libpng.txt

一个国人自己实现图像库的程序（有参考价值）

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 logical 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