Rain in movies is always a tricky thing. Natural rain is so subtle that it is hardly noticable in the picture. It is also usually not available on command and one has to worry about drowning the expensive equipment. Therefore, natural rain is usually substituted by large raining devices which create especially large streams of rain. Another alternative is computer generated rain.
The article “Making a CG Rainstorm with After Effects Plug-ins and Automated Rotoscoping” discusses the latter option. Based on a commercial for BMW, it shows how a scene shot in sunlight is transformed into a fierce rainstorm.
Several techniques are employed. Rain and falling stones are generated by the plugin Particular. One can use it to render 3D particle simulations directly in After Effects. To postprocess the filmed BMW the team used a rotoscoping software called Motor. Seemingly, it can automate the rotoscoping process quite a lot.
For illustration the article includes two videos comparing original shot and postprocessed result. Have fun. 🙂