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The default projection type will use the projection specified either in the CameraCreate node or Alembic_In camera.
Projection Modifiers
Projection modifiers can change one or more aspects of the camera projection. For example, PxrRollingShutter modifies the time samples on the camera rays, without adjusting the ray origin or direction. Projection modifiers can only be used with a default projection - perspective or orthographic.
Custom Projection Plugins
Custom Projection plugins, such as PxrCamera, are created with a Material or PrmanShadingNode and assigned to /root with a MaterialAssign node. Just like projection modifiers, only one custom projection plugin can be used at a time in conjunction with a default projection.
