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