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Difference

This node produces the difference between two images as a matte. It takes two inputs:

input B contains the subject with the background, and

input A contains the background without the subject (a clean plate).

You can use this node to produce a rough matte for a moving subject shot in front of a static background, for example. However, because of lighting discrepancies, grain, and tiny movements of the camera or background, you may need to use other nodes to refine the results.