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In movies and commercials, we often need to composite the CG objects over a live action plate. To convince the audience that the CG objects are real and belong to the scene, we need to use a holdout object to collect the CG objects' shadow and reflection that match very closely to the shadow and reflection in the live action plate.
In RenderMan, holdouts objects are geometry with
In RenderMan, holdout objects are indictated by enabling the trace attribute holdout.