RENDERMAN 22 DOCUMENTATION

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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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Geometry

In RenderMan, holdout objects are specified by enabling the trace attribute holdout.

Attribute "trace" "int holdout" [1]

Integrators and AOVs

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

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

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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.

To create a holdout light (a light source that directly illuminates other holdouts), create a light using "__illumholdout" as the light group suffix. This tells integrators to consider this light as a new light source when illuminating holdout geometry.

Light "PxrSphereLight" "spherelight" "string lightGroup" ["light__illumholdout"]

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Alpha, Shadows, and Compositing

A holdout shadow pass can be output using the PxrShadowFilter plug-in. Occluded and unoccluded input AOVs should be rendered using the holdouts LPE prefix.

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

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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.

Error rendering macro 'carousel-slider-macro'

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Additional Considerations

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