There's nothing to "paste". This is simple basic masking/ keying of one layer on top of another.
Thanks for the reply!
hmmm okay, I was looking up online about motion tracking and I think I understand how to track the object I wish to track. Lets say for example I have someones head tracked in one video and I want to put that persons head on someone else's body in another video. How would I got about doing that? That's the part where I get confused..
You would track the head in the shot that you want to replace then apply the tracking info to a null.
In another composition you would stabilize the shot of the replacement head so that the head does not move in the frame, then mask around the head. It's important that the head does not move very much in the frame. Then, if you were being efficient, you would set the "Region of Interest" for the comp to just include the stabilized head and then crop the comp to the Region of Interest. (If you don't know what I'm talking about then either look it up using the Search Help field at the top right corner of AE or don't worry about it).
Next you nest your stabilized head comp inside the main comp, set the anchor point to the bottom of the neck, position the replacement head on the body and then parent the replacement head to the null.
Hey Rick, thanks for the reply - I think I have a better understanding of it now!
Can I just ask you, why do I need to apply the tracking info to a null? I'm sorry if I'm asking a stupid question, I'm not too familiar with AE yet!
It's just easier. The null doesn't have to be accurately positioned. If you applied tracking to the stabilized head and you needed to adjust the position or do any other animation you would have a mess on your hands. Parenting solves this problem. Once the tracking data is applied it's there for any and all layers to use as needed. Nulls also don't add anything to the rendering overhead.