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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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By default, light sources are considered holdouts and 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 holdout light (i.e. a light source that does directly illuminates illuminate holdout objects), create a light using use "__illumholdout" as the light group suffix. This tells integrators integrator plug-ins to consider this light as a non-holdout light source when illuminating holdout geometrydetermining illumination.

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

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Alpha, Shadows, and Compositing

Holdout contributions contribution can be composited over a film plate using an alpha and shadow pass as the masking termmasks. The shadow pass can be output is generated using the PxrShadowFilter PxrShadowDisplayFilter plug-in and can be optionally written to its own AOV or added to the alpha. 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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Compositing can be interactively previewed in RenderMan by using the PxrImagePlaneFilter PxrImageDisplayFilter plug-in. This will composite the film plate with the beauty channel, using the alpha channel and holdout shadow AOV to mask the plate.

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