The idea is to put a projector and a camera in a dark room. The projector would show a greyish background along with computer generated rain (simple stuff, not photorealistic) and cast the shadow of the people passing against a wall.
Using the camera and some Processing/OpenFrameworks kung-fu the rain should start bouncing against shadows' edges.
Here's a screenshot of the first sketch. Just trying out some ideas with a static and simple image:
The rendering (without the OPENGL option) is quite slow, as you can see in the applet linked below. That's due to some quick'n'dirty hacks to get the job done in less time. I also lowered the number of particles created per-frame.
It also could take a lot to load because I'm using a 4MB mp3 for the background "music". I should definitely cut it :D
Rainy Day (Applet)