A Magic Eye picture hides a depth map inside a repeating pattern. Points that sit closer to you are drawn slightly closer together. Measure that spacing everywhere and the hidden surface falls out — no eye-crossing required.
Drop a stereogram image here, or paste one with ⌘V / Ctrl+V.
Sample stereograms by Fred Hsu (CC BY-SA 3.0) and Gary W. Priester (CC BY-SA 4.0), via Wikimedia Commons.
If it looks like noise: the period is probably wrong. Drag the slider slowly — the correct value is the distance the wallpaper pattern repeats over, and the picture snaps into a solid shape within a pixel or two of it. Photos of posters must be shot square-on; perspective skew misaligns the rows the method depends on.
