Similar to range rejection, but using a specific rather than percentage value. Use case is nightscape photography, where the foreground object gently drifts in front of the background - I would like to be able to start shooting when the background has a nice alignment with the foreground, keep shooting as the foreground drifts away from the background and becomes uncovered, and stack to the initial image without worrying about whether or not this or that bit in the foreground will leave streaks in my stack
