I created a system in Processing which takes in black and white image sequences and fills the white portion of the image with a particle system. To show it off I created a psychedelic animation that leads into a short shot by Ralph Mastromonaco.

I import an image sequence and push all the white pixels into an array with a nested for loop. The particles check to see if their next position remains in the white pixels and if not they search back for the edge between white and black. The tricky part is if the next frame is a big enough jump that the particle gets left in the black. Since it doesn't know the direction the animation is moving it creates a grid (the red dots) to search for the white and takes the closest position. It's not perfect but is close enough and not obvious when there are many particles.

Here's some reference video I shot for animating.

Here is the animation that I made in Flash. I exported every layer as a black and white sequence.

Here is one of my first tests with the animation.