Apple Shake 4 - Manual (Page 429)

Chapter 15 Image Processing Basics 429
If you zoom into the Viewer and look very closely, you’ll notice a dark rim appears
around the edge.
3 In the Node View, select the IMult3 node and press I.
The node is ignored, and the same nasty edges appear as before.
4 Press I again so the node is no longer ignored.
So, what is the solution? Simple math will help resolve this issue. Dividing something
by a number and then multiplying by the same number is the same as multiplying by
1—the equivalent of no change at all. If you multiply the foreground by the mask one
too many times, divide it by the alpha to balance it out.
Mathematically, here’s how it looks:
Comp = (Fg * A / A * A) + ((1-A)*Bg)
Or, abbreviating the equation, you return to the proper formula, since the two extra
alphas (A) cancel out:
Comp = (Fg * A) + ((1-A)*Bg)
To translate this into the node tree:
1 In the Node View, select the IMult1 node, then click Layer–IDiv.
An IDiv1 node is attached to the IMult1 node.
Add switched to ContrastLum IAdd1 detail
IAdd1 detail with IMult3 ignored