How do I clean up this difference matte?

[Steve Roberts]

You might have to add another keying effect to the effects stack above the diff matte effect to get rid of some colours before keying with the diff matte.

Otherwise, if parts of the talent match parts of the background exactly, you have no choice but to do some masking.

[Zygotesoup]

hmmmm…. never done any rotoscoping before. not really sure how to animate a mask either (im a noob). I’ll play around with it and I’m sure I’ll have some questions for you guys soon.

[Jim Dodson]

This is a great job for “Auto-Trace” under “Layer–> Auto-Trace”

Use Luminance as your “channel”

and here’s the trick:
set “Minimum Area” to 3000 (that’s the max I think)

Now AE will make one or more masks around your rough matte.

It may make additional masks for the smaller areas, if it does just delete those masks– you want the single biggest mask and it will also clean up the non anti-aliased problems that the difference matte introduced…

Jim Dodson

Quad 2.5 gig Mac – OSX