Glowdoodle is an interactive light painting system I created in processing (plus some PHP for image uploads).  I was inspired by the classic long-exposure photography technique of light painting.  Basically my system lets you do something very similar, but with a webcam, and you see the results in real time as you paint.  The system lends itself well to painting in a dark room with all sorts of different glowing objects- LEDs and flashlights of course, but glowsticks and lighters and cellphones etc too.  You can also use it with the lights on, and paint with any object brighter than the background- you can paint with your teeth, with a finger, a flower, some Lego bricks, or whatever else is around.  Then, just press one key to upload your image to the website, where you can see thousands of others.  There's already an incredible variety of inventive glowdoodles on the site.

You can find it by visiting glowdoodle dot com (apparently I am not allowed to post links since this is my first post).

I learned a lot in making this which I would be happy to share.  Here are some things I can post more about if you're interested:

- I got image uploading to work via http POST and PHP.  It works asynchronously, so I can display an animation during upload.  It took cobbling together code from several different places (credited in the source, which you can find at the site).

- Turns out the built in image filters are pretty slow, and it runs much faster if you re-implement them yourself with bit-shifting etc.  fun.  I have a faster version of glowdoodle not yet released (which will include an "inverted" version of the algorithm for "darkpainting")

- The processing video libraries don't work out of the box on windows, so you need to instruct people to install some extra junk (I've heard this will improve soon)