pngwrite.c

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      switch (filters & (PNG_ALL_FILTERS | 0x07))
      {
         case 5:
         case 6:
         case 7: png_warning(png_ptr, "Unknown row filter for method 0");
         case PNG_FILTER_VALUE_NONE:  png_ptr->do_filter=PNG_FILTER_NONE; break;
         case PNG_FILTER_VALUE_SUB:   png_ptr->do_filter=PNG_FILTER_SUB;  break;
         case PNG_FILTER_VALUE_UP:    png_ptr->do_filter=PNG_FILTER_UP;   break;
         case PNG_FILTER_VALUE_AVG:   png_ptr->do_filter=PNG_FILTER_AVG;  break;
         case PNG_FILTER_VALUE_PAETH: png_ptr->do_filter=PNG_FILTER_PAETH;break;
         default: png_ptr->do_filter = (png_byte)filters; break;
      }

      /* If we have allocated the row_buf, this means we have already started
       * with the image and we should have allocated all of the filter buffers
       * that have been selected.  If prev_row isn't already allocated, then
       * it is too late to start using the filters that need it, since we
       * will be missing the data in the previous row.  If an application
       * wants to start and stop using particular filters during compression,
       * it should start out with all of the filters, and then add and
       * remove them after the start of compression.
       */
      if (png_ptr->row_buf != NULL)
      {
         if ((png_ptr->do_filter & PNG_FILTER_SUB) && png_ptr->sub_row == NULL)
         {
            png_ptr->sub_row = (png_bytep)png_malloc(png_ptr,
              (png_ptr->rowbytes + 1));
            png_ptr->sub_row[0] = PNG_FILTER_VALUE_SUB;
         }

         if ((png_ptr->do_filter & PNG_FILTER_UP) && png_ptr->up_row == NULL)
         {
            if (png_ptr->prev_row == NULL)
            {
               png_warning(png_ptr, "Can't add Up filter after starting");
               png_ptr->do_filter &= ~PNG_FILTER_UP;
            }
            else
            {
               png_ptr->up_row = (png_bytep)png_malloc(png_ptr,
                  (png_ptr->rowbytes + 1));
               png_ptr->up_row[0] = PNG_FILTER_VALUE_UP;
            }
         }

         if ((png_ptr->do_filter & PNG_FILTER_AVG) && png_ptr->avg_row == NULL)
         {
            if (png_ptr->prev_row == NULL)
            {
               png_warning(png_ptr, "Can't add Average filter after starting");
               png_ptr->do_filter &= ~PNG_FILTER_AVG;
            }
            else
            {
               png_ptr->avg_row = (png_bytep)png_malloc(png_ptr,
                  (png_ptr->rowbytes + 1));
               png_ptr->avg_row[0] = PNG_FILTER_VALUE_AVG;
            }
         }

         if ((png_ptr->do_filter & PNG_FILTER_PAETH) &&
             png_ptr->paeth_row == NULL)
         {
            if (png_ptr->prev_row == NULL)
            {
               png_warning(png_ptr, "Can't add Paeth filter after starting");
               png_ptr->do_filter &= (png_byte)(~PNG_FILTER_PAETH);
            }
            else
            {
               png_ptr->paeth_row = (png_bytep)png_malloc(png_ptr,
                  (png_ptr->rowbytes + 1));
               png_ptr->paeth_row[0] = PNG_FILTER_VALUE_PAETH;
            }
         }

         if (png_ptr->do_filter == PNG_NO_FILTERS)
            png_ptr->do_filter = PNG_FILTER_NONE;
      }
   }
   else
      png_error(png_ptr, "Unknown custom filter method");
}

/* This allows us to influence the way in which libpng chooses the "best"
 * filter for the current scanline.  While the "minimum-sum-of-absolute-
 * differences metric is relatively fast and effective, there is some
 * question as to whether it can be improved upon by trying to keep the
 * filtered data going to zlib more consistent, hopefully resulting in
 * better compression.
 */
#if defined(PNG_WRITE_WEIGHTED_FILTER_SUPPORTED)      /* GRR 970116 */
void PNGAPI
png_set_filter_heuristics(png_structp png_ptr, int heuristic_method,
   int num_weights, png_doublep filter_weights,
   png_doublep filter_costs)
{
   int i;

   png_debug(1, "in png_set_filter_heuristics\n");
   if (heuristic_method >= PNG_FILTER_HEURISTIC_LAST)
   {
      png_warning(png_ptr, "Unknown filter heuristic method");
      return;
   }

   if (heuristic_method == PNG_FILTER_HEURISTIC_DEFAULT)
   {
      heuristic_method = PNG_FILTER_HEURISTIC_UNWEIGHTED;
   }

   if (num_weights < 0 || filter_weights == NULL ||
      heuristic_method == PNG_FILTER_HEURISTIC_UNWEIGHTED)
   {
      num_weights = 0;
   }

   png_ptr->num_prev_filters = (png_byte)num_weights;
   png_ptr->heuristic_method = (png_byte)heuristic_method;

   if (num_weights > 0)
   {
      if (png_ptr->prev_filters == NULL)
      {
         png_ptr->prev_filters = (png_bytep)png_malloc(png_ptr,
            (png_uint_32)(png_sizeof(png_byte) * num_weights));

         /* To make sure that the weighting starts out fairly */
         for (i = 0; i < num_weights; i++)
         {
            png_ptr->prev_filters[i] = 255;
         }
      }

      if (png_ptr->filter_weights == NULL)
      {
         png_ptr->filter_weights = (png_uint_16p)png_malloc(png_ptr,
            (png_uint_32)(png_sizeof(png_uint_16) * num_weights));

         png_ptr->inv_filter_weights = (png_uint_16p)png_malloc(png_ptr,
            (png_uint_32)(png_sizeof(png_uint_16) * num_weights));
         for (i = 0; i < num_weights; i++)
         {
            png_ptr->inv_filter_weights[i] =
            png_ptr->filter_weights[i] = PNG_WEIGHT_FACTOR;
         }
      }

      for (i = 0; i < num_weights; i++)
      {
         if (filter_weights[i] < 0.0)
         {
            png_ptr->inv_filter_weights[i] =
            png_ptr->filter_weights[i] = PNG_WEIGHT_FACTOR;
         }
         else
         {
            png_ptr->inv_filter_weights[i] =
               (png_uint_16)((double)PNG_WEIGHT_FACTOR*filter_weights[i]+0.5);
            png_ptr->filter_weights[i] =
               (png_uint_16)((double)PNG_WEIGHT_FACTOR/filter_weights[i]+0.5);
         }
      }
   }

   /* If, in the future, there are other filter methods, this would
    * need to be based on png_ptr->filter.
    */
   if (png_ptr->filter_costs == NULL)
   {
      png_ptr->filter_costs = (png_uint_16p)png_malloc(png_ptr,
         (png_uint_32)(png_sizeof(png_uint_16) * PNG_FILTER_VALUE_LAST));

      png_ptr->inv_filter_costs = (png_uint_16p)png_malloc(png_ptr,
         (png_uint_32)(png_sizeof(png_uint_16) * PNG_FILTER_VALUE_LAST));

      for (i = 0; i < PNG_FILTER_VALUE_LAST; i++)
      {
         png_ptr->inv_filter_costs[i] =
         png_ptr->filter_costs[i] = PNG_COST_FACTOR;
      }
   }

   /* Here is where we set the relative costs of the different filters.  We
    * should take the desired compression level into account when setting
    * the costs, so that Paeth, for instance, has a high relative cost at low
    * compression levels, while it has a lower relative cost at higher
    * compression settings.  The filter types are in order of increasing
    * relative cost, so it would be possible to do this with an algorithm.
    */
   for (i = 0; i < PNG_FILTER_VALUE_LAST; i++)
   {
      if (filter_costs == NULL || filter_costs[i] < 0.0)
      {
         png_ptr->inv_filter_costs[i] =
         png_ptr->filter_costs[i] = PNG_COST_FACTOR;
      }
      else if (filter_costs[i] >= 1.0)
      {
         png_ptr->inv_filter_costs[i] =
            (png_uint_16)((double)PNG_COST_FACTOR / filter_costs[i] + 0.5);
         png_ptr->filter_costs[i] =
            (png_uint_16)((double)PNG_COST_FACTOR * filter_costs[i] + 0.5);
      }
   }
}
#endif /* PNG_WRITE_WEIGHTED_FILTER_SUPPORTED */

void PNGAPI
png_set_compression_level(png_structp png_ptr, int level)
{
   png_debug(1, "in png_set_compression_level\n");
   png_ptr->flags |= PNG_FLAG_ZLIB_CUSTOM_LEVEL;
   png_ptr->zlib_level = level;
}

void PNGAPI
png_set_compression_mem_level(png_structp png_ptr, int mem_level)
{
   png_debug(1, "in png_set_compression_mem_level\n");
   png_ptr->flags |= PNG_FLAG_ZLIB_CUSTOM_MEM_LEVEL;
   png_ptr->zlib_mem_level = mem_level;
}

void PNGAPI
png_set_compression_strategy(png_structp png_ptr, int strategy)
{
   png_debug(1, "in png_set_compression_strategy\n");
   png_ptr->flags |= PNG_FLAG_ZLIB_CUSTOM_STRATEGY;
   png_ptr->zlib_strategy = strategy;
}

void PNGAPI
png_set_compression_window_bits(png_structp png_ptr, int window_bits)
{
   if (window_bits > 15)
      png_warning(png_ptr, "Only compression windows <= 32k supported by PNG");
   else if (window_bits < 8)
      png_warning(png_ptr, "Only compression windows >= 256 supported by PNG");
#ifndef WBITS_8_OK
   /* avoid libpng bug with 256-byte windows */
   if (window_bits == 8)
     {
       png_warning(png_ptr, "Compression window is being reset to 512");
       window_bits=9;
     }
#endif
   png_ptr->flags |= PNG_FLAG_ZLIB_CUSTOM_WINDOW_BITS;
   png_ptr->zlib_window_bits = window_bits;
}

void PNGAPI
png_set_compression_method(png_structp png_ptr, int method)
{
   png_debug(1, "in png_set_compression_method\n");
   if (method != 8)
      png_warning(png_ptr, "Only compression method 8 is supported by PNG");
   png_ptr->flags |= PNG_FLAG_ZLIB_CUSTOM_METHOD;
   png_ptr->zlib_method = method;
}

void PNGAPI
png_set_write_status_fn(png_structp png_ptr, png_write_status_ptr write_row_fn)
{
   png_ptr->write_row_fn = write_row_fn;
}

#if defined(PNG_WRITE_USER_TRANSFORM_SUPPORTED)
void PNGAPI
png_set_write_user_transform_fn(png_structp png_ptr, png_user_transform_ptr
   write_user_transform_fn)
{
   png_debug(1, "in png_set_write_user_transform_fn\n");
   png_ptr->transformations |= PNG_USER_TRANSFORM;
   png_ptr->write_user_transform_fn = write_user_transform_fn;
}
#endif


#if defined(PNG_INFO_IMAGE_SUPPORTED)
void PNGAPI
png_write_png(png_structp png_ptr, png_infop info_ptr,
              int transforms, voidp params)
{
#if defined(PNG_WRITE_INVERT_ALPHA_SUPPORTED)
   /* invert the alpha channel from opacity to transparency */
   if (transforms & PNG_TRANSFORM_INVERT_ALPHA)
       png_set_invert_alpha(png_ptr);
#endif

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

   /* ------ these transformations don't touch the info structure ------- */

#if defined(PNG_WRITE_INVERT_SUPPORTED)
   /* invert monochrome pixels */
   if (transforms & PNG_TRANSFORM_INVERT_MONO)
       png_set_invert_mono(png_ptr);
#endif

#if defined(PNG_WRITE_SHIFT_SUPPORTED)
   /* Shift the pixels up to a legal bit depth and fill in
    * as appropriate to correctly scale the image.
    */
   if ((transforms & PNG_TRANSFORM_SHIFT)
               && (info_ptr->valid & PNG_INFO_sBIT))
       png_set_shift(png_ptr, &info_ptr->sig_bit);
#endif

#if defined(PNG_WRITE_PACK_SUPPORTED)
   /* pack pixels into bytes */
   if (transforms & PNG_TRANSFORM_PACKING)
       png_set_packing(png_ptr);
#endif

#if defined(PNG_WRITE_SWAP_ALPHA_SUPPORTED)
   /* swap location of alpha bytes from ARGB to RGBA */
   if (transforms & PNG_TRANSFORM_SWAP_ALPHA)
       png_set_swap_alpha(png_ptr);
#endif

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

#if defined(PNG_WRITE_BGR_SUPPORTED)
   /* flip BGR pixels to RGB */
   if (transforms & PNG_TRANSFORM_BGR)
       png_set_bgr(png_ptr);
#endif

#if defined(PNG_WRITE_SWAP_SUPPORTED)
   /* swap bytes of 16-bit files to most significant byte first */
   if (transforms & PNG_TRANSFORM_SWAP_ENDIAN)
       png_set_swap(png_ptr);
#endif

#if defined(PNG_WRITE_PACKSWAP_SUPPORTED)
   /* swap bits of 1, 2, 4 bit packed pixel formats */
   if (transforms & PNG_TRANSFORM_PACKSWAP)
       png_set_packswap(png_ptr);
#endif

   /* ----------------------- end of transformations ------------------- */

   /* write the bits */
   if (info_ptr->valid & PNG_INFO_IDAT)
       png_write_image(png_ptr, info_ptr->row_pointers);

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

   if(transforms == 0 || params == NULL)
      /* quiet compiler warnings */ return;
}
#endif
#endif /* PNG_WRITE_SUPPORTED */