pngwtran.c

一个国人自己实现图像库的程序（有参考价值）

      {
         png_bytep bp = row;
         png_uint_32 i;
         png_uint_32 istop = channels * row_info->width;

         for (i = 0; i < istop; i++, bp++)
         {

            png_uint_16 v;
            int j;
            int c = (int)(i%channels);

            v = *bp;
            *bp = 0;
            for (j = shift_start[c]; j > -shift_dec[c]; j -= shift_dec[c])
            {
               if (j > 0)
                  *bp |= (png_byte)((v << j) & 0xff);
               else
                  *bp |= (png_byte)((v >> (-j)) & 0xff);
            }
         }
      }
      else
      {
         png_bytep bp;
         png_uint_32 i;
         png_uint_32 istop = channels * row_info->width;

         for (bp = row, i = 0; i < istop; i++)
         {
            int c = (int)(i%channels);
            png_uint_16 value, v;
            int j;

            v = (png_uint_16)(((png_uint_16)(*bp) << 8) + *(bp + 1));
            value = 0;
            for (j = shift_start[c]; j > -shift_dec[c]; j -= shift_dec[c])
            {
               if (j > 0)
                  value |= (png_uint_16)((v << j) & (png_uint_16)0xffff);
               else
                  value |= (png_uint_16)((v >> (-j)) & (png_uint_16)0xffff);
            }
            *bp++ = (png_byte)(value >> 8);
            *bp++ = (png_byte)(value & 0xff);
         }
      }
   }
}
#endif

#if defined(PNG_WRITE_SWAP_ALPHA_SUPPORTED)
void /* PRIVATE */
png_do_write_swap_alpha(png_row_infop row_info, png_bytep row)
{
   png_debug(1, "in png_do_write_swap_alpha\n");
#if defined(PNG_USELESS_TESTS_SUPPORTED)
   if (row != NULL && row_info != NULL)
#endif
   {
      if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
      {
         /* This converts from ARGB to RGBA */
         if (row_info->bit_depth == 8)
         {
            png_bytep sp, dp;
            png_uint_32 i;
            png_uint_32 row_width = row_info->width;
            for (i = 0, sp = dp = row; i < row_width; i++)
            {
               png_byte save = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = save;
            }
         }
         /* This converts from AARRGGBB to RRGGBBAA */
         else
         {
            png_bytep sp, dp;
            png_uint_32 i;
            png_uint_32 row_width = row_info->width;

            for (i = 0, sp = dp = row; i < row_width; i++)
            {
               png_byte save[2];
               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 AG to GA */
         if (row_info->bit_depth == 8)
         {
            png_bytep sp, dp;
            png_uint_32 i;
            png_uint_32 row_width = row_info->width;

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

            for (i = 0, sp = dp = row; i < row_width; i++)
            {
               png_byte save[2];
               save[0] = *(sp++);
               save[1] = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = save[0];
               *(dp++) = save[1];
            }
         }
      }
   }
}
#endif

#if defined(PNG_WRITE_INVERT_ALPHA_SUPPORTED)
void /* PRIVATE */
png_do_write_invert_alpha(png_row_infop row_info, png_bytep row)
{
   png_debug(1, "in png_do_write_invert_alpha\n");
#if defined(PNG_USELESS_TESTS_SUPPORTED)
   if (row != NULL && row_info != NULL)
#endif
   {
      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, dp;
            png_uint_32 i;
            png_uint_32 row_width = row_info->width;
            for (i = 0, sp = dp = row; i < row_width; i++)
            {
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = (png_byte)(255 - *(sp++));
            }
         }
         /* This inverts the alpha channel in RRGGBBAA */
         else
         {
            png_bytep sp, dp;
            png_uint_32 i;
            png_uint_32 row_width = row_info->width;

            for (i = 0, sp = dp = row; i < row_width; i++)
            {
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = *(sp++);
               *(dp++) = (png_byte)(255 - *(sp++));
               *(dp++) = (png_byte)(255 - *(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, dp;
            png_uint_32 i;
            png_uint_32 row_width = row_info->width;

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

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

#if defined(PNG_MNG_FEATURES_SUPPORTED)
/* undoes intrapixel differencing  */
void /* PRIVATE */
png_do_write_intrapixel(png_row_infop row_info, png_bytep row)
{
   png_debug(1, "in png_do_write_intrapixel\n");
   if (
#if defined(PNG_USELESS_TESTS_SUPPORTED)
       row != NULL && row_info != NULL &&
#endif
       (row_info->color_type & PNG_COLOR_MASK_COLOR))
   {
      int bytes_per_pixel;
      png_uint_32 row_width = row_info->width;
      if (row_info->bit_depth == 8)
      {
         png_bytep rp;
         png_uint_32 i;

         if (row_info->color_type == PNG_COLOR_TYPE_RGB)
            bytes_per_pixel = 3;
         else if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
            bytes_per_pixel = 4;
         else
            return;

         for (i = 0, rp = row; i < row_width; i++, rp += bytes_per_pixel)
         {
            *(rp)   = (png_byte)((*rp     - *(rp+1))&0xff);
            *(rp+2) = (png_byte)((*(rp+2) - *(rp+1))&0xff);
         }
      }
      else if (row_info->bit_depth == 16)
      {
         png_bytep rp;
         png_uint_32 i;

         if (row_info->color_type == PNG_COLOR_TYPE_RGB)
            bytes_per_pixel = 6;
         else if (row_info->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
            bytes_per_pixel = 8;
         else
            return;

         for (i = 0, rp = row; i < row_width; i++, rp += bytes_per_pixel)
         {
            png_uint_32 s0=*(rp  )<<8 | *(rp+1);
            png_uint_32 s1=*(rp+2)<<8 | *(rp+3);
            png_uint_32 s2=*(rp+4)<<8 | *(rp+5);
            png_uint_32 red=(s0-s1)&0xffff;
            png_uint_32 blue=(s2-s1)&0xffff;
            *(rp  ) = (png_byte)((red>>8)&0xff);
            *(rp+1) = (png_byte)(red&0xff);
            *(rp+4) = (png_byte)((blue>>8)&0xff);
            *(rp+5) = (png_byte)(blue&0xff);
         }
      }
   }
}
#endif /* PNG_MNG_FEATURES_SUPPORTED */
#endif /* PNG_WRITE_SUPPORTED */