[Harriet Farren]

you’ve probably already moved on from this, but for people viewing in the future via google search. BUT although i don’t know how to do this in aftereffects, what i would to, is the following:

if your planning on ‘tracing’ a still image
(so if you were tracing a picture of a cat face but it’s morphing, and you’re using the still image as a reference point), pop the still image into photoshop. go to window>animation (you then get a long bar at the bottom of your work panel, these are frames, which you can then draw onto layers, and set visibility to each frame you create (for more details, search ‘rotoscoping using photoshop’). then export as a video and import into your after effects project.

‘Tracing’ a video
this is only do able if you’ve got 64-bit adobe photoshop cs5. do as above, but to import the video, go to file>import>video frames to layers. you can then adjust the fps rate to desired (i’d reccomend 12fps for high quality, 6fps for low quality) but only import 15 seconds of footage at a time other wise photoshop has trouble handling the file. (15s of video footage at 12fps is aprox equal to 150-180 images).

again, this is only my solution from a ‘non aftereffects point of view’. i’m sure there is a way to ‘trace’ in after effects somewhere!