2 thoughts:
Are you using “Keylight” to pull the key? If so, twirl open “screen matte” and under “Replace colour” choose “source”…
Also, you can pre-treat your footage with “Remove Grain” as follows:
To quickly reduce the noise of a sequence that has a lot of buzzing noise, set the Noise Reduction value to zero and the Temporal Filtering Amount to 100%, and render the sequence.
Do the remove grain in a precomp then pull a key on the precomped layer….
The key will have significantly less “chatter” and your shirt will be far less “noisy”….
As for file size, I usually let After Effects make its huge render file (so I have good quality) then using Quicktime export a smaller h264 codec for internet/emailing etc, but if you prefer, in the Render Cue, if you click on your current Output module then go to “Format Options” you can choose h264 which makes a very nice web-sized file for you….
Jim Dodson
8 Core Intel — Mac – OSX