Apple Shake 4 - Manual (Page 695)

Chapter 24 Keying 695
Problem 1: Edge Ringing
When the blur is applied, a blue edge is introduced along the woman’s neck line.
Problem 2: Accidentally Blurring Both Layers
One might be tempted to place the Blur node after the Keylight node in the tree. This
also produces an incorrect result—the background is blurred as well.
Problem 3: Artifacts Introduced by Masking
Lastly, one might be tempted to mask the blur using the key from Keylight. This also
produces an incorrect result.