Theres also a version of this for Quake II, which can be a lot of fun to play. The difficulty is recognizing distant enemies and close-up doors - it’s not always apparent when something might be a switch or a door other than maybe a small outset frame, so it’s best for maps with which you’re already familiar.
Also: don’t go “cross-eyed” - you need to focus more distantly than the screen in order to cause the common dots to align correctly. Cross-eye stereograms invert the spacial distribution, which will cause near things to look far and vice versa, which will be particularly confusing if not headache inducing. Look at something 3m (~10’) beyond your screen, then whilst keeping that focus move the screen into the view, et voila.
