Having a real difficult time with Key Light

Question

I'm reading Mark Christiansen's After Effects CS4 Studio Techniques and Ive read the section on using Key Light 5 times and still cant get a key like his examples.  I'm trying to conquer two things that just are seeming impossible. One to retain the whisps of hair blowing in the wind and 2, to get rid of the green spill on the edges.  I've tried the 3 Pass Method, which he does not very cleary explain,and I cant get the edge matte that it is supposed to produce.  He leaves some of the important infomation written in the side notes, and other things are just plain confusing and he does not make clear reference to the examples on the DVd.
  Can anyone help me conquer this keying or at least point me in the direction so that I can learn this Step by CLEAR Step.
Thanks.

Answer 1

Hi there,
The principle here is simple - it sounds as though the execution is what's tripping you up.
First, let's get the principle straight. The whole reason to do this is to avoid using Keylight for anything but your edges in order to give them the subtlest possible threshold. This means breaking apart the image, and the most elementary way to do it is to start with three passes: a garbage matte to get rid of background noise/rig that doesn't cross the edges, a holdout matte to get rid of holes in the matte, and finally the edge pass. In more complicated shots it is necessary to break the image down further, for example doing one edge pass for hair and another for the body, etc.
So you can start by just applying Keylight and creating your key.
What I have you doing if that doesn't simply solve your key is to spread the resulting matte (using Simpler Choker with a negative value) so that it's well outside the edges to create the garbage matte. In the previous CS3 version of the book I instructed that you do this on a separate layer and use Stencil Alpha blending mode to apply the result to the layers below, one for the edge matte and one for the holdout matte. You can still do it that way if it's clearer.
Otherwise, on a single layer the steps are
1. apply Keylight, refine the matte and then leave the View menu set to Intermediate Result - that's important, it insures no color shift
2. spread the matte with Simple Choker, set to a negative value (usually in the low double-digits)
3. apply Keylight again to get your edge matte
And after that, if there are holes in the matte, fix them by masking out a separate layer of the same source, below all of the others.
I hope that helps.
Mark Christiansen
Author, After Effects Studio Techniques