png_read_filter_row_up(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
   png_size_t i;
   png_size_t istop = row_info->rowbytes;
   png_bytep rp = row;
   png_const_bytep pp = prev_row;

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