The Lenticulator is a web toy that helps you print and fold eye-baffling illusions whereby one picture blends into another as the observer moves from one side to the other.

Take two photos, feed them into the app then print, score and fold. Nothing leaves your computer, and it is all open source

I used to make these 20 years ago very laboriously by slicing and dicing images in a graphics package, and have been meaning to make the app ever since.

This version is written in HTML, CSS and 80 lines of JavaScript (see the code on GitHub).