Click the image to play with this one.

This is my first go at the "Propagation" portion of the UnOfficial Four Elements Contest. Not complicated; click and drag your mouse around the bitmap and watch the particles spawn and flow downhill.

Simple mechanic: each particle sniffs each of the eight pixels around it, and goes toward the darkest one. If the darkest neighbor is the same color as the current pixel, it stops moving. I check the neighbors from upper left to lower right, so there is a bias toward the upper left in case of a tie between more two or more neighbors. Adding vector data to the particles will take care of that.

It will take a lot more work to make this usable in a game. It also gave me an idea for making a simple erosion simulator which will make the terrain much more realistic...if it works.