int shift[4];
      int channels = 0;
      int c;
      png_uint_16 value = 0;
      png_uint_32 row_width = row_info->width;

      if (row_info->color_type & PNG_COLOR_MASK_COLOR)
      {
         shift[channels++] = row_info->bit_depth - sig_bits->red;
         shift[channels++] = row_info->bit_depth - sig_bits->green;
         shift[channels++] = row_info->bit_depth - sig_bits->blue;
      }
      else
      {
         shift[channels++] = row_info->bit_depth - sig_bits->gray;
      }
      if (row_info->color_type & PNG_COLOR_MASK_ALPHA)
      {
         shift[channels++] = row_info->bit_depth - sig_bits->alpha;
      }

      for (c = 0; c < channels; c++)
      {
         if (shift[c] <= 0)
            shift[c] = 0;
         else
            value = 1;
      }

      if (!value)
         return;

      switch (row_info->bit_depth)
      {
         case 2:
         {
            png_bytep bp;
            png_uint_32 i;
            png_uint_32 istop = row_info->rowbytes;

            for (bp = row, i = 0; i < istop; i++)
            {
               *bp >>= 1;
               *bp++ &= 0x55;
            }
            break;
         }
         case 4:
         {
            png_bytep bp = row;
            png_uint_32 i;
            png_uint_32 istop = row_info->rowbytes;
            png_byte mask = (png_byte)((((int)0xf0 >> shift[0]) & (int)0xf0) |
               (png_byte)((int)0xf >> shift[0]));

            for (i = 0; i < istop; i++)
            {
               *bp >>= shift[0];
               *bp++ &= mask;
            }
            break;
         }
         case 8:
         {
            png_bytep bp = row;
            png_uint_32 i;
            png_uint_32 istop = row_width * channels;

            for (i = 0; i < istop; i++)
            {
               *bp++ >>= shift[i%channels];
            }
            break;
         }
         case 16:
         {
            png_bytep bp = row;
            png_uint_32 i;
            png_uint_32 istop = channels * row_width;

            for (i = 0; i < istop; i++)
            {
               value = (png_uint_16)((*bp << 8) + *(bp + 1));
               value >>= shift[i%channels];
               *bp++ = (png_byte)(value >> 8);
               *bp++ = (png_byte)(value & 0xff);
            }
            break;
         }
      }
   }
}
#endif

#if defined(PNG_READ_16_TO_8_SUPPORTED)
/* chop rows of bit depth 16 down to 8 */
void /* PRIVATE */
png_do_chop(png_row_infop row_info, png_bytep row)
{
   png_debug(1, "in png_do_chop\n");
#if defined(PNG_USELESS_TESTS_SUPPORTED)
   if (row != NULL && row_info != NULL && row_info->bit_depth == 16)
#else
   if (row_info->bit_depth == 16)
#endif
   {
      png_bytep sp = row;
      png_bytep dp = row;
      png_uint_32 i;
      png_uint_32 istop = row_info->width * row_info->channels;

      for (i = 0; i<istop; i++, sp += 2, dp++)
      {
#if defined(PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED)
      /* This does a more accurate scaling of the 16-bit color
       * value, rather than a simple low-byte truncation.
       *
       * What the ideal calculation should be:
       *   *dp = (((((png_uint_32)(*sp) << 8) |
       *          (png_uint_32)(*(sp + 1))) * 255 + 127) / (png_uint_32)65535L;
       *
       * GRR: no, I think this is what it really should be:
       *   *dp = (((((png_uint_32)(*sp) << 8) |
       *           (png_uint_32)(*(sp + 1))) + 128L) / (png_uint_32)257L;
       *
       * GRR: here's the exact calculation with shifts:
       *   temp = (((png_uint_32)(*sp) << 8) | (png_uint_32)(*(sp + 1))) + 128L;
       *   *dp = (temp - (temp >> 8)) >> 8;
       *
       * Approximate calculation with shift/add instead of multiply/divide:
       *   *dp = ((((png_uint_32)(*sp) << 8) |
       *          (png_uint_32)((int)(*(sp + 1)) - *sp)) + 128) >> 8;
       *
       * What we actually do to avoid extra shifting and conversion:
       */

         *dp = *sp + ((((int)(*(sp + 1)) - *sp) > 128) ? 1 : 0);
#else
       /* Simply discard the low order byte */
         *dp = *sp;
#endif
      }
      row_info->bit_depth = 8;
      row_info->pixel_depth = (png_byte)(8 * row_info->channels);
      row_info->rowbytes = row_info->width * row_info->channels;
   }
}
#endif

#if defined(PNG_READ_SWAP_ALPHA_SUPPORTED)
void /* PRIVATE */
png_do_read_swap_alpha(png_row_infop row_info, png_bytep row)
{
   png_debug(1, "in png_do_read_swap_alpha\n");
#if defined(PNG_USELESS_TESTS_SUPPORTED)
   if (row != NULL && row_info != NULL)
#endif
   {
      png_uint_32 row_width = row_info->width;
      if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
      {
         /* This converts from RGBA to ARGB */
         if (row_info->bit_depth == 8)
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_byte save;
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               save = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = save;
            }
         }
         /* This converts from RRGGBBAA to AARRGGBB */
         else
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_byte save[2];
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               save[0] = *(--sp);
               save[1] = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = save[0];
               *(--dp) = save[1];
            }
         }
      }
      else if (row_info->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
      {
         /* This converts from GA to AG */
         if (row_info->bit_depth == 8)
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_byte save;
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               save = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = save;
            }
         }
         /* This converts from GGAA to AAGG */
         else
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_byte save[2];
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               save[0] = *(--sp);
               save[1] = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = save[0];
               *(--dp) = save[1];
            }
         }
      }
   }
}
#endif

#if defined(PNG_READ_INVERT_ALPHA_SUPPORTED)
void /* PRIVATE */
png_do_read_invert_alpha(png_row_infop row_info, png_bytep row)
{
   png_debug(1, "in png_do_read_invert_alpha\n");
#if defined(PNG_USELESS_TESTS_SUPPORTED)
   if (row != NULL && row_info != NULL)
#endif
   {
      png_uint_32 row_width = row_info->width;
      if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
      {
         /* This inverts the alpha channel in RGBA */
         if (row_info->bit_depth == 8)
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               *(--dp) = (png_byte)(255 - *(--sp));

/*             This does nothing:
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               We can replace it with:
*/
               sp-=3;
               dp=sp;
            }
         }
         /* This inverts the alpha channel in RRGGBBAA */
         else
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               *(--dp) = (png_byte)(255 - *(--sp));
               *(--dp) = (png_byte)(255 - *(--sp));

/*             This does nothing:
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
               We can replace it with:
*/
               sp-=6;
               dp=sp;
            }
         }
      }
      else if (row_info->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
      {
         /* This inverts the alpha channel in GA */
         if (row_info->bit_depth == 8)
         {
            png_bytep sp = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               *(--dp) = (png_byte)(255 - *(--sp));
               *(--dp) = *(--sp);
            }
         }
         /* This inverts the alpha channel in GGAA */
         else
         {
            png_bytep sp  = row + row_info->rowbytes;
            png_bytep dp = sp;
            png_uint_32 i;

            for (i = 0; i < row_width; i++)
            {
               *(--dp) = (png_byte)(255 - *(--sp));
               *(--dp) = (png_byte)(255 - *(--sp));
/*
               *(--dp) = *(--sp);
               *(--dp) = *(--sp);
*/
               sp-=2;
               dp=sp;
            }
         }
      }
   }
}
#endif

#if defined(PNG_READ_FILLER_SUPPORTED)
/* Add filler channel if we have RGB color */
void /* PRIVATE */
png_do_read_filler(png_row_infop row_info, png_bytep row,
   png_uint_32 filler, png_uint_32 flags)
{
   png_uint_32 i;
   png_uint_32 row_width = row_info->width;

   png_byte hi_filler = (png_byte)((filler>>8) & 0xff);
   png_byte lo_filler = (png_byte)(filler & 0xff);

   png_debug(1, "in png_do_read_filler\n");
   if (
#if defined(PNG_USELESS_TESTS_SUPPORTED)
       row != NULL  && row_info != NULL &&
#endif
       row_info->color_type == PNG_COLOR_TYPE_GRAY)
   {
      if(row_info->bit_depth == 8)
      {
         /* This changes the data from G to GX */
         if (flags & PNG_FLAG_FILLER_AFTER)
         {
            png_bytep sp = row + (png_size_t)row_width;
            png_bytep dp =  sp + (png_size_t)row_width;
            for (i = 1; i < row_width; i++)
            {
               *(--dp) = lo_filler;
               *(--dp) = *(--sp);
            }
            *(--dp) = lo_filler;
            row_info->channels = 2;