When I first got into Processing I ran across some work done by Craig Mackenzie. For whatever reason I found it quite fascinating, and wanted to try my hand at it.
In a nutshell, the sketch analyzes an image: For a random pixel, find every other pixel in the image that is the same color, and then draw a line to it. With a bit of embellishment of course. Furthermore, in my colorSearch sketch, I can define a specific range of colors to look for, that way the whole image isn’t analyzed.
The bulk of the work is a class I call ImageData that allows for easy querying of the color data in an image. Notes specifically for that class over on my Processing Wiki.