stereograms
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A stereogram is an optical illusion of depth created from flat, two-dimensional image or images. Originally, stereogram referred to a pair of stereo images which could be viewed using stereoscope. Other types of stereograms include anaglyphs and autostereograms.

An autostereogram is an optical illusion of depth usually observed by allowing the eyes to focus behind an image (diverge), but also sometimes in front of the image (converge).

You can see in my blog a link of stereogram tetris game.