example.c

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       png_voidp user_error_ptr, user_error_fn, user_warning_fn);

   if (*png_ptr == NULL)
   {
      *info_ptr = NULL;
      return (ERROR);
   }

   *info_ptr = png_create_info_struct(png_ptr);

   if (*info_ptr == NULL)
   {
      png_destroy_read_struct(png_ptr, info_ptr, 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 will need to provide all three
    * function callbacks, even if you aren't using them all.
    * 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().
    * These functions shouldn't be dependent on global or
    * static variables if you are decoding several images
    * simultaneously.  You should store stream specific data
    * in a separate struct, given as the second parameter,
    * and retrieve the pointer from inside the callbacks using
    * the function png_get_progressive_ptr(png_ptr).
    */
   png_set_progressive_read_fn(*png_ptr, (void *)stream_data,
      info_callback, row_callback, end_callback);

   return (OK);
}

int
process_data(png_structp *png_ptr, png_infop *info_ptr,
   png_bytep buffer, png_uint_32 length)
{
   if (setjmp(png_jmpbuf((*png_ptr))))
   {
      /* Free the png_ptr and info_ptr memory on error */
      png_destroy_read_struct(png_ptr, info_ptr, png_infopp_NULL);
      return (ERROR);
   }

   /* This one's new also.  Simply give it chunks of data as
    * they arrive from the data stream (in order, of course).
    * On Segmented 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, but I haven't tried with less
    * than 256 bytes yet).  When this function returns, you may
    * want to display any rows that were generated in the row
    * callback, if you aren't already displaying them there.
    */
   png_process_data(*png_ptr, *info_ptr, buffer, length);
   return (OK);
}

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.
 */
}

row_callback(png_structp png_ptr, png_bytep new_row,
   png_uint_32 row_num, int pass)
{
/*
 * This function is called for every row in the image.  If the
 * image is interlaced, and you turned on the interlace handler,
 * this function will be called for every row in every pass.
 *
 * In this function you will receive a pointer to new row data from
 * libpng called new_row that is to replace a corresponding row (of
 * the same data format) in a buffer allocated by your application.
 * 
 * The new row data pointer new_row may be NULL, indicating there is
 * no new data to be replaced (in cases of interlace loading).
 * 
 * If new_row is not NULL then you need to call
 * png_progressive_combine_row() to replace the corresponding row as
 * shown below:
 */
   /* Check if row_num is in bounds. */
   if((row_num >= 0) && (row_num < height))
   {
     /* Get pointer to corresponding row in our
      * PNG read buffer.
      */
     png_bytep old_row = ((png_bytep *)our_data)[row_num];

     /* If both rows are allocated then copy the new row
      * data to the corresponding row data.
      */
     if((old_row != NULL) && (new_row != NULL))
     png_progressive_combine_row(png_ptr, old_row, new_row);
   }
/*
 * 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 new row and the
 * old row, as demonstrated above.  You can call this function for
 * NULL rows (it will just return) and for non-interlaced images
 * (it just does the png_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 previous rows.  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 as new_row, and the function will combine
 * the old row and the new row.
 */
}

end_callback(png_structp png_ptr, png_infop info)
{
/* this function is called when 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.
 */
}

/* write a png file */
void write_png(char *file_name /* , ... other image information ... */)
{
   FILE *fp;
   png_structp png_ptr;
   png_infop info_ptr;
   png_colorp palette;

   /* open the file */
   fp = fopen(file_name, "wb");
   if (fp == NULL)
      return (ERROR);

   /* Create and initialize the png_struct with the desired error handler
    * functions.  If you want to use the default stderr and longjump method,
    * you can supply NULL for the last three parameters.  We also check that
    * the library version is compatible with the one used at compile time,
    * in case we are using dynamically linked libraries.  REQUIRED.
    */
   png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
      png_voidp user_error_ptr, user_error_fn, user_warning_fn);

   if (png_ptr == NULL)
   {
      fclose(fp);
      return (ERROR);
   }

   /* Allocate/initialize the image information data.  REQUIRED */
   info_ptr = png_create_info_struct(png_ptr);
   if (info_ptr == NULL)
   {
      fclose(fp);
      png_destroy_write_struct(&png_ptr,  png_infopp_NULL);
      return (ERROR);
   }

   /* Set error handling.  REQUIRED if you aren't supplying your own
    * error handling functions in the png_create_write_struct() call.
    */
   if (setjmp(png_jmpbuf(png_ptr)))
   {
      /* If we get here, we had a problem reading the file */
      fclose(fp);
      png_destroy_write_struct(&png_ptr, &info_ptr);
      return (ERROR);
   }

   /* One of the following I/O initialization functions is REQUIRED */
#ifdef streams /* I/O initialization method 1 */
   /* set up the output control if you are using standard C streams */
   png_init_io(png_ptr, fp);
#else no_streams /* I/O initialization method 2 */
   /* If you are using replacement read functions, instead of calling
    * png_init_io() here you would call */
   png_set_write_fn(png_ptr, (void *)user_io_ptr, user_write_fn,
      user_IO_flush_function);
   /* where user_io_ptr is a structure you want available to the callbacks */
#endif no_streams /* only use one initialization method */

#ifdef hilevel
   /* This is the easy way.  Use it if you already have all the
    * image info living info in the structure.  You could "|" many
    * PNG_TRANSFORM flags into the png_transforms integer here.
    */
   png_write_png(png_ptr, info_ptr, png_transforms, png_voidp_NULL);
#else
   /* This is the hard way */

   /* Set the image information here.  Width and height are up to 2^31,
    * bit_depth is one of 1, 2, 4, 8, or 16, but valid values also depend on
    * the color_type selected. color_type is one of PNG_COLOR_TYPE_GRAY,
    * PNG_COLOR_TYPE_GRAY_ALPHA, PNG_COLOR_TYPE_PALETTE, PNG_COLOR_TYPE_RGB,
    * or PNG_COLOR_TYPE_RGB_ALPHA.  interlace is either PNG_INTERLACE_NONE or
    * PNG_INTERLACE_ADAM7, and the compression_type and filter_type MUST
    * currently be PNG_COMPRESSION_TYPE_BASE and PNG_FILTER_TYPE_BASE. REQUIRED
    */
   png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth, PNG_COLOR_TYPE_???,
      PNG_INTERLACE_????, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);

   /* set the palette if there is one.  REQUIRED for indexed-color images */
   palette = (png_colorp)png_malloc(png_ptr, PNG_MAX_PALETTE_LENGTH
             * png_sizeof (png_color));
   /* ... set palette colors ... */
   png_set_PLTE(png_ptr, info_ptr, palette, PNG_MAX_PALETTE_LENGTH);
   /* You must not free palette here, because png_set_PLTE only makes a link to
      the palette that you malloced.  Wait until you are about to destroy
      the png structure. */

   /* optional significant bit chunk */
   /* if we are dealing with a grayscale image then */
   sig_bit.gray = true_bit_depth;
   /* otherwise, if we are dealing with a color image then */
   sig_bit.red = true_red_bit_depth;
   sig_bit.green = true_green_bit_depth;
   sig_bit.blue = true_blue_bit_depth;
   /* if the image has an alpha channel then */
   sig_bit.alpha = true_alpha_bit_depth;
   png_set_sBIT(png_ptr, info_ptr, sig_bit);


   /* Optional gamma chunk is strongly suggested if you have any guess
    * as to the correct gamma of the image.
    */
   png_set_gAMA(png_ptr, info_ptr, gamma);

   /* Optionally write comments into the image */
   text_ptr[0].key = "Title";
   text_ptr[0].text = "Mona Lisa";
   text_ptr[0].compression = PNG_TEXT_COMPRESSION_NONE;
   text_ptr[1].key = "Author";
   text_ptr[1].text = "Leonardo DaVinci";
   text_ptr[1].compression = PNG_TEXT_COMPRESSION_NONE;
   text_ptr[2].key = "Description";
   text_ptr[2].text = "<long text>";
   text_ptr[2].compression = PNG_TEXT_COMPRESSION_zTXt;
#ifdef PNG_iTXt_SUPPORTED
   text_ptr[0].lang = NULL;
   text_ptr[1].lang = NULL;
   text_ptr[2].lang = NULL;
#endif
   png_set_text(png_ptr, info_ptr, text_ptr, 3);

   /* other optional chunks like cHRM, bKGD, tRNS, tIME, oFFs, pHYs, */
   /* note that if sRGB is present the gAMA and cHRM chunks must be ignored
    * on read and must be written in accordance with the sRGB profile */

   /* Write the file header information.  REQUIRED */
   png_write_info(png_ptr, info_ptr);

   /* If you want, you can write the info in two steps, in case you need to
    * write your private chunk ahead of PLTE:
    *
    *   png_write_info_before_PLTE(write_ptr, write_info_ptr);
    *   write_my_chunk();
    *   png_write_info(png_ptr, info_ptr);
    *
    * However, given the level of known- and unknown-chunk support in 1.1.0
    * and up, this should no longer be necessary.
    */

   /* Once we write out the header, the compression type on the text
    * chunks gets changed to PNG_TEXT_COMPRESSION_NONE_WR or
    * PNG_TEXT_COMPRESSION_zTXt_WR, so it doesn't get written out again
    * at the end.
    */

   /* set up the transformations you want.  Note that these are
    * all optional.  Only call them if you want them.
    */

   /* invert monochrome pixels */
   png_set_invert_mono(png_ptr);

   /* Shift the pixels up to a legal bit depth and fill in
    * as appropriate to correctly scale the image.
    */
   png_set_shift(png_ptr, &sig_bit);

   /* pack pixels into bytes */
   png_set_packing(png_ptr);

   /* swap location of alpha bytes from ARGB to RGBA */
   png_set_swap_alpha(png_ptr);

   /* Get rid of filler (OR ALPHA) bytes, pack XRGB/RGBX/ARGB/RGBA into
    * RGB (4 channels -> 3 channels). The second parameter is not used.
    */
   png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

   /* flip BGR pixels to RGB */
   png_set_bgr(png_ptr);

   /* swap bytes of 16-bit files to most significant byte first */
   png_set_swap(png_ptr);

   /* swap bits of 1, 2, 4 bit packed pixel formats */
   png_set_packswap(png_ptr);

   /* turn on interlace handling if you are not using png_write_image() */
   if (interlacing)
      number_passes = png_set_interlace_handling(png_ptr);
   else
      number_passes = 1;

   /* The easiest way to write the image (you may have a different memory
    * layout, however, so choose what fits your needs best).  You need to
    * use the first method if you aren't handling interlacing yourself.
    */
   png_uint_32 k, height, width;
   png_byte image[height][width*bytes_per_pixel];
   png_bytep row_pointers[height];

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

   for (k = 0; k < height; k++)
     row_pointers[k] = image + k*width*bytes_per_pixel;

   /* One of the following output methods is REQUIRED */
#ifdef entire /* write out the entire image data in one call */
   png_write_image(png_ptr, row_pointers);

   /* the other way to write the image - deal with interlacing */

#else no_entire /* write out the image data by one or more scanlines */
   /* The number of passes is either 1 for non-interlaced images,
    * or 7 for interlaced images.
    */
   for (pass = 0; pass < number_passes; pass++)
   {
      /* Write a few rows at a time. */
      png_write_rows(png_ptr, &row_pointers[first_row], number_of_rows);

      /* If you are only writing one row at a time, this works */
      for (y = 0; y < height; y++)
      {
         png_write_rows(png_ptr, &row_pointers[y], 1);
      }
   }
#endif no_entire /* use only one output method */

   /* You can write optional chunks like tEXt, zTXt, and tIME at the end
    * as well.  Shouldn't be necessary in 1.1.0 and up as all the public
    * chunks are supported and you can use png_set_unknown_chunks() to
    * register unknown chunks into the info structure to be written out.
    */

   /* It is REQUIRED to call this to finish writing the rest of the file */
   png_write_end(png_ptr, info_ptr);
#endif hilevel

   /* If you png_malloced a palette, free it here (don't free info_ptr->palette,
      as recommended in versions 1.0.5m and earlier of this example; if
      libpng mallocs info_ptr->palette, libpng will free it).  If you
      allocated it with malloc() instead of png_malloc(), use free() instead
      of png_free(). */
   png_free(png_ptr, palette);
   palette=NULL;

   /* Similarly, if you png_malloced any data that you passed in with
      png_set_something(), such as a hist or trans array, free it here,
      when you can be sure that libpng is through with it. */
   png_free(png_ptr, trans);
   trans=NULL;

   /* clean up after the write, and free any memory allocated */
   png_destroy_write_struct(&png_ptr, &info_ptr);

   /* close the file */
   fclose(fp);

   /* that's it */
   return (OK);
}

#endif /* if 0 */