A half-hour-or-so sketch that uses the 3D library PeasyCam, to display 4 simple translucent boxes that leave tracks as you drag them around the central (red) box.
In the original you can click-drag to ‘paint’, right-drag or mouse wheel to move in and out.
Java no longer works in browsers so there’s no interaction, but you can run the .pde code in Processing.
View source code (2 Dec 2012). Built with Processing