Hi Ho,
If you're talking about rotoscoping hair, then yes that is a tricky situation. What's the final result you're after? What's the background like?
If you haven't shot on a greenscreen, then I hope there's good contrast beteen background and foreground. Here's what I've done in the past:
i) Create a hi-con matte, using either luma key, or Calculations and Levels. This will not be perfect, but it is designed to give you a good edge key, to keep in details such as hair.
ii) Create an inner mask using rotoscoping. You can use mocha if you wish, or After Effects' own tools depending on the subject. The idea here is to fill in gaps and remove extraneous objects not keyed by your hi-con matte.
The reason I do it in this order is because roto is time consuming. You want to do as much of the heavy lifting as possible with your key/hi-con matte.
Good luck,
Ben
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