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Here, it is important to distinguish between two types of plugins: ones that need to create many lightweight instances over the course of a render, and ones that do not. RixBxdf, RixDisplacement, and RixLight represent represent lightweight instances that may be created many times during the course of a single render, and therefore are not directly subclasses of RixShadingPlugin. Instead, instancees of those classes are returned by the appropriate Factory (e.g. RixBxdfFactory), with the Factory itself being the subclass of RixShadingPlugin.

RixIntegrator, RixDisplayFilter, RixLightFilter, RixPattern, RixProjection, and RixSampleFilter do do not create many lightweight instances. As such, these classes are directly subclasses of RixShadingPlugin.

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