Sometimes you have a shot where you want to attach a background to a tracking marker, but there isn’t one tracking marker that remains in view the entire time. While I’m not sure this is the best way to deal with that problem, here’s what worked for me (using After Effects CS6).
1. Track the first marker normally, and apply its motion to a null object (“Edit Target”, select null object, then “Apply”).
2. Find a frame where both tracking markers are in view, and add another tracker. Then before you track it, move your cursor over the crosshair in the center of the tracker – you should be able to move it outside of the tracking box. Move it over on top of the first tracking marker, and switch back and forth between the two until the anchors appear to be in exactly the same spot. Finish tracking, and then apply that data to another null object.
3. Now you’ve got tracking data centered over the first marker through the entire shot, even when it moves out of frame, but it’s in two separate null objects. The way to fix this is to copy the position data from the second null object into the first null object. Open the second tracking marker and find the position property (or press “P”), drag to select the keyframes, and copy the keyframes. Then open the position property on the first tracking marker, find the section that doesn’t have any keyframes yet, and paste them there. You can then delete the second null object.
4. Now you have one null object that you can parent things to, and they’ll stick through the entire shot.
If anyone else has any simpler ideas on how to do this, please let me know in the comments!
Update: Thanks to rakete in the comments for this much much simpler method –
just track something like you do, and if you need to track another part of the picture, just press and hold alt, drag the feature and continue tracking. that’s it.
I’ll give that method a shot the next time, and update this post.