For people working in computer vision, openCv is godsend. The following is code implementation for creating an optical illusion. This involves creating a pencil sketch and some more simple filtering application. Check out the illusion here.
Direct Access Header: It makes accessing individual pixels in openCv way easier
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EDIT: I changed the code a bit so that the strips appear horizontal rather than vertical. The advantage of it is that the hidden image can be revealed just by rapidly scrolling up and down...(while you do so fix your eye at the center). The illusion is really cool.… Read the full article