Function threshold 

pub fn threshold(gray: &Gray, percentile: Percentile) -> u16

The sample value the bitmaps are cut at. A u16 because it can land one past the range: an image of nothing but 255s gives 256, as it does in OpenCV.

A result of 0 or 256 means every sample fell on one side of the cut, so the threshold bitmap is empty and the exposure carries nothing to align on. More than half a frame clipped does this. Callers wanting to detect it can compare against those two values before aligning.