Solve the Problem Internally

Solve the Problem Internally

The really gnarly fact is that premultiplication errors can
be introduced within a composition, typically by applying a
matte to footage that is already somehow blended—multiplied—
with a background.
If you see fringing in your edges, you can try the Remove
Color Matting effect (Figure 3.12). This effect has one setting
only, for background color, because all it does is apply
the unpremultiply calculation (the antidote to premultiplication)
in the same manner that it would be applied in
Interpret Footage.
An even better option in cases where you have an element
against black and no alpha channel is UnMult, originally
created by John Knoll and included on the book’s disc in
the Red Giant Software folder. This effect uses black areas
of the image to create transparency and removes the multiplied
black from the resulting transparent pixels