png_read_filter_row_paeth_1byte_pixel(png_row_infop row_info, png_bytep row,
    png_const_bytep prev_row)
{
   png_bytep rp_end = row + row_info->rowbytes;
   int a, c;

   /* First pixel/byte */
   c = *prev_row++;
   a = *row + c;
   *row++ = (png_byte)a;

   /* Remainder */
   while (row < rp_end)
   {
      int b, pa, pb, pc, p;

      a &= 0xff; /* From previous iteration or start */
      b = *prev_row++;

      p = b - c;
      pc = a - c;

#ifdef PNG_USE_ABS
      pa = abs(p);
      pb = abs(pc);
      pc = abs(p + pc);
#else
      pa = p < 0 ? -p : p;
      pb = pc < 0 ? -pc : pc;
      pc = (p + pc) < 0 ? -(p + pc) : p + pc;
#endif

      /* Find the best predictor, the least of pa, pb, pc favoring the earlier
       * ones in the case of a tie.
       */
      if (pb < pa)
      {
         pa = pb; a = b;
      }
      if (pc < pa) a = c;

      /* Calculate the current pixel in a, and move the previous row pixel to c
       * for the next time round the loop
       */
      c = b;
      a += *row;
      *row++ = (png_byte)a;
   }
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.