Apple Shake 4 - Manual (Page 616)

616 Chapter 23 Color Correction
In the following example, a computer-generated graphic is composited with a
background image. The addition of a ContrastLum node (with a value of .6) results in
premultiplication issues—manifested as a fringing around the edges of the image.
To eliminate this problem, unpremultiply the graphic prior to color correction by
inserting an MDiv node prior to any color-correction nodes in the tree. Later, at the end
of the chain of color-correction nodes you apply in the tree, make sure you once again
premultiply the graphic by adding an MMult node (this can also be done by turning on
the preMultiply parameter in the Over node).