Table of Contents

Introduction

A RixShadingContext encapsulates a shading context - a description of the environment in which shading is to be performed. It is the primary domain of operation for the shader classes RixBxdf,  RixDisplacement,  RixLight,  RixPattern, and RixVolume, and is also of high importance to RixIntegrator. Implementors of these classes and their associated plugins will thus need to be intimately familiar with the RixShadingContext class.

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In some circumstances, users of EvalParam() that do not know the paramId (perhaps due to the use of a dynamic parameter table) or the type of the parameter can introspect for the paramId or information about the parameter via the GetParamId() and the GetParamInfo() methods.  

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allocate allocate

Memory Management

RixShadingContext provides a fast memory allocation service tailored to shaders that need to execute quickly. The main provider of this service is the Allocate() routine; the New() routine and the class Allocator are wrappers around Allocate(). The backing store for this memory allocation are memory pools which are tailored specifically to the lifetime requirements of RixPattern and RixBxdf, with the latter category equating to the the lifetime of the RixShadingContext. Clients that use these memory management services should use "placement new" semantics and should not rely on the invocation of a destructor.

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getnearesthits getnearesthits

Tracing Rays

RixShadingContext provides a limited service for tracing rays. The GetNearestHits() routine allows shaders to trace rays to determine the nearest hit. These rays do not trigger shading on the hit geometry, and should thus be considered geometric probe rays: the only information that can be returned from GetNearestHits is a RtHitPoint struct, which contains a minimal set of geometric information including the distance, the P, Ng, u, v builtins of the hit geometry, and the filter and micropolygon sizes. A ray that missed is indicated by the distance being set to 0. While limited, this service is sufficient to allow for calculation of effects such as ambient occlusion.

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Transformations Transformations

Transformations

Transformation of RtFloat3 data between two coordinate systems known to the renderer can be accomplished using the Transform() method. The interpretation of the RtFloat3 data as point, vector, or normal data is specified using the TransformInterpretation enum. The array of data must be numPts in size and the data will be transformed in place. A non-zero return value indicates a failed transformation, typically due to unknown coordinate systems being specified.

In addition, transform matrices between two coordinate systems can be returned directly via the GetTransform() method. The matrix output parameter points at storage allocated by the renderer containing the matrices. The size of this storage is indicated by the numMatrices output parameter. If the transformation is time varying, the numMatrices returned will be the same as numPts. If the transformation is uniform, numMatrices will be set to 1. A non-zero return value indicates a failed transformation, typically due to unknown coordinate systems being specified.