How it works

A stereogram repeats its pattern across the picture, and the hidden shape is made by shifting that repeat slightly where the shape is. Your eyes merge the repeats to see depth. CacheCrack does the same thing with math: it finds the repeat distance, compares the picture with itself moved by that distance, and shows the parts that do not line up. Those parts are the hidden shape, usually as a clear outline or block.

If the result looks doubled or faint, turn off the automatic setting and try a few pixels either side of the suggested shift.