weight is negative, default

                      weights (21268, 71514) are used.



If you have set error_action = 1 or 2, you can

later check whether the image really was gray, after processing

the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.

It will return a png_byte that is zero if the image was gray or

1 if there were any non-gray pixels.  bKGD and sBIT data

will be silently converted to grayscale, using the green channel

data, regardless of the error_action setting.



With red_weight+green_weight<=100000,

the normalized graylevel is computed:



    int rw = red_weight * 65536;

    int gw = green_weight * 65536;

    int bw = 65536 - (rw + gw);

    gray = (rw*red + gw*green + bw*blue)/65536;



The default values approximate those recommended in the Charles

Poynton's Color FAQ, <http://www.inforamp.net/~poynton/>

Copyright (c) 1998-01-04 Charles Poynton <poynton at inforamp.net>



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



Libpng approximates this with



    Y = 0.21268 * R    + 0.7151 * G    + 0.07217 * B



which can be expressed with integers as



    Y = (6969 * R + 23434 * G + 2365 * B)/32768



The calculation is done in a linear colorspace, if the image gamma

is known.



If you have a grayscale and you are using png_set_expand_depth(),

png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to

a higher bit-depth, you must either supply the background color as a gray

value at the original file bit-depth (need_expand = 1) or else supply the

background color as an RGB triplet at the final, expanded bit depth

(need_expand = 0).  Similarly, if you are reading a paletted image, you

must either supply the background color as a palette index (need_expand = 1)

or as an RGB triplet that may or may not be in the palette (need_expand = 0).



    png_color_16 my_background;

    png_color_16p image_background;



    if (png_get_bKGD(png_ptr, info_ptr, &image_background))

        png_set_background(png_ptr, image_background,

          PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);

    else

        png_set_background(png_ptr, &my_background,

          PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);



The png_set_background() function tells libpng to composite images

with alpha or simple transparency against the supplied background

color.  If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),

you may use this color, or supply another color more suitable for

the current display (e.g., the background color from a web page).  You

need to tell libpng whether the color is in the gamma space of the

display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file

(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one

that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't

know why anyone would use this, but it's here).



To properly display PNG images on any kind of system, the application needs

to know what the display gamma is.  Ideally, the user will know this, and

the application will allow them to set it.  One method of allowing the user

to set the display gamma separately for each system is to check for a

SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which will hopefully be

correctly set.



Note that display_gamma is the overall gamma correction required to produce

pleasing results, which depends on the lighting conditions in the surrounding

environment.  In a dim or brightly lit room, no compensation other than

the physical gamma exponent of the monitor is needed, while in a dark room

a slightly smaller exponent is better.



   double gamma, screen_gamma;



   if (/* We have a user-defined screen

       gamma value */)

   {

      screen_gamma = user_defined_screen_gamma;

   }

   /* One way that applications can share the same

      screen gamma value */

   else if ((gamma_str = getenv("SCREEN_GAMMA"))

      != NULL)

   {

      screen_gamma = (double)atof(gamma_str);

   }

   /* If we don't have another value */

   else

   {

      screen_gamma = 2.2; /* A good guess for a

           PC monitor in a bright office or a dim room */

      screen_gamma = 2.0; /* A good guess for a

           PC monitor in a dark room */

      screen_gamma = 1.7 or 1.0;  /* A good

           guess for Mac systems */

   }



The png_set_gamma() function handles gamma transformations of the data.

Pass both the file gamma and the current screen_gamma.  If the file does

not have a gamma value, you can pass one anyway if you have an idea what

it is (usually 0.45455 is a good guess for GIF images on PCs).  Note

that file gammas are inverted from screen gammas.  See the discussions

on gamma in the PNG specification for an excellent description of what

gamma is, and why all applications should support it.  It is strongly

recommended that PNG viewers support gamma correction.



   if (png_get_gAMA(png_ptr, info_ptr, &gamma))

      png_set_gamma(png_ptr, screen_gamma, gamma);

   else

      png_set_gamma(png_ptr, screen_gamma, 0.45455);



If you need to reduce an RGB file to a paletted file, or if a paletted

file has more entries then will fit on your screen, png_set_dither()

will do that.  Note that this is a simple match dither that merely

finds the closest color available.  This should work fairly well with

optimized palettes, and fairly badly with linear color cubes.  If you

pass a palette that is larger then maximum_colors, the file will

reduce the number of colors in the palette so it will fit into

maximum_colors.  If there is a histogram, it will use it to make

more intelligent choices when reducing the palette.  If there is no

histogram, it may not do as good a job.



   if (color_type & PNG_COLOR_MASK_COLOR)

   {

      if (png_get_valid(png_ptr, info_ptr,

         PNG_INFO_PLTE))

      {

         png_uint_16p histogram = NULL;



         png_get_hIST(png_ptr, info_ptr,

            &histogram);

         png_set_dither(png_ptr, palette, num_palette,

            max_screen_colors, histogram, 1);

      }

      else

      {

         png_color std_color_cube[MAX_SCREEN_COLORS] =

            { ... colors ... };



         png_set_dither(png_ptr, std_color_cube,

            MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,

            NULL,0);

      }

   }



PNG files describe monochrome as black being zero and white being one.

The following code will reverse this (make black be one and white be

zero):



   if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)

      png_set_invert_mono(png_ptr);



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



   if (color_type == PNG_COLOR_TYPE_GRAY ||

        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)

      png_set_invert_mono(png_ptr);



PNG files store 16 bit pixels in network byte order (big-endian,

ie. most significant bits first).  This code changes the storage to the

other way (little-endian, i.e. least significant bits first, the

way PCs store them):



    if (bit_depth == 16)

        png_set_swap(png_ptr);



If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you

need to change the order the pixels are packed into bytes, you can use:



    if (bit_depth < 8)

       png_set_packswap(png_ptr);



Finally, you can write your own transformation function if none of

the existing ones meets your needs.  This is done by setting a callback

with



    png_set_read_user_transform_fn(png_ptr,

       read_transform_fn);



You must supply the function



    void read_transform_fn(png_ptr ptr, row_info_ptr

       row_info, png_bytep data)



See pngtest.c for a working example.  Your function will be called

after all of the other transformations have been processed.



You can also set up a pointer to a user structure for use by your

callback function, and you can inform libpng that your transform

function will change the number of channels or bit depth with the

function



    png_set_user_transform_info(png_ptr, user_ptr,

       user_depth, user_channels);



The user's application, not libpng, is responsible for allocating and

freeing any memory required for the user structure.



You can retrieve the pointer via the function

png_get_user_transform_ptr().  For example:



    voidp read_user_transform_ptr =

       png_get_user_transform_ptr(png_ptr);



The last thing to handle is interlacing; this is covered in detail below,

but you must call the function here if you want libpng to handle expansion

of the interlaced image.



    number_of_passes = png_set_interlace_handling(png_ptr);



After setting the transformations, libpng can update your png_info

structure to reflect any transformations you've requested with this

call.  This is most useful to update the info structure's rowbytes

field so you can use it to allocate your image memory.  This function

will also update your palette with the correct screen_gamma and

background if these have been given with the calls above.



    png_read_update_info(png_ptr, info_ptr);



After you call png_read_update_info(), you can allocate any

memory you need to hold the image.  The row data is simply

raw byte data for all forms of images.  As the actual allocation

varies among applications, no example will be given.  If you

are allocating one large chunk, you will need to build an

array of pointers to each row, as it will be needed for some

of the functions below.



Reading image data



After you've allocated memory, you can read the image data.

The simplest way to do this is in one function call.  If you are

allocating enough memory to hold the whole image, you can just

call png_read_image() and libpng will read in all the image data

and put it in the memory area supplied.  You will need to pass in

an array of pointers to each row.



This function automatically handles interlacing, so you don't need

to call png_set_interlace_handling() or call this function multiple

times, or any of that other stuff necessary with png_read_rows().



   png_read_image(png_ptr, row_pointers);



where row_pointers is:



   png_bytep row_pointers[height];



You can point to void or char or whatever you use for pixels.



If you don't want to read in the whole image at once, you can

use png_read_rows() instead.  If there is no interlacing (check

interlace_type == PNG_INTERLACE_NONE), this is simple:



    png_read_rows(png_ptr, row_pointers, NULL,

       number_of_rows);



where row_pointers is the same as in the png_read_image() call.



If you are doing this just one row at a time, you can do this with

a single row_pointer instead of an array of row_pointers:



    png_bytep row_pointer = row;

    png_read_row(png_ptr, row_pointer, NULL);



If the file is interlaced (interlace_type != 0 in the IHDR chunk), things

get somewhat harder.  The only current (PNG Specification version 1.2)

interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7)

is a somewhat complicated 2D interlace scheme, known as Adam7, that

breaks down an image into seven smaller images of varying size, based

on an 8x8 grid.



libpng can fill out those images or it can give them to you "as is".

If you want them filled out, there are two ways to do that.  The one

mentioned in the PNG specification is to expand each pixel to cover

those pixels that have not been read yet (the "rectangle" method).

This results in a blocky image for the first pass, which gradually

smooths out as more pixels are read.  The other method is the "sparkle"

method, where pixels are drawn only in their final locations, with the

rest of the image remaining whatever colors they were initialized to

before the start of the read.  The first method usually looks better,

but tends to be slower, as there are more pixels to put in the rows.



If you don't want libpng to handle the interlacing details, just call

png_read_rows() seven times to read in all seven images.  Each of the

images is a valid image by itself, or they can all be combined on an

8x8 grid to form a single image (although if you intend to combine them

you would be far better off using the libpng interlace handling).



The first pass will return an image 1/8 as wide as the entire image

(every 8th column starting in column 0) and 1/8 as high as the original

(every 8th row starting in row 0), the second will be 1/8 as wide

(starting in column 4) and 1/8 as high (also starting in row 0).  The

third pass will be 1/4 as wide (every 4th pixel starting in column 0) and

1/8 as high (every 8th row starting in row 4), and the fourth pass will

be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,

and every 4th row starting in row 0).  The fifth pass will return an

image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),

while the sixth pass will be 1/2 as wide and 1/2 as high as the original

(starting in column 1 and row 0).  The seventh and final pass will be as

wide as the original, and 1/2 as high, containing all of the odd

numbered scanlines.  Phew!



If you want libpng to expand the images, call this before calling

png_start_read_image() or png_read_update_info():



    if (interlace_type == PNG_INTERLACE_ADAM7)

        number_of_passes

           = png_set_interlace_handling(png_ptr);



This will return the number of passes needed.  Currently, this

is seven, but may change if another interlace type is added.

This function can be called even if the file is not interlaced,

where it will return one pass.



If you are not going to display the image after each pass, but are

going to wait until the entire image is read in, use the sparkle

effect.  This effect is faster and the end result of either method

is exactly the same.  If you are planning on displaying the image

after each pass, the "rectangle" effect is generally considered the

better looking one.



If you only want the "sparkle" effect, just call png_read_rows() as

normal, with the third parameter NULL.  Make sure you make pass over

the image number_of_passes times, and you don't change the data in the

rows between calls.  You can change the locations of the data, just

not the data.  Each pass only writes the pixels appropriate for that

pass, and assumes the data from previous passes is still valid.



    png_read_rows(png_ptr, row_pointers, NULL,

       number_of_rows);



If you only want the first effect (the rectangles), do the same as

before except pass the row buffer in the third parameter, and leave

the second parameter NULL.



    png_read_rows(png_ptr, NULL, row_pointers,

       number_of_rows);



Finishing a sequential read



After you are finished reading the image through the

low-level interface, you can finish reading the file.  If you are

interested in comments or time, which may be stored either before or

after the image data, you should pass the separate png_info struct if

you want to keep the comments from before and after the image

separate.  If you are not interested, you can pass NULL.



   png_read_end(png_ptr, end_info);



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



   png_destroy_read_struct(&png_ptr, &info_ptr,

       &end_info);



It is also possible to individually free the info_ptr members that

point to libpng-allocated storage with the following function:



    png_free_data(png_ptr, info_ptr, mask, seq)

    mask - identifies data to be freed, a mask

           containing the bitwise OR of one or

           more of

             PNG_FREE_PLTE, PNG_FREE_TRNS,

             PNG_FREE_HIST, PNG_FREE_ICCP,

             PNG_FREE_PCAL, PNG_FREE_ROWS,

             PNG_FREE_SCAL, PNG_FREE_SPLT,

             PNG_FREE_TEXT, PNG_FREE_UNKN,

           or simply PNG_FREE_ALL

    seq  - sequence number of item to be freed

           (-1 for all items)



This function may be safely called when the relevant storage has

already been freed, or has not yet been allocated, or was allocated

by the user and not by libpng,  and will in those

cases do nothing.  The "seq" parameter is ignored if only one item

of the selected data type, such as PLTE, is allowed.  If "seq" is not

-1, and multiple items are allowed for the data type identified in

the mask, such as text or sPLT, only the n'th item in the structure

is freed, where n is "seq".



The default behavior is only to free data that was allocated internally

by libpng.  This can be changed, so that libpng will not free the data,

or so that it will free data that was allocated by the user with png_malloc()

or png_zalloc() and passed in via a png_set_*() function, with



    png_data_freer(png_ptr, info_ptr, freer, mask)

    mask   - which data elements are affected

             same choices as in png_free_data()

    freer  - one of

               PNG_DESTROY_WILL_FREE_DATA

               PNG_SET_WILL_FREE_DATA

               PNG_USER_WILL_FREE_DATA



This function only affects data that has already been allocated.

You can call this function after reading the PNG data but before calling

any png_set_*() functions, to control whether the user or the png_set_*()

function is responsible for freeing any existing data that might be present,

and again after the png_set_*() functions to control whether the user

or png_destroy_*() is supposed to free the data.  When the user assumes

responsibility for libpng-allocated data, the application must use

png_free() to free it, and when the user transfers responsibility to libpng

for data that the user has allocated, the user must have used png_malloc()

or png_zalloc() to allocate it.



If you allocated your row_pointers in a single block, as suggested above in

the description of the high level read interface, you must not transfer

responsibility for freeing it to the png_set_rows or png_read_destroy function,

because they would also try to free the individual row_pointers[i].



If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword

separately, do not transfer responsibility for freeing text_ptr to libpng,

because when libpng fills a png_text structure it combines these members with

the key member, and png_free_data() will free only text_ptr.key.  Similarly,

if you transfer responsibility for free'ing text_ptr from libpng to your

application, your application must not separately free those members.



The png_free_data() function will turn off the "valid" flag for anything

it frees.  If you need to turn the flag off for a chunk that was freed by your

application instead of by libpng, you can use



    png_set_invalid(png_ptr, info_ptr, mask);

    mask - identifies the chunks to be made invalid,

           containing the bitwise OR of one or

           more of

             PNG_INFO_gAMA, PNG_INFO_sBIT,

             PNG_INFO_cHRM, PNG_INFO_PLTE,

             PNG_INFO_tRNS, PNG_INFO_bKGD,

             PNG_INFO_hIST, PNG_INFO_pHYs,

             PNG_INFO_oFFs, PNG_INFO_tIME,

             PNG_INFO_pCAL, PNG_INFO_sRGB,

             PNG_INFO_iCCP, PNG_INFO_sPLT,

             PNG_INFO_sCAL, PNG_INFO_IDAT



For a more compact example of reading a PNG image, see the file example.c.



Reading PNG files progressively



The progressive reader is slightly different then the non-progressive

reader.  Instead of calling png_read_info(), png_read_rows(), and

png_read_end(), you make one call to png_process_data(), which calls

callbacks when it has the info, a row, or the end of the image.  You

set up these callbacks with png_set_progressive_read_fn().  You don't

have to worry about the input/output functions of libpng, as you are

giving the library the data directly in png_process_data().  I will

assume that you have read the section on reading PNG files above,

so I will only highlight the differences (although I will show

all of the code).



png_structp png_ptr;

png_infop info_ptr;



 /*  An example code fragment of how you would

     initialize the progressive reader in your

     application. */

 int

 initialize_png_reader()

 {

    png_ptr = png_create_read_struct

        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,

         user_error_fn, user_warning_fn);

    if (!png_ptr)

        return (ERROR);

    info_ptr = png_create_info_struct(png_ptr);

    if (!info_ptr)

    {

        png_destroy_read_struct(&png_ptr, (png_infopp)NULL,

           (png_infopp)NULL);

        return (ERROR);

    }



    if (setjmp(png_jmpbuf(png_ptr)))

    {

        png_destroy_read_struct(&png_ptr, &info_ptr,

           (png_infopp)NULL);

        return (ERROR);

    }



    /* This one's new.  You can provide functions

       to be called when the header info is valid,

       when each row is completed, and when the image

       is finished.  If you aren't using all functions,

       you can specify NULL parameters.  Even when all

       three functions are NULL, you need to call

       png_set_progressive_read_fn().  You can use

       any struct as the user_ptr (cast to a void pointer

       for the function call), and retrieve the pointer

       from inside the callbacks using the function



          png_get_progressive_ptr(png_ptr);



       which will return a void pointer, which you have

       to cast appropriately.

     */

    png_set_progressive_read_fn(png_ptr, (void *)user_ptr,

        info_callback, row_callback, end_callback);



    return 0;

 }



 /* A code fragment that you call as you receive blocks

   of data */

 int

 process_data(png_bytep buffer, png_uint_32 length)

 {

    if (setjmp(png_jmpbuf(png_ptr)))

    {

        png_destroy_read_struct(&png_ptr, &info_ptr,

           (png_infopp)NULL);

        return (ERROR);

    }



    /* This one's new also.  Simply give it a chunk

       of data from the file stream (in order, of

       course).  On machines with segmented memory

       models machines, don't give it any more than

       64K.  The library seems to run fine with sizes

       of 4K. Although you can give it much less if

       necessary (I assume you can give it chunks of

       1 byte, I haven't tried less then 256 bytes

       yet).  When this function returns, you may

       want to display any rows that were generated

       in the row callback if you don't already do

       so there.

     */

    png_process_data(png_ptr, info_ptr, buffer, length);

    return 0;

 }



 /* This function is called (as set by

    png_set_progressive_read_fn() above) when enough data

    has been supplied so all of the header has been

    read.

 */

 void

 info_callback(png_structp png_ptr, png_infop info)

 {

    /* Do any setup here, including setting any of

       the transformations mentioned in the Reading

       PNG files section.  For now, you _must_ call

       either png_start_read_image() or

       png_read_update_info() after all the

       transformations are set (even if you don't set

       any).  You may start getting rows before

       png_process_data() returns, so this is your

       last chance to prepare for that.

     */

 }



 /* This function is called when each row of image

    data is complete */

 void

 row_callback(png_structp png_ptr, png_bytep new_row,

    png_uint_32 row_num, int pass)

 {

    /* If the image is interlaced, and you turned

       on the interlace handler, this function will

       be called for every row in every pass.  Some

       of these rows will not be changed from the

       previous pass.  When the row is not changed,

       the new_row variable will be NULL.  The rows

       and passes are called in order, so you don't

       really need the row_num and pass, but I'm

       supplying them because it may make your life

       easier.



       For the non-NULL rows of interlaced images,

       you must call png_progressive_combine_row()

       passing in the row and the old row.  You can

       call this function for NULL rows (it will just

       return) and for non-interlaced images (it just

       does the memcpy for you) if it will make the

       code easier.  Thus, you can just do this for

       all cases:

     */



        png_progressive_combine_row(png_ptr, old_row,

          new_row);



    /* where old_row is what was displayed for

       previously for the row.  Note that the first

       pass (pass == 0, really) will completely cover

       the old row, so the rows do not have to be

       initialized.  After the first pass (and only

       for interlaced images), you will have to pass

       the current row, and the function will combine

       the old row and the new row.

    */

 }



 void

 end_callback(png_structp png_ptr, png_infop info)

 {

    /* This function is called after the whole image

       has been read, including any chunks after the

       image (up to and including the IEND).  You

       will usually have the same info chunk as you

       had in the header, although some data may have

       been added to the comments and time fields.



       Most people won't do much here, perhaps setting

       a flag that marks the image as finished.

     */

 }







IV. Writing



Much of this is very similar to reading.  However, everything of

importance is repeated here, so you won't have to constantly look

back up in the reading section to understand writing.



Setup



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

so if it doesn't work, you don't have anything to undo. If you are not

using the standard I/O functions, you will need to replace them with

custom writing functions.  See the discussion under Customizing libpng.



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

    if (!fp)

    {

       return (ERROR);

    }



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

As these can be both relatively large, you may not want to store these

on the stack, unless you have stack space to spare.  Of course, you

will want to check if they return NULL.  If you are also reading,

you won't want to name your read structure and your write structure

both "png_ptr"; you can call them anything you like, such as

"read_ptr" and "write_ptr".  Look at pngtest.c, for example.



    png_structp png_ptr = png_create_write_struct

       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,

        user_error_fn, user_warning_fn);

    if (!png_ptr)

       return (ERROR);



    png_infop info_ptr = png_create_info_struct(png_ptr);

    if (!info_ptr)

    {

       png_destroy_write_struct(&png_ptr,

         (png_infopp)NULL);

       return (ERROR);

    }



If you want to use your own memory allocation routines,

define PNG_USER_MEM_SUPPORTED and use

png_create_write_struct_2() instead of png_create_write_struct():



    png_structp png_ptr = png_create_write_struct_2

       (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,

        user_error_fn, user_warning_fn, (png_voidp)

        user_mem_ptr, user_malloc_fn, user_free_fn);



After you have these structures, you will need to set up the

error handling.  When libpng encounters an error, it expects to

longjmp() back to your routine.  Therefore, you will need to call

setjmp() and pass the png_jmpbuf(png_ptr).  If you

write the file from different routines, you will need to update

the png_jmpbuf(png_ptr) every time you enter a new routine that will

call a png_*() function.  See your documentation of setjmp/longjmp

for your compiler for more information on setjmp/longjmp.  See

the discussion on libpng error handling in the Customizing Libpng

section below for more information on the libpng error handling.



    if (setjmp(png_jmpbuf(png_ptr)))

    {

       png_destroy_write_struct(&png_ptr, &info_ptr);

       fclose(fp);

       return (ERROR);

    }

    ...

    return;



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

you can compile libpng with PNG_SETJMP_NOT_SUPPORTED, in which case

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



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

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

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

opened in binary mode.  Again, if you wish to handle writing data in

another way, see the discussion on libpng I/O handling in the Customizing

Libpng section below.



    png_init_io(png_ptr, fp);



If you are embedding your PNG into a datastream such as MNG, and don't

want libpng to write the 8-byte signature, or if you have already

written the signature in your application, use



    png_set_sig_bytes(png_ptr, 8);



to inform libpng that it should not write a signature.



Write callbacks



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

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

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

You must supply a function



    void write_row_callback(png_ptr, png_uint_32 row,

       int pass);

    {

      /* put your code here */

    }



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



To inform libpng about your function, use



    png_set_write_status_fn(png_ptr, write_row_callback);



You now have the option of modifying how the compression library will

run.  The following functions are mainly for testing, but may be useful

in some cases, like if you need to write PNG files extremely fast and

are willing to give up some compression, or if you want to get the

maximum possible compression at the expense of slower writing.  If you

have no special needs in this area, let the library do what it wants by

not calling this function at all, as it has been tuned to deliver a good

speed/compression ratio. The second parameter to png_set_filter() is

the filter method, for which the only valid values are 0 (as of the

July 1999 PNG specification, version 1.2) or 64 (if you are writing

a PNG datastream that is to be embedded in a MNG datastream).  The third

parameter is a flag that indicates which filter type(s) are to be tested

for each scanline.  See the PNG specification for details on the specific filter

types.





    /* turn on or off filtering, and/or choose

       specific filters.  You can use either a single

       PNG_FILTER_VALUE_NAME or the bitwise OR of one

       or more PNG_FILTER_NAME masks. */

    png_set_filter(png_ptr, 0,

       PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |

       PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |

       PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |

       PNG_FILTER_AVE   | PNG_FILTER_VALUE_AVE  |

       PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|

       PNG_ALL_FILTERS);



If an application

wants to start and stop using particular filters during compression,

it should start out with all of the filters (to ensure that the previous

row of pixels will be stored in case it's needed later), and then add

and remove them after the start of compression.



If you are writing a PNG datastream that is to be embedded in a MNG

datastream, the second parameter can be either 0 or 64.



The png_set_compression_*() functions interface to the zlib compression

library, and should mostly be ignored unless you really know what you are

doing.  The only generally useful call is png_set_compression_level()

which changes how much time zlib spends on trying to compress the image

data.  See the Compression Library (zlib.h and algorithm.txt, distributed

with zlib) for details on the compression levels.



    /* set the zlib compression level */

    png_set_compression_level(png_ptr,

        Z_BEST_COMPRESSION);



    /* set other zlib parameters */

    png_set_compression_mem_level(png_ptr, 8);

    png_set_compression_strategy(png_ptr,

        Z_DEFAULT_STRATEGY);

    png_set_compression_window_bits(png_ptr, 15);

    png_set_compression_method(png_ptr, 8);

    png_set_compression_buffer_size(png_ptr, 8192)



extern PNG_EXPORT(void,png_set_zbuf_size)



Setting the contents of info for output



You now need to fill in the png_info structure with all the data you

wish to write before the actual image.  Note that the only thing you

are allowed to write after the image is the text chunks and the time

chunk (as of PNG Specification 1.2, anyway).  See png_write_end() and

the latest PNG specification for more information on that.  If you

wish to write them before the image, fill them in now, and flag that

data as being valid.  If you want to wait until after the data, don't

fill them until png_write_end().  For all the fields in png_info and

their data types, see png.h.  For explanations of what the fields

contain, see the PNG specification.



Some of the more important parts of the png_info are:



    png_set_IHDR(png_ptr, info_ptr, width, height,

       bit_depth, color_type, interlace_type,

       compression_type, filter_method)

    width          - holds the width of the image

                     in pixels (up to 2^31).

    height         - holds the height of the image

                     in pixels (up to 2^31).

    bit_depth      - holds the bit depth of one of the

                     image channels.

                     (valid values are 1, 2, 4, 8, 16

                     and depend also on the

                     color_type.  See also significant

                     bits (sBIT) below).

    color_type     - describes which color/alpha

                     channels are present.

                     PNG_COLOR_TYPE_GRAY

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

                     PNG_COLOR_TYPE_GRAY_ALPHA

                        (bit depths 8, 16)

                     PNG_COLOR_TYPE_PALETTE

                        (bit depths 1, 2, 4, 8)

                     PNG_COLOR_TYPE_RGB

                        (bit_depths 8, 16)

                     PNG_COLOR_TYPE_RGB_ALPHA

                        (bit_depths 8, 16)



                     PNG_COLOR_MASK_PALETTE

                     PNG_COLOR_MASK_COLOR

                     PNG_COLOR_MASK_ALPHA



    interlace_type - PNG_INTERLACE_NONE or

                     PNG_INTERLACE_ADAM7

    compression_type - (must be

                     PNG_COMPRESSION_TYPE_DEFAULT)

    filter_method  - (must be PNG_FILTER_TYPE_DEFAULT

                     or, if you are writing a PNG to

                     be embedded in a MNG datastream,

                     can also be

                     PNG_INTRAPIXEL_DIFFERENCING)



    png_set_PLTE(png_ptr, info_ptr, palette,

       num_palette);

    palette        - the palette for the file

                     (array of png_color)

    num_palette    - number of entries in the palette



    png_set_gAMA(png_ptr, info_ptr, gamma);

    gamma          - the gamma the image was created

                     at (PNG_INFO_gAMA)



    png_set_sRGB(png_ptr, info_ptr, srgb_intent);

    srgb_intent    - the rendering intent

                     (PNG_INFO_sRGB) The presence of

                     the sRGB chunk means that the pixel

                     data is in the sRGB color space.

                     This chunk also implies specific

                     values of gAMA and cHRM.  Rendering

                     intent is the CSS-1 property that

                     has been defined by the International

                     Color Consortium

                     (http://www.color.org).

                     It can be one of

                     PNG_sRGB_INTENT_SATURATION,

                     PNG_sRGB_INTENT_PERCEPTUAL,

                     PNG_sRGB_INTENT_ABSOLUTE, or

                     PNG_sRGB_INTENT_RELATIVE.





    png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,

       srgb_intent);

    srgb_intent    - the rendering intent

                     (PNG_INFO_sRGB) The presence of the

                     sRGB chunk means that the pixel

                     data is in the sRGB color space.

                     This function also causes gAMA and

                     cHRM chunks with the specific values

                     that are consistent with sRGB to be

                     written.



    png_set_iCCP(png_ptr, info_ptr, name, compression_type,

                      profile, proflen);

    name            - The profile name.

    compression     - The compression type; always

                      PNG_COMPRESSION_TYPE_BASE for PNG 1.0.

                      You may give NULL to this argument to

                      ignore it.

    profile         - International Color Consortium color

                      profile data. May contain NULs.

    proflen         - length of profile data in bytes.



    png_set_sBIT(png_ptr, info_ptr, sig_bit);

    sig_bit        - the number of significant bits for

                     (PNG_INFO_sBIT) each of the gray, red,

                     green, and blue channels, whichever are

                     appropriate for the given color type

                     (png_color_16)



    png_set_tRNS(png_ptr, info_ptr, trans, num_trans,

       trans_values);

    trans          - array of transparent entries for

                     palette (PNG_INFO_tRNS)

    trans_values   - graylevel or color sample values of

                     the single transparent color for

                     non-paletted images (PNG_INFO_tRNS)

    num_trans      - number of transparent entries

                     (PNG_INFO_tRNS)



    png_set_hIST(png_ptr, info_ptr, hist);

                    (PNG_INFO_hIST)

    hist           - histogram of palette (array of

                     png_uint_16)



    png_set_tIME(png_ptr, info_ptr, mod_time);

    mod_time       - time image was last modified

                     (PNG_VALID_tIME)



    png_set_bKGD(png_ptr, info_ptr, background);

    background     - background color (PNG_VALID_bKGD)



    png_set_text(png_ptr, info_ptr, text_ptr, num_text);

    text_ptr       - array of png_text holding image

                     comments

    text_ptr[i].compression - type of compression used

                 on "text" PNG_TEXT_COMPRESSION_NONE

                           PNG_TEXT_COMPRESSION_zTXt

                           PNG_ITXT_COMPRESSION_NONE

                           PNG_ITXT_COMPRESSION_zTXt

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

                 1-79 characters.

    text_ptr[i].text  - text comments for current

                         keyword.  Can be NULL or empty.

    text_ptr[i].text_length - length of text string,

                 after decompression, 0 for iTXt

    text_ptr[i].itxt_length - length of itxt string,

                 after decompression, 0 for tEXt/zTXt

    text_ptr[i].lang  - language of comment (NULL or

                         empty for unknown).

    text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL

                         or empty for unknown).

    num_text       - number of comments



    png_set_sPLT(png_ptr, info_ptr, &palette_ptr,

       num_spalettes);

    palette_ptr    - array of png_sPLT_struct structures

                     to be added to the list of palettes

                     in the info structure.

    num_spalettes  - number of palette structures to be

                     added.



    png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,

        unit_type);

    offset_x  - positive offset from the left

                     edge of the screen

    offset_y  - positive offset from the top

                     edge of the screen

    unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER



    png_set_pHYs(png_ptr, info_ptr, res_x, res_y,

        unit_type);

    res_x       - pixels/unit physical resolution

                  in x direction

    res_y       - pixels/unit physical resolution

                  in y direction

    unit_type   - PNG_RESOLUTION_UNKNOWN,

                  PNG_RESOLUTION_METER



    png_set_sCAL(png_ptr, info_ptr, unit, width, height)

    unit        - physical scale units (an integer)

    width       - width of a pixel in physical scale units

    height      - height of a pixel in physical scale units

                  (width and height are doubles)



    png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)

    unit        - physical scale units (an integer)

    width       - width of a pixel in physical scale units

    height      - height of a pixel in physical scale units

                 (width and height are strings like "2.54")



    png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,

       num_unknowns)

    unknowns          - array of png_unknown_chunk

                        structures holding unknown chunks

    unknowns[i].name  - name of unknown chunk

    unknowns[i].data  - data of unknown chunk

    unknowns[i].size  - size of unknown chunk's data

    unknowns[i].location - position to write chunk in file

                           0: do not write chunk

                           PNG_HAVE_IHDR: before PLTE

                           PNG_HAVE_PLTE: before IDAT

                           PNG_AFTER_IDAT: after IDAT



The "location" member is set automatically according to

what part of the output file has already been written.

You can change its value after calling png_set_unknown_chunks()

as demonstrated in pngtest.c.  Within each of the "locations",

the chunks are sequenced according to their position in the

structure (that is, the value of "i", which is the order in which

the chunk was either read from the input file or defined with

png_set_unknown_chunks).



A quick word about text and num_text.  text is an array of png_text

structures.  num_text is the number of valid structures in the array.

Each png_text structure holds a language code, a keyword, a text value,

and a compression type.



The compression types have the same valid numbers as the compression

types of the image data.  Currently, the only valid number is zero.

However, you can store text either compressed or uncompressed, unlike

images, which always have to be compressed.  So if you don't want the

text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.

Because tEXt and zTXt chunks don't have a language field, if you

specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt

any language code or translated keyword will not be written out.



Until text gets around 1000 bytes, it is not worth compressing it.

After the text has been written out to the file, the compression type

is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,

so that it isn't written out again at the end (in case you are calling

png_write_end() with the same struct.



The keywords that are given in the PNG Specification are:



    Title            Short (one line) title or

                     caption for image

    Author           Name of image's creator

    Description      Description of image (possibly long)

    Copyright        Copyright notice

    Creation Time    Time of original image creation

                     (usually RFC 1123 format, see below)

    Software         Software used to create the image

    Disclaimer       Legal disclaimer

    Warning          Warning of nature of content

    Source           Device used to create the image

    Comment          Miscellaneous comment; conversion

                     from other image format



The keyword-text pairs work like this.  Keywords should be short

simple descriptions of what the comment is about.  Some typical

keywords are found in the PNG specification, as is some recommendations

on keywords.  You can repeat keywords in a file.  You can even write

some text before the image and some after.  For example, you may want

to put a description of the image before the image, but leave the

disclaimer until after, so viewers working over modem connections

don't have to wait for the disclaimer to go over the modem before

they start seeing the image.  Finally, keywords should be full

words, not abbreviations.  Keywords and text are in the ISO 8859-1

(Latin-1) character set (a superset of regular ASCII) and can not

contain NUL characters, and should not contain control or other

unprintable characters.  To make the comments widely readable, stick

with basic ASCII, and avoid machine specific character set extensions

like the IBM-PC character set.  The keyword must be present, but

you can leave off the text string on non-compressed pairs.

Compressed pairs must have a text string, as only the text string

is compressed anyway, so the compression would be meaningless.



PNG supports modification time via the png_time structure.  Two

conversion routines are provided, png_convert_from_time_t() for

time_t and png_convert_from_struct_tm() for struct tm.  The

time_t routine uses gmtime().  You don't have to use either of

these, but if you wish to fill in the png_time structure directly,

you should provide the time in universal time (GMT) if possible

instead of your local time.  Note that the year number is the full

year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and

that months start with 1.



If you want to store the time of the original image creation, you should

use a plain tEXt chunk with the "Creation Time" keyword.  This is

necessary because the "creation time" of a PNG image is somewhat vague,

depending on whether you mean the PNG file, the time the image was

created in a non-PNG format, a still photo from which the image was

scanned, or possibly the subject matter itself.  In order to facilitate

machine-readable dates, it is recommended that the "Creation Time"

tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),

although this isn't a requirement.  Unlike the tIME chunk, the

"Creation Time" tEXt chunk is not expected to be automatically changed

by the software.  To facilitate the use of RFC 1123 dates, a function

png_convert_to_rfc1123(png_timep) is provided to convert from PNG

time to an RFC 1123 format string.



Writing unknown chunks



You can use the png_set_unknown_chunks function to queue up chunks

for writing.  You give it a chunk name, raw data, and a size; that's

all there is to it.  The chunks will be written by the next following

png_write_info_before_PLTE, png_write_info, or png_write_end function.

Any chunks previously read into the info structure's unknown-chunk

list will also be written out in a sequence that satisfies the PNG

specification's ordering rules.



The high-level write interface



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

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

You can use the high-level interface if your image data is present

in the info structure.  All defined output

transformations are permitted, enabled by the following masks.



    PNG_TRANSFORM_IDENTITY      No transformation

    PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples

    PNG_TRANSFORM_PACKSWAP      Change order of packed

                                pixels to LSB first

    PNG_TRANSFORM_INVERT_MONO   Invert monochrome images

    PNG_TRANSFORM_SHIFT         Normalize pixels to the

                                sBIT depth

    PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA

                                to BGRA

    PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA

                                to AG

    PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity

                                to transparency

    PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples

    PNG_TRANSFORM_STRIP_FILLER  Strip out filler bytes.



If you have valid image data in the info structure (you can use

png_set_rows() to put image data in the info structure), simply do this:



    png_write_png(png_ptr, info_ptr, png_transforms, NULL)



where png_transforms is an integer containing the bitwise OR of some set of

transformation flags.  This call is equivalent to png_write_info(),

followed the set of transformations indicated by the transform mask,

then png_write_image(), and finally png_write_end().



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

to transformation parameters required by some future output transform.)



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

when you use png_write_png().



The low-level write interface



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

write all the file information up to the actual image data.  You do

this with a call to png_write_info().



    png_write_info(png_ptr, info_ptr);



Note that there is one transformation you may need to do before

png_write_info().  In PNG files, the alpha channel in an image is the

level of opacity.  If your data is supplied as a level of

transparency, you can invert the alpha channel before you write it, so

that 0 is fully transparent and 255 (in 8-bit or paletted images) or

65535 (in 16-bit images) is fully opaque, with



    png_set_invert_alpha(png_ptr);



This must appear before png_write_info() instead of later with the

other transformations because in the case of paletted images the tRNS

chunk data has to be inverted before the tRNS chunk is written.  If

your image is not a paletted image, the tRNS data (which in such cases

represents a single color to be rendered as transparent) won't need to

be changed, and you can safely do this transformation after your

png_write_info() call.



If you need to write a private chunk that you want to appear before

the PLTE chunk when PLTE is present, you can write the PNG info in

two steps, and insert code to write your own chunk between them:



    png_write_info_before_PLTE(png_ptr, info_ptr);

    png_set_unknown_chunks(png_ptr, info_ptr, ...);

    png_write_info(png_ptr, info_ptr);



After you've written the file information, you can set up the library

to handle any special transformations of the image data.  The various

ways to transform the data will be described in the order that they

should occur.  This is important, as some of these change the color

type and/or bit depth of the data, and some others only work on

certain color types and bit depths.  Even though each transformation

checks to see if it has data that it can do something with, you should

make sure to only enable a transformation if it will be valid for the

data.  For example, don't swap red and blue on grayscale data.



PNG files store RGB pixels packed into 3 or 6 bytes.  This code tells

the library to strip input data that has 4 or 8 bytes per pixel down

to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2

bytes per pixel).



    png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);



where the 0 is unused, and the location is either PNG_FILLER_BEFORE or

PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel

is stored XRGB or RGBX.



PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as

they can, resulting in, for example, 8 pixels per byte for 1 bit files.

If the data is supplied at 1 pixel per byte, use this code, which will

correctly pack the pixels into a single byte:



    png_set_packing(png_ptr);



PNG files reduce possible bit depths to 1, 2, 4, 8, and 16.  If your

data is of another bit depth, you can write an sBIT chunk into the

file so that decoders can recover the original data if desired.



    /* Set the true bit depth of the image data */

    if (color_type & PNG_COLOR_MASK_COLOR)

    {

        sig_bit.red = true_bit_depth;

        sig_bit.green = true_bit_depth;

        sig_bit.blue = true_bit_depth;

    }

    else

    {

        sig_bit.gray = true_bit_depth;

    }

    if (color_type & PNG_COLOR_MASK_ALPHA)

    {

        sig_bit.alpha = true_bit_depth;

    }



    png_set_sBIT(png_ptr, info_ptr, &sig_bit);



If the data is stored in the row buffer in a bit depth other than

one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),

this will scale the values to appear to be the correct bit depth as

is required by PNG.



    png_set_shift(png_ptr, &sig_bit);



PNG files store 16 bit pixels in network byte order (big-endian,

ie. most significant bits first).  This code would be used if they are

supplied the other way (little-endian, i.e. least significant bits

first, the way PCs store them):



    if (bit_depth > 8)

       png_set_swap(png_ptr);



If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you

need to change the order the pixels are packed into bytes, you can use:



    if (bit_depth < 8)

       png_set_packswap(png_ptr);



PNG files store 3 color pixels in red, green, blue order.  This code

would be used if they are supplied as blue, green, red:



    png_set_bgr(png_ptr);



PNG files describe monochrome as black being zero and white being

one. This code would be used if the pixels are supplied with this reversed

(black being one and white being zero):



    png_set_invert_mono(png_ptr);



Finally, you can write your own transformation function if none of

the existing ones meets your needs.  This is done by setting a callback

with



    png_set_write_user_transform_fn(png_ptr,

       write_transform_fn);



You must supply the function



    void write_transform_fn(png_ptr ptr, row_info_ptr

       row_info, png_bytep data)



See pngtest.c for a working example.  Your function will be called

before any of the other transformations are processed.



You can also set up a pointer to a user structure for use by your

callback function.



    png_set_user_transform_info(png_ptr, user_ptr, 0, 0);



The user_channels and user_depth parameters of this function are ignored

when writing; you can set them to zero as shown.



You can retrieve the pointer via the function png_get_user_transform_ptr().

For example:



    voidp write_user_transform_ptr =

       png_get_user_transform_ptr(png_ptr);



It is possible to have libpng flush any pending output, either manually,

or automatically after a certain number of lines have been written.  To

flush the output stream a single time call:



    png_write_flush(png_ptr);



and to have libpng flush the output stream periodically after a certain

number of scanlines have been written, call:



    png_set_flush(png_ptr, nrows);



Note that the distance between rows is from the last time png_write_flush()

was called, or the first row of the image if it has never been called.

So if you write 50 lines, and then png_set_flush 25, it will flush the

output on the next scanline, and every 25 lines thereafter, unless

png_write_flush() is called before 25 more lines have been written.

If nrows is too small (less than about 10 lines for a 640 pixel wide

RGB image) the image compression may decrease noticeably (although this

may be acceptable for real-time applications).  Infrequent flushing will

only degrade the compression performance by a few percent over images

that do not use flushing.



Writing the image data



That's it for the transformations.  Now you can write the image data.

The simplest way to do this is in one function call.  If you have the

whole image in memory, you can just call png_write_image() and libpng

will write the image.  You will need to pass in an array of pointers to

each row.  This function automatically handles interlacing, so you don't

need to call png_set_interlace_handling() or call this function multiple

times, or any of that other stuff necessary with png_write_rows().



    png_write_image(png_ptr, row_pointers);



where row_pointers is:



    png_byte *row_pointers[height];



You can point to void or char or whatever you use for pixels.



If you don't want to write the whole image at once, you can

use png_write_rows() instead.  If the file is not interlaced,

this is simple:



    png_write_rows(png_ptr, row_pointers,

       number_of_rows);



row_pointers is the same as in the png_write_image() call.



If you are just writing one row at a time, you can do this with

a single row_pointer instead of an array of row_pointers:



    png_bytep row_pointer = row;



    png_write_row(png_ptr, row_pointer);



When the file is interlaced, things can get a good deal more

complicated.  The only currently (as of the PNG Specification

version 1.2, dated July 1999) defined interlacing scheme for PNG files

is the "Adam7" interlace scheme, that breaks down an

image into seven smaller images of varying size.  libpng will build

these images for you, or you can do them yourself.  If you want to

build them yourself, see the PNG specification for details of which

pixels to write when.



If you don't want libpng to handle the interlacing details, just

use png_set_interlace_handling() and call png_write_rows() the

correct number of times to write all seven sub-images.



If you want libpng to build the sub-images, call this before you start

writing any rows:



    number_of_passes =

       png_set_interlace_handling(png_ptr);



This will return the number of passes needed.  Currently, this

is seven, but may change if another interlace type is added.



Then write the complete image number_of_passes times.



    png_write_rows(png_ptr, row_pointers,

       number_of_rows);



As some of these rows are not used, and thus return immediately,

you may want to read about interlacing in the PNG specification,

and only update the rows that are actually used.



Finishing a sequential write



After you are finished writing the image, you should finish writing

the file.  If you are interested in writing comments or time, you should

pass an appropriately filled png_info pointer.  If you are not interested,

you can pass NULL.



    png_write_end(png_ptr, info_ptr);



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



    png_destroy_write_struct(&png_ptr, &info_ptr);



It is also possible to individually free the info_ptr members that

point to libpng-allocated storage with the following function:



    png_free_data(png_ptr, info_ptr, mask, seq)

    mask  - identifies data to be freed, a mask

            containing the bitwise OR of one or

            more of

              PNG_FREE_PLTE, PNG_FREE_TRNS,

              PNG_FREE_HIST, PNG_FREE_ICCP,

              PNG_FREE_PCAL, PNG_FREE_ROWS,

              PNG_FREE_SCAL, PNG_FREE_SPLT,

              PNG_FREE_TEXT, PNG_FREE_UNKN,

            or simply PNG_FREE_ALL

    seq   - sequence number of item to be freed

            (-1 for all items)



This function may be safely called when the relevant storage has

already been freed, or has not yet been allocated, or was allocated

by the user  and not by libpng,  and will in those

cases do nothing.  The "seq" parameter is ignored if only one item

of the selected data type, such as PLTE, is allowed.  If "seq" is not

-1, and multiple items are allowed for the data type identified in

the mask, such as text or sPLT, only the n'th item in the structure

is freed, where n is "seq".



If you allocated data such as a palette that you passed

in to libpng with png_set_*, you must not free it until just before the call to

png_destroy_write_struct().



The default behavior is only to free data that was allocated internally

by libpng.  This can be changed, so that libpng will not free the data,

or so that it will free data that was allocated by the user with png_malloc()

or png_zalloc() and passed in via a png_set_*() function, with



    png_data_freer(png_ptr, info_ptr, freer, mask)

    mask   - which data elements are affected

             same choices as in png_free_data()

    freer  - one of

               PNG_DESTROY_WILL_FREE_DATA

               PNG_SET_WILL_FREE_DATA

               PNG_USER_WILL_FREE_DATA



For example, to transfer responsibility for some data from a read structure

to a write structure, you could use



    png_data_freer(read_ptr, read_info_ptr,

       PNG_USER_WILL_FREE_DATA,

       PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

    png_data_freer(write_ptr, write_info_ptr,

       PNG_DESTROY_WILL_FREE_DATA,

       PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)



thereby briefly reassigning responsibility for freeing to the user but

immediately afterwards reassigning it once more to the write_destroy

function.  Having done this, it would then be safe to destroy the read

structure and continue to use the PLTE, tRNS, and hIST data in the write

structure.



This function only affects data that has already been allocated.

You can call this function before calling after the png_set_*() functions

to control whether the user or png_destroy_*() is supposed to free the data.

When the user assumes responsibility for libpng-allocated data, the

application must use

png_free() to free it, and when the user transfers responsibility to libpng

for data that the user has allocated, the user must have used png_malloc()

or png_zalloc() to allocate it.



If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword

separately, do not transfer responsibility for freeing text_ptr to libpng,

because when libpng fills a png_text structure it combines these members with

the key member, and png_free_data() will free only text_ptr.key.  Similarly,

if you transfer responsibility for free'ing text_ptr from libpng to your

application, your application must not separately free those members.

For a more compact example of writing a PNG image, see the file example.c.



V. Modifying/Customizing libpng:



There are three issues here.  The first is changing how libpng does

standard things like memory allocation, input/output, and error handling.

The second deals with more complicated things like adding new chunks,

adding new transformations, and generally changing how libpng works.

Both of those are compile-time issues; that is, they are generally

determined at the time the code is written, and there is rarely a need

to provide the user with a means of changing them.  The third is a

run-time issue:  choosing between and/or tuning one or more alternate

versions of computationally intensive routines; specifically, optimized

assembly-language (and therefore compiler- and platform-dependent)

versions.



Memory allocation, input/output, and error handling



All of the memory allocation, input/output, and error handling in libpng

goes through callbacks that are user-settable.  The default routines are

in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively.  To change

these functions, call the appropriate png_set_*_fn() function.



Memory allocation is done through the functions png_malloc()

and png_free().  These currently just call the standard C functions.  If

your pointers can't access more then 64K at a time, you will want to set

MAXSEG_64K in zlib.h.  Since it is unlikely that the method of handling

memory allocation on a platform will change between applications, these

functions must be modified in the library at compile time.  If you prefer

to use a different method of allocating and freeing data, you can use

png_create_read_struct_2() or png_create_write_struct_2() to register

your own functions as described above.

These functions also provide a void pointer that can be retrieved via



    mem_ptr=png_get_mem_ptr(png_ptr);



Your replacement memory functions must have prototypes as follows:



    png_voidp malloc_fn(png_structp png_ptr,

       png_size_t size);

    void free_fn(png_structp png_ptr, png_voidp ptr);



Your malloc_fn() must return NULL in case of failure.  The png_malloc()

function will normally call png_error() if it receives a NULL from the

system memory allocator or from your replacement malloc_fn().



Input/Output in libpng is done through png_read() and png_write(),

which currently just call fread() and fwrite().  The FILE * is stored in

png_struct and is initialized via png_init_io().  If you wish to change

the method of I/O, the library supplies callbacks that you can set

through the function png_set_read_fn() and png_set_write_fn() at run

time, instead of calling the png_init_io() function.  These functions

also provide a void pointer that can be retrieved via the function

png_get_io_ptr().  For example:



    png_set_read_fn(png_structp read_ptr,

        voidp read_io_ptr, png_rw_ptr read_data_fn)



    png_set_write_fn(png_structp write_ptr,

        voidp write_io_ptr, png_rw_ptr write_data_fn,

        png_flush_ptr output_flush_fn);



    voidp read_io_ptr = png_get_io_ptr(read_ptr);

    voidp write_io_ptr = png_get_io_ptr(write_ptr);



The replacement I/O functions must have prototypes as follows:



    void user_read_data(png_structp png_ptr,

        png_bytep data, png_size_t length);

    void user_write_data(png_structp png_ptr,

        png_bytep data, png_size_t length);

    void user_flush_data(png_structp png_ptr);



Supplying NULL for the read, write, or flush functions sets them back

to using the default C stream functions.  It is an error to read from

a write stream, and vice versa.



Error handling in libpng is done through png_error() and png_warning().

Errors handled through png_error() are fatal, meaning that png_error()

should never return to its caller.  Currently, this is handled via

setjmp() and longjmp() (unless you have compiled libpng with

PNG_SETJMP_NOT_SUPPORTED, in which case it is handled via PNG_ABORT()),

but you could change this to do things like exit() if you should wish.



On non-fatal errors, png_warning() is called

to print a warning message, and then control returns to the calling code.

By default png_error() and png_warning() print a message on stderr via

fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined

(because you don't want the messages) or PNG_NO_STDIO defined (because

fprintf() isn't available).  If you wish to change the behavior of the error

functions, you will need to set up your own message callbacks.  These

functions are normally supplied at the time that the png_struct is created.

It is also possible to redirect errors and warnings to your own replacement

functions after png_create_*_struct() has been called by calling:



    png_set_error_fn(png_structp png_ptr,

        png_voidp error_ptr, png_error_ptr error_fn,

        png_error_ptr warning_fn);



    png_voidp error_ptr = png_get_error_ptr(png_ptr);



If NULL is supplied for either error_fn or warning_fn, then the libpng

default function will be used, calling fprintf() and/or longjmp() if a

problem is encountered.  The replacement error functions should have

parameters as follows:



    void user_error_fn(png_structp png_ptr,

        png_const_charp error_msg);

    void user_warning_fn(png_structp png_ptr,

        png_const_charp warning_msg);



The motivation behind using setjmp() and longjmp() is the C++ throw and

catch exception handling methods.  This makes the code much easier to write,

as there is no need to check every return code of every function call.

However, there are some uncertainties about the status of local variables

after a longjmp, so the user may want to be careful about doing anything after

setjmp returns non-zero besides returning itself.  Consult your compiler

documentation for more details.  For an alternative approach, you may wish

to use the "cexcept" facility (see http://cexcept.sourceforge.net).



Custom chunks



If you need to read or write custom chunks, you may need to get deeper

into the libpng code.  The library now has mechanisms for storing

and writing chunks of unknown type; you can even declare callbacks

for custom chunks.  However, this may not be good enough if the

library code itself needs to know about interactions between your

chunk and existing `intrinsic' chunks.



If you need to write a new intrinsic chunk, first read the PNG

specification. Acquire a first level of

understanding of how it works.  Pay particular attention to the

sections that describe chunk names, and look at how other chunks were

designed, so you can do things similarly.  Second, check out the

sections of libpng that read and write chunks.  Try to find a chunk

that is similar to yours and use it as a template.  More details can

be found in the comments inside the code.  It is best to handle unknown

chunks in a generic method, via callback functions, instead of by

modifying libpng functions.



If you wish to write your own transformation for the data, look through

the part of the code that does the transformations, and check out some of

the simpler ones to get an idea of how they work.  Try to find a similar

transformation to the one you want to add and copy off of it.  More details

can be found in the comments inside the code itself.



Configuring for 16 bit platforms



You will want to look into zconf.h to tell zlib (and thus libpng) that

it cannot allocate more then 64K at a time.  Even if you can, the memory

won't be accessible.  So limit zlib and libpng to 64K by defining MAXSEG_64K.



Configuring for DOS



For DOS users who only have access to the lower 640K, you will

have to limit zlib's memory usage via a png_set_compression_mem_level()

call.  See zlib.h or zconf.h in the zlib library for more information.



Configuring for Medium Model



Libpng's support for medium model has been tested on most of the popular

compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets

defined, and FAR gets defined to far in pngconf.h, and you should be

all set.  Everything in the library (except for zlib's structure) is

expecting far data.  You must use the typedefs with the p or pp on

the end for pointers (or at least look at them and be careful).  Make

note that the rows of data are defined as png_bytepp, which is an

unsigned char far * far *.



Configuring for gui/windowing platforms:



You will need to write new error and warning functions that use the GUI

interface, as described previously, and set them to be the error and

warning functions at the time that png_create_*_struct() is called,

in order to have them available during the structure initialization.

They can be changed later via png_set_error_fn().  On some compilers,

you may also have to change the memory allocators (png_malloc, etc.).



Configuring for compiler xxx:



All includes for libpng are in pngconf.h.  If you need to add/change/delete

an include, this is the place to do it.  The includes that are not

needed outside libpng are protected by the PNG_INTERNAL definition,

which is only defined for those routines inside libpng itself.  The

files in libpng proper only include png.h, which includes pngconf.h.



Configuring zlib:



There are special functions to configure the compression.  Perhaps the

most useful one changes the compression level, which currently uses

input compression values in the range 0 - 9.  The library normally

uses the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests

have shown that for a large majority of images, compression values in

the range 3-6 compress nearly as well as higher levels, and do so much

faster.  For online applications it may be desirable to have maximum speed

(Z_BEST_SPEED = 1).  With versions of zlib after v0.99, you can also

specify no compression (Z_NO_COMPRESSION = 0), but this would create

files larger than just storing the raw bitmap.  You can specify the

compression level by calling:



    png_set_compression_level(png_ptr, level);



Another useful one is to reduce the memory level used by the library.

The memory level defaults to 8, but it can be lowered if you are

short on memory (running DOS, for example, where you only have 640K).

Note that the memory level does have an effect on compression; among

other things, lower levels will result in sections of incompressible

data being emitted in smaller stored blocks, with a correspondingly

larger relative overhead of up to 15% in the worst case.



    png_set_compression_mem_level(png_ptr, level);



The other functions are for configuring zlib.  They are not recommended

for normal use and may result in writing an invalid PNG file.  See

zlib.h for more information on what these mean.



    png_set_compression_strategy(png_ptr,

        strategy);

    png_set_compression_window_bits(png_ptr,

        window_bits);

    png_set_compression_method(png_ptr, method);

    png_set_compression_buffer_size(png_ptr, size);



Controlling row filtering



If you want to control whether libpng uses filtering or not, which

filters are used, and how it goes about picking row filters, you

can call one of these functions.  The selection and configuration

of row filters can have a significant impact on the size and

encoding speed and a somewhat lesser impact on the decoding speed

of an image.  Filtering is enabled by default for RGB and grayscale

images (with and without alpha), but not for paletted images nor

for any images with bit depths less than 8 bits/pixel.



The 'method' parameter sets the main filtering method, which is

currently only '0' in the PNG 1.2 specification.  The 'filters'

parameter sets which filter(s), if any, should be used for each

scanline.  Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS

to turn filtering on and off, respectively.



Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,

PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise

ORed together with '|' to specify one or more filters to use.

These filters are described in more detail in the PNG specification.

If you intend to change the filter type during the course of writing

the image, you should start with flags set for all of the filters

you intend to use so that libpng can initialize its internal

structures appropriately for all of the filter types.  (Note that this

means the first row must always be adaptively filtered, because libpng

currently does not allocate the filter buffers until png_write_row()

is called for the first time.)



    filters = PNG_FILTER_NONE | PNG_FILTER_SUB

              PNG_FILTER_UP | PNG_FILTER_AVE |

              PNG_FILTER_PAETH | PNG_ALL_FILTERS;



    png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,

       filters);

              The second parameter can also be

              PNG_INTRAPIXEL_DIFFERENCING if you are

              writing a PNG to be embedded in a MNG

              datastream.  This parameter must be the

              same as the value of filter_method used

              in png_set_IHDR().



It is also possible to influence how libpng chooses from among the

available filters.  This is done in one or both of two ways - by

telling it how important it is to keep the same filter for successive

rows, and by telling it the relative computational costs of the filters.



    double weights[3] = {1.5, 1.3, 1.1},

       costs[PNG_FILTER_VALUE_LAST] =

       {1.0, 1.3, 1.3, 1.5, 1.7};



    png_set_filter_heuristics(png_ptr,

       PNG_FILTER_HEURISTIC_WEIGHTED, 3,

       weights, costs);



The weights are multiplying factors that indicate to libpng that the

row filter should be the same for successive rows unless another row filter

is that many times better than the previous filter.  In the above example,

if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a

"sum of absolute differences" 1.5 x 1.3 times higher than other filters

and still be chosen, while the NONE filter could have a sum 1.1 times

higher than other filters and still be chosen.  Unspecified weights are

taken to be 1.0, and the specified weights should probably be declining

like those above in order to emphasize recent filters over older filters.



The filter costs specify for each filter type a relative decoding cost

to be considered when selecting row filters.  This means that filters

with higher costs are less likely to be chosen over filters with lower

costs, unless their "sum of absolute differences" is that much smaller.

The costs do not necessarily reflect the exact computational speeds of

the various filters, since this would unduly influence the final image

size.



Note that the numbers above were invented purely for this example and

are given only to help explain the function usage.  Little testing has

been done to find optimum values for either the costs or the weights.



Removing unwanted object code



There are a bunch of #define's in pngconf.h that control what parts of

libpng are compiled.  All the defines end in _SUPPORTED.  If you are

never going to use a capability, you can change the #define to #undef

before recompiling libpng and save yourself code and data space, or

you can turn off individual capabilities with defines that begin with

PNG_NO_.



You can also turn all of the transforms and ancillary chunk capabilities

off en masse with compiler directives that define

PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,

or all four,

along with directives to turn on any of the capabilities that you do

want.  The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable

the extra transformations but still leave the library fully capable of reading

and writing PNG files with all known public chunks

Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive

produces a library that is incapable of reading or writing ancillary chunks.

If you are not using the progressive reading capability, you can

turn that off with PNG_NO_PROGRESSIVE_READ (don't confuse

this with the INTERLACING capability, which you'll still have).



All the reading and writing specific code are in separate files, so the

linker should only grab the files it needs.  However, if you want to

make sure, or if you are building a stand alone library, all the

reading files start with pngr and all the writing files start with

pngw.  The files that don't match either (like png.c, pngtrans.c, etc.)

are used for both reading and writing, and always need to be included.

The progressive reader is in pngpread.c



If you are creating or distributing a dynamically linked library (a .so

or DLL file), you should not remove or disable any parts of the library,

as this will cause applications linked with different versions of the

library to fail if they call functions not available in your library.

The size of the library itself should not be an issue, because only

those sections that are actually used will be loaded into memory.



Requesting debug printout



The macro definition PNG_DEBUG can be used to request debugging

printout.  Set it to an integer value in the range 0 to 3.  Higher

numbers result in increasing amounts of debugging information.  The

information is printed to the "stderr" file, unless another file

name is specified in the PNG_DEBUG_FILE macro definition.



When PNG_DEBUG > 0, the following functions (macros) become available:



   png_debug(level, message)

   png_debug1(level, message, p1)

   png_debug2(level, message, p1, p2)



in which "level" is compared to PNG_DEBUG to decide whether to print

the message, "message" is the formatted string to be printed,

and p1 and p2 are parameters that are to be embedded in the string

according to printf-style formatting directives.  For example,



   png_debug1(2, "foo=%d\n", foo);



is expanded to



   if(PNG_DEBUG > 2)

     fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);



When PNG_DEBUG is defined but is zero, the macros aren't defined, but you

can still use PNG_DEBUG to control your own debugging:



   #ifdef PNG_DEBUG

       fprintf(stderr, ...

   #endif



When PNG_DEBUG = 1, the macros are defined, but only png_debug statements

having level = 0 will be printed.  There aren't any such statements in

this version of libpng, but if you insert some they will be printed.



VI.  Runtime optimization



A new feature in libpng 1.2.0 is the ability to dynamically switch between

standard and optimized versions of some routines.  Currently these are

limited to three computationally intensive tasks when reading PNG files:

decoding row filters, expanding interlacing, and combining interlaced or

transparent row data with previous row data.  Currently the optimized

versions are available only for x86 (Intel, AMD, etc.) platforms with

MMX support, though this may change in future versions.  (For example,

the non-MMX assembler optimizations for zlib might become similarly

runtime-selectable in future releases, in which case libpng could be

extended to support them.  Alternatively, the compile-time choice of

floating-point versus integer routines for gamma correction might become

runtime-selectable.)



Because such optimizations tend to be very platform- and compiler-dependent,

both in how they are written and in how they perform, the new runtime code

in libpng has been written to allow programs to query, enable, and disable

either specific optimizations or all such optimizations.  For example, to

enable all possible optimizations (bearing in mind that some "optimizations"

may actually run more slowly in rare cases):



    #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)

       png_uint_32 mask, flags;



       flags = png_get_asm_flags(png_ptr);

       mask = png_get_asm_flagmask(PNG_SELECT_READ | PNG_SELECT_WRITE);

       png_set_asm_flags(png_ptr, flags | mask);

    #endif



To enable only optimizations relevant to reading PNGs, use PNG_SELECT_READ

by itself when calling png_get_asm_flagmask(); similarly for optimizing

only writing.  To disable all optimizations:



    #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)

       flags = png_get_asm_flags(png_ptr);

       mask = png_get_asm_flagmask(PNG_SELECT_READ | PNG_SELECT_WRITE);

       png_set_asm_flags(png_ptr, flags & ~mask);

    #endif



To enable or disable only MMX-related features, use png_get_mmx_flagmask()

in place of png_get_asm_flagmask().  The mmx version takes one additional

parameter:



    #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)

       int selection = PNG_SELECT_READ | PNG_SELECT_WRITE;

       int compilerID;



       mask = png_get_mmx_flagmask(selection, &compilerID);

    #endif



On return, compilerID will indicate which version of the MMX assembler

optimizations was compiled.  Currently two flavors exist:  Microsoft

Visual C++ (compilerID == 1) and GNU C (a.k.a. gcc/gas, compilerID == 2).

On non-x86 platforms or on systems compiled without MMX optimizations, a

value of -1 is used.



Note that both png_get_asm_flagmask() and png_get_mmx_flagmask() return

all valid, settable optimization bits for the version of the library that's

currently in use.  In the case of shared (dynamically linked) libraries,

this may include optimizations that did not exist at the time the code was

written and compiled.  It is also possible, of course, to enable only known,

specific optimizations; for example:



    #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)

       flags = PNG_ASM_FLAG_MMX_READ_COMBINE_ROW  \

             | PNG_ASM_FLAG_MMX_READ_INTERLACE    \

             | PNG_ASM_FLAG_MMX_READ_FILTER_SUB   \

             | PNG_ASM_FLAG_MMX_READ_FILTER_UP    \

             | PNG_ASM_FLAG_MMX_READ_FILTER_AVG   \

             | PNG_ASM_FLAG_MMX_READ_FILTER_PAETH ;

       png_set_asm_flags(png_ptr, flags);

    #endif



This method would enable only the MMX read-optimizations available at the

time of libpng 1.2.0's release, regardless of whether a later version of

the DLL were actually being used.  (Also note that these functions did not

exist in versions older than 1.2.0, so any attempt to run a dynamically

linked app on such an older version would fail.)



To determine whether the processor supports MMX instructions at all, use

the png_mmx_support() function:



    #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)

       mmxsupport = png_mmx_support();

    #endif



It returns -1 if MMX support is not compiled into libpng, 0 if MMX code

is compiled but MMX is not supported by the processor, or 1 if MMX support

is fully available.  Note that png_mmx_support(), png_get_mmx_flagmask(),

and png_get_asm_flagmask() all may be called without allocating and ini-

tializing any PNG structures (for example, as part of a usage screen or

"about" box).



The following code can be used to prevent an application from using the

thread_unsafe features, even if libpng was built with PNG_THREAD_UNSAFE_OK

defined:



#if defined(PNG_USE_PNGGCCRD) && defined(PNG_ASSEMBLER_CODE_SUPPORTED) \

  && defined(PNG_THREAD_UNSAFE_OK)

    /* Disable thread-unsafe features of pnggccrd */

    if (png_access_version_number() >= 10200)

    {

      png_uint_32 mmx_disable_mask = 0;

      png_uint_32 asm_flags;



      mmx_disable_mask |= ( PNG_ASM_FLAG_MMX_READ_COMBINE_ROW  \

                          | PNG_ASM_FLAG_MMX_READ_FILTER_SUB   \

                          | PNG_ASM_FLAG_MMX_READ_FILTER_AVG   \

                          | PNG_ASM_FLAG_MMX_READ_FILTER_PAETH );

      asm_flags = png_get_asm_flags(png_ptr);

      png_set_asm_flags(png_ptr, asm_flags & ~mmx_disable_mask);

    }

#endif



For more extensive examples of runtime querying, enabling and disabling

of optimized features, see contrib/gregbook/readpng2.c in the libpng

source-code distribution.



VII.  MNG support



The MNG specification (available at http://www.libpng.org/pub/mng) allows

certain extensions to PNG for PNG images that are embedded in MNG datastreams.

Libpng can support some of these extensions.  To enable them, use the

png_permit_mng_features() function:



   feature_set = png_permit_mng_features(png_ptr, mask)

   mask is a png_uint_32 containing the bitwise OR of the

        features you want to enable.  These include

        PNG_FLAG_MNG_EMPTY_PLTE

        PNG_FLAG_MNG_FILTER_64

        PNG_ALL_MNG_FEATURES

   feature_set is a png_uint_32 that is the bitwise AND of

      your mask with the set of MNG features that is

      supported by the version of libpng that you are using.



It is an error to use this function when reading or writing a standalone

PNG file with the PNG 8-byte signature.  The PNG datastream must be wrapped

in a MNG datastream.  As a minimum, it must have the MNG 8-byte signature

and the MHDR and MEND chunks.  Libpng does not provide support for these

or any other MNG chunks; your application must provide its own support for

them.  You may wish to consider using libmng (available at

http://www.libmng.com) instead.



VIII.  Changes to Libpng from version 0.88



It should be noted that versions of libpng later than 0.96 are not

distributed by the original libpng author, Guy Schalnat, nor by

Andreas Dilger, who had taken over from Guy during 1996 and 1997, and

distributed versions 0.89 through 0.96, but rather by another member

of the original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas are

still alive and well, but they have moved on to other things.



The old libpng functions png_read_init(), png_write_init(),

png_info_init(), png_read_destroy(), and png_write_destroy() have been

moved to PNG_INTERNAL in version 0.95 to discourage their use.  These

functions will be removed from libpng version 2.0.0.



The preferred method of creating and initializing the libpng structures is

via the png_create_read_struct(), png_create_write_struct(), and

png_create_info_struct() because they isolate the size of the structures

from the application, allow version error checking, and also allow the

use of custom error handling routines during the initialization, which

the old functions do not.  The functions png_read_destroy() and

png_write_destroy() do not actually free the memory that libpng

allocated for these structs, but just reset the data structures, so they

can be used instead of png_destroy_read_struct() and

png_destroy_write_struct() if you feel there is too much system overhead

allocating and freeing the png_struct for each image read.



Setting the error callbacks via png_set_message_fn() before

png_read_init() as was suggested in libpng-0.88 is no longer supported

because this caused applications that do not use custom error functions

to fail if the png_ptr was not initialized to zero.  It is still possible

to set the error callbacks AFTER png_read_init(), or to change them with

png_set_error_fn(), which is essentially the same function, but with a new

name to force compilation errors with applications that try to use the old

method.



Starting with version 1.0.7, you can find out which version of the library

you are using at run-time:



   png_uint_32 libpng_vn = png_access_version_number();



The number libpng_vn is constructed from the major version, minor

version with leading zero, and release number with leading zero,

(e.g., libpng_vn for version 1.0.7 is 10007).



You can also check which version of png.h you used when compiling your

application:



   png_uint_32 application_vn = PNG_LIBPNG_VER;



IX. Y2K Compliance in libpng



January 31, 2007



Since the PNG Development group is an ad-hoc body, we can't make

an official declaration.



This is your unofficial assurance that libpng from version 0.71 and

upward through 1.2.16 are Y2K compliant.  It is my belief that earlier

versions were also Y2K compliant.



Libpng only has three year fields.  One is a 2-byte unsigned integer that

will hold years up to 65535.  The other two hold the date in text

format, and will hold years up to 9999.



The integer is

    "png_uint_16 year" in png_time_struct.



The strings are

    "png_charp time_buffer" in png_struct and

    "near_time_buffer", which is a local character string in png.c.



There are seven time-related functions:



    png_convert_to_rfc_1123() in png.c

      (formerly png_convert_to_rfc_1152() in error)

    png_convert_from_struct_tm() in pngwrite.c, called

      in pngwrite.c

    png_convert_from_time_t() in pngwrite.c

    png_get_tIME() in pngget.c

    png_handle_tIME() in pngrutil.c, called in pngread.c

    png_set_tIME() in pngset.c

    png_write_tIME() in pngwutil.c, called in pngwrite.c



All appear to handle dates properly in a Y2K environment.  The

png_convert_from_time_t() function calls gmtime() to convert from system

clock time, which returns (year - 1900), which we properly convert to

the full 4-digit year.  There is a possibility that applications using

libpng are not passing 4-digit years into the png_convert_to_rfc_1123()

function, or that they are incorrectly passing only a 2-digit year

instead of "year - 1900" into the png_convert_from_struct_tm() function,

but this is not under our control.  The libpng documentation has always

stated that it works with 4-digit years, and the APIs have been

documented as such.



The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned

integer to hold the year, and can hold years as large as 65535.



zlib, upon which libpng depends, is also Y2K compliant.  It contains

no date-related code.





   Glenn Randers-Pehrson

   libpng maintainer

   PNG Development Group

