Apple Color 1.5 - Manual (Page 299)

Any grayscale image can be used to create a matte that you can connect to the Alpha
input, for a variety of effects. In the following example, a Curve node is used to manipulate
the contrast of an image so that an Edge Detector node can better isolate the edges to
create a grayscale matte; a Blur node is used to soften the result, and an Invert node is
used to reverse the black and white areas of the matte so that the edges of the face
become the areas of the matte that are transparent, or not to be adjusted.
This matte is connected to the Alpha input of the Alpha Blend node (the third input). A
Blur node is then connected to the Source 2 input.
299 Chapter 11 The Color FX Room