Help Using Timewarp effect with a Matte

Hi all,

I have a short 15-frame stop motion clip that I would like to slow down and smooth out using optical flow. I know the timewarp effect in After Effects uses this technology, and can make use of a matte layer to help smooth artifacts, but I am having a hard time getting it to work properly, and the AE documentation is a bit sparse as far as this goes.

I figured I could duplicate the stop-motion layer, and draw some masks to roto the foreground elements - to give timewarp something to work with. But when I tell the plugin to look at that layer's alpha, I see no difference in the results - certain items in the frame are getting distorted still as they pass one another.

Do I need to slow down my matte layer as well to match the new duration of the timewarp'd layer? Am I right in using the alpha of the masked layer?

Any help would be appreciated.

Scott