Flutterby™! : Hiding Cat Pictures in Windows

Hiding Images in Plain Sight: The Physics of Magic Windows, in which a guy uses caustics and a bunch of optical calculations to use a CNC router to mill a piece of acrylic to make a clear piece that one can see through (with some waviness), but that projects a cat picture when light is shone through it.

Via MeFi.

Source code for the height field generator is at https://github.com/MattFerraro/causticsEngineering