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VFX artists often need to composite the CG objects over a live action film plate. To convince the audience that the CG objects are real and belong to the scene, holdouts are used to collect the CG objects' shadow and reflection that match very closely to the shadow and reflection in the live action plate.

Error rendering macro 'carousel-slider-macro'

query parameter is required.

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.

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.

Error rendering macro 'carousel-slider-macro'

query parameter is required.

LPE workflow remains unchanged for holdout geometry.

Error rendering macro 'carousel-slider-macro'

query parameter is required.

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

Lights

By default, light sources are considered holdouts are do not illuminate other 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 light source that does directly illuminate holdout objects, use "__illumholdout" as the light group suffix. This tells integrator plug-ins to consider this light as a non-holdout light source when determining illumination.

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

Error rendering macro 'carousel-slider-macro'

query parameter is required.

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.

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

Error rendering macro 'carousel-slider-macro'

query parameter is required.

Compositing can be interactively previewed in RenderMan by using the PxrImagePlaneFilter plug-in. This will composite the film plate with the beauty channel, using the alpha channel and holdout shadow AOV to mask the plate.

Error rendering macro 'carousel-slider-macro'

query parameter is required.