png_read_filter_row_sub(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
   png_size_t i;
   png_size_t istop = row_info->rowbytes;
   unsigned int bpp = (row_info->pixel_depth + 7) >> 3;
   png_bytep rp = row + bpp;

   PNG_UNUSED(prev_row)

   for (i = bpp; i < istop; i++)
   {
      *rp = (png_byte)(((int)(*rp) + (int)(*(rp-bpp))) & 0xff);
      rp++;
   }
}
Unknown