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The seasoned RenderMan user will notice this is all very similar to how displacement was done in the classic RenderMan Shading Language (RSL) – see for example the "Advanced RenderMan" book by Apodaca and Gritz, section 8.2.  One notable difference is that the displacement calculations used to be in camera space, but in RenderMan 22 they are in object space.

The RixDisplacement class

For most users, it will be sufficient to write OSL patterns to calculate displacement amounts, and simply pass their output to the standard PxrDisplace displacement shader.  However, PxrDisplace is just one example of a RixDisplacement class displacement shader.  The RixDisplacement interface characterizes the displacement of points on the surface of an object.

If a developer wishes to write their own displacement plug-in, two classes are of interest: RixDisplacementFactory and RixDisplacement.

The RixDisplacementFactory interface is a subclass of RixShadingPlugin, and defines a shading plugin responsible for creating a RixDisplacement object from a shading context (RixShadingContext) and the set of connected patterns (RixPattern).  Since RixDisplacementFactory is a subclass of RixShadingPlugin, it shares the same initialization, synchronization, and parameter table logic as other shading plugins. Therefore to start developing your own displacement plug-in, you can #include "RixDisplacement.h" and make sure your displacement factory class implements the required methods inherited from the RixShadingPlugin interface: Init(), Finalize(), Synchronize(), GetParamTable(), BeginDisplacement(), and EndDisplacement(). Generally, there is one shading plugin instance of a RixDisplacementFactory per bound RiDisplacement (RIB) request. This instance may be active in multiple threads simultaneously.

Once a RixDisplacement object is obtained, the renderer may invoke its GetDisplacement() method.  The GetDisplacement() method loops over the surface points in a RixShadingContext and moves them according to e.g. surface normal and various input parameters.  Operating on a collection of shading points allows maximizing shading coherency and supporting SIMD computation.

For more details, please see the built-in PxrDisplace displacement shader implementation – it is a good example of how a displacement plug-in is structured.