png_setup_sub_row(png_structrp png_ptr, const png_uint_32 bpp,
    const png_size_t row_bytes, const png_size_t lmins)
{
   png_bytep rp, dp, lp;
   png_size_t i;
   png_size_t sum = 0;
   int v;

   png_ptr->try_row[0] = PNG_FILTER_VALUE_SUB;

   for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1; i < bpp;
        i++, rp++, dp++)
   {
      v = *dp = *rp;
      sum += (v < 128) ? v : 256 - v;
   }

   for (lp = png_ptr->row_buf + 1; i < row_bytes;
      i++, rp++, lp++, dp++)
   {
      v = *dp = (png_byte)(((int)*rp - (int)*lp) & 0xff);
      sum += (v < 128) ? v : 256 - v;

      if (sum > lmins)  /* We are already worse, don't continue. */
        break;
   }

   return (sum);
}
png_setup_sub_row(png_structrp png_ptr, const png_uint_32 bpp,
    const png_size_t row_bytes, const png_size_t lmins)
{
   png_bytep rp, dp, lp;
   png_size_t i;
   png_size_t sum = 0;
   int v;

   png_ptr->try_row[0] = PNG_FILTER_VALUE_SUB;

   for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1; i < bpp;
        i++, rp++, dp++)
   {
      v = *dp = *rp;
      sum += (v < 128) ? v : 256 - v;
   }

   for (lp = png_ptr->row_buf + 1; i < row_bytes;
      i++, rp++, lp++, dp++)
   {
      v = *dp = (png_byte)(((int)*rp - (int)*lp) & 0xff);
      sum += (v < 128) ? v : 256 - v;

      if (sum > lmins)   
        break;
   }

   return (sum);
}