…g up the alpha range by 1 and then subtracting 1 to force rounding down was doing more harm than good.
Also, saturating the result to 255 avoids overflows that eventually happen due to some of the liberties taken.
All in all the absolute difference in result from fixed point and floating point deviate, on average, by less than 0.03.
The fixed point code can detect a couple more images than the previous code; however it also detects a couple less. Zero sum I suppose? However it's much faster, so still a win ;)