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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.

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Holdouts are an integrator feature implemented at the integrator level. Available integrator that support holdouts are: PxrPathTracer, PxrVCM, and PxrUPBP.

// The same holdout render featuring caustics and thin shadows

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Because holdouts are considered an intrinsic geometric property when calculating illumination, LPE workflow remain unchanged for holdout geometry.

// A render of a beauty, diffuse, specular AOV

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Lights

By default, light sources do not illuminate holdout objects directly. This prevents double contribution between 3d lights used to illuminate non-holdout geometry and the illumination already captured on the plate image.

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Code Block
DisplayChannel "color occluded" "string source" ["color lpe:holdouts;C[DS]+[LO]"]
DisplayChannel "color unoccluded" "string source" ["color lpe:holdouts;unoccluded;C[DS]+[LO]"]

// render showing holdout shadow pass

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Compositing can be previewed in RenderMan by using PxrImagePlaneFilter SampleFilter plug-in. This will composite the beauty render over the specified image plane, using the alpha channel and holdout shadow AOV to mask the plate.

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