This demo shows off the OpenCV Patch from kineme.net. It tracks where the user clicks, and draws lines connecting the points.
It also makes use of a few other custom patches for simplicity.
After the background is blanked, rapid motion causes it to slowly lose tracking, causing the jumbled mess of lines. Better lighting addresses this, but I left it in for artistic effect.
You can find the composition here: kineme.net/OpenCVPatch20080207beta#comment-988 (you'll need an account to access stuff that's in beta)