I am using AE CS4 to blur out faces in a video. I understand how to use a null object, adjustment layer and masking to do this on one object, but how does this work when I have more than one object to track and blur? Should I create a duplicate source video and apply a second motion track to that? It appears that I lose the tracking on the first video when I do that.
Track one face, apply the tracking info to a null object, add another tracker, and repeat.
When you click to track the footage a second time, it should create a second tracker. However, if it doesn't, you've already got your tracking info from the first track saved on your null.
Track as many faces as you need to then use your favorite blur technique using the nulls as your track point info.
One way to do it would be to precomp all those moving nulls, add little feathered shape layers into the precomp (the same size as the faces - see where I'm going with this?), parent them to the nulls, and then use that precomp as a track matte for a blur adjustment layer.
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