Ok so the screenshot above is the scene i am working on.. I want to create a slight rain effect behind the two characters. I figured its not that hard but I really need help from AE experts.. I am not that great with AE.. So the appraoch i took was, I took the video added CC rain.. I know it looks somewhat cheap but I don't need the details I just need light rain in the background and its blurry anyway. I added CC rain to the video. Then I played with setting till I was happy. Then I added gaussian blur in order to blur up the rain drops and not make them look cheap. It worked, I got what I wanted. After that I added a second layer to the composition and inesrted the same video on top of the one i added rain to. Here is where my problem starts. I figured I would mask out the background so the rain from layer below would show.. Again it works.. However Two of them and the umbrella keep on moving all the time. Not much but sometimes it really shows.. Is there a way i can somehow add motion tracking to the mask. I really don't feel like keyframing the mask for every time they move. It will take forever. Shot is about 4 minutes long..... What would be the best solution?
The best solution would have been either shoot green screen to allow you the control you want OR have locked the camera down-shot a clean background plate for the rain then composite the actors shot with the camera still locked.
Now you have to deal with not only possible camera movement, but movement of each actor AND the umbrella. Unfortunately I think you have condemned yourself to the animating of mattes (a kind of rotoscoping). I dont think trying keying would help since there are light and dark values in your background and foreground.
If you are skilled with Mocha you could track actor 1, then create a shape mask and link it to the track, but you would still need to roto the shape where needed. Ditto for Actor 2 and the umbrella.
On a long shot you could use the Rotobrush and see how that works. Theres a chance with time, patience and lots of luck that could work for you! I think Rotobrush was added in AE 5.5
Watch these 2 tutorial videos. They will help.
Rotobrush with a soft edge is your best solution. It's going to be very tough because there's not any real edge detail in your screenshot with the black hair of the girl.
The biggest problem I see with the shot is that if it was raining there would be reflections from the lights in the background. Fixing up the background plate to simulate wet streets and backlighting rain from the lights is going to take more than a single layer and a single effect.