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Geometry

Holdout objects are used in place of real world object to catch light interactions from 3D objects. In RenderMan, holdout objects are specified by enabling the trace holdout attribute.

Code Block
Attribute "trace" "int holdout" [1]

Integrators and AOVs

Holdouts are an illumination feature implemented at the integrator level. They are able to catch a variety of integrator-specific illumination effects such as diffuse and specular reflections, transmissions, and caustics. Available integrator that support holdouts are: PxrPathTracer, PxrVCM, and PxrUPBP.

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Note

Because holdouts workflow involves multiple AOVs, rendering with adapt all enabled can reduce noise.

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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Alpha, Shadows, and Compositing

Holdout contributions can be composited over a film plate using an alpha and shadow pass as the masking term. The shadow pass can be output using the PxrShadowFilter plug-in. The occluded and unoccluded input AOVs should be rendered using the holdouts LPE prefix so that they only contain the shadowing term for holdout geometry.

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