Tracker2Mask

download the script at aescripts.com/tracker2mask
(pay what you want - I suggest 10$ but you can even download it for free by changing the price to 0$. Here is more info about the pay what you want system)

Tracker2Mask is an After Effects script that computes the movement of maskshapes based on the movement of trackpoints. It is easy and flexible to use. In particular you don't need one trackpoint for each maskpoint.

The workflow is as follows:

1. track a few points (something between one and four points is reasonable, even for complex mask shapes) that represent the basic movement of the object which you want to draw a mask around
   
2. keyframe the mask by hand in the first and last frame
   
3. use the script to predict keyframes in between (the script uses the tracker information for this prediction which is hence much better than the simple interpolation done by after effects itself)
   

Step 3) can be interleaved with manual corrections which leads to a quite fast, assisted rotoscoping workflow.

Tutorial 1

In this first tutorial I show how to rotoscope the shape of a moving car based on only four trackpoints, for simpler shapes often one trackpoint is sufficient.

Tracker2Mask Tutorial from Mathias Möhl on Vimeo.

Tutorial 2

In the second tutorial I describe some new features of Tracker2Mask that allow local modifications of masks.
Tracker2Mask Tutorial 2 - new features from Mathias Möhl on Vimeo.

Tracker2Mask works with After Effecs CS2 (7.0) and higher.