Apple Color 1.0 - Manual (Page 246)

246 Chapter 11 Color FX
Isolating Portions of an Image
Another common method of creating a layered effect is to use a grayscale matte to
control where in an image two inputs are added together. The Alpha Blend node has
three inputs that work together to create exactly this effect.
This node blends the Source 2 input to the Source 1 input in all the areas where the
Source 3 Alpha input image is white. Where the Alpha input image is black, only the
Source 1 input is shown.
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.