I obtained the alpha channel by duplicating the clip of the actor that had been keyed out using Keylight. Instead of turning on Final Result, I turned on Screen Matte and used the clip that way. In another composition I placed my screen matte clip over still another clip of the actor and told the clip of the actor to use the screen matte clip as a track matte. This combination was then screened over yet another clip of the actor. This is all a bit complicated and almost requires a tutorial of it's own but if you have used track mattes before it will make some sense. A lot of things in After Effects are done by copying the same clip over and over and placing it on top of it's self using different modes. You can also make lots of compositions and put them into other compositions. In this case I think I used a composition called track matte described above that was put into a composition called Rich in car which was then placed into another composition called richard driving BRSony comp 1. which was the final render comp.
We were all very pleased by the test as it meant we could shoot the dialogue and action inside the car in the quiet of the studio. We would not have to rent the very expensive antique cars, and we could tailor the backgrounds to match our 50s period. Imagine trying to drive around waiting for the right combination of performance, usable background, right lighting and good audio.
You can see the total composite with music in our Flash video below. (To download a full size version of this video without music in .M4V mpg, click here.)
