I'm coming across a problem while trying to pull a key from a not very well lit greenscreen shot. I'm employing the technique of using Keylight in its combined matte output mode to produce a black and white imnage which serves as an alpha chanel for my footage - I've placed it in the layer above my footage and set the track matte on the footage to Luma Matte. So far so good. The difficulty arises when I precomepose the layer I'm using as a matte and duplicate the layers inside this comp and use masks to tweak the keylight settings for different parts of the frame. This works fine apart from the problem that the masks create a dark border which is clearly visible in the keyed footage. Feathering the mask only makes matters worse. Any idea what's going on here and how I can work round this?
Thanks for the suggestion, Roland. I've tried Matte Choker, Refine Matte and Simple Choker. They all turn everything outside my mask black and none of the choke controls seem to have any effect at all. I've tried experimenting with blend modes too but this hasn't worked either.