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Introduction

This documentation is intended to instruct developers in the authoring of custom bxdfs (previously referred to as surface shaders). Developers should also consult the RixBxdf.h header file for complete details.

The RixBxdfFactory interface is a subclass of RixShadingPlugin, and defines a shading plugin responsible for creating a RixBxdf object from a shading context (RixShadingContext) and the set of connected patterns (RixPattern).

The RixBxdf interface characterizes the light-scattering behavior (sometimes referred to as material response) for a given point on the surface of an object.

RixBxdfFactory

Implementing the RixBxdfFactory Interface

RixBxdfFactory is a subclass of RixShadingPlugin, and therefore shares the same initializationsynchronization, and parameter table logic as other shading plugins. Therefore to start developing your own Bxdf, you can #include "RixBxdf.h" and make sure your bxdf factory class implements the required methods inherited from the RixShadingPlugin interface: Init()Finalize()Synchronize()GetParamTable(), and CreateInstanceData(). Generally, there is one shading plugin instance of a RixBxdfFactory per bound RiBxdf (RIB) request. This instance may be active in multiple threads simultaneously.

Integrators (RixIntegrator) use RixBxdfFactory objects by invoking RixBxdfFactory::BeginScatter()  to obtain a  RixBxdf. Because a RixBxdf is expected to be a lightweight object that may be created many times over the course of the render, RixBxdfFactory is expected to take advantage of the lightweight instancing services provided by RixShadingPlugin. In particular, BeginScatter() is provided a pointer to an instance data that is created by RixBxdfFactory::CreateInstanceData(), which is called once per shading plugin instance, as defined by the unique set of parameters supplied to the material description. It is expected that the instance data will point to a private cached representation of any expensive setup which depends on the parameters, and BeginScatter() will reuse this cached representation many times over the course of the render to create RixBxdf objects. 

The RIX_BXDFPLUGINCREATE() macro defines the CreateRixBxdfFactory() method, which is called by the renderer to create an instance of the bxdf plugin. Generally, the implementation of this method should simply return a new allocated copy of your bxdf factory class. Similarly, the RIX_BXDFPLUGINDESTROY() macro defines the DestroyRixBxdfFactory() function called by the renderer to delete an instance of the bxdf plugin; a typical implementation of this function is to delete the passed in bxdf pointer:

RIX_BXDFPLUGINCREATE
{
    return new MyBxdfFactory();
}
RIX_BXDFPLUGINDESTROY
{
    delete ((MyBxdfFactory*)bxdf);
}

RixBxdfFactory::BeginScatter()

As mentioned above, integrators invoke RixBxdfFactory::BeginScatter() to obtain a RixBxdf. The renderer's operating model is that the Bxdf that is obtained this way is a closure, with the closure functions being GenerateSampleEvaluateSample, EvaluateSamplesAtIndex(), and EmitLocal. The RixBxdfFactory should stash state in the RixBxdf object and consider that the RixBxdf lifetime is under control of the integrator. Generally integrators will attempt to minimize the number of live  RixBxdf  objects but may nonetheless require a large number. For this reason, the RixBxdf  instances should attempt to minimize  memory consumption and construction / deconstruction costs.

Any computations that the Bxdf need in order to efficient evaluate its closure functions should be computed once inside RixBxdfFactory::BeginScatter(), and then saved in the overridden RixBxdf class. Critically, these computations include upstream evaluation of any pattern networks. Therefore, it is typical for BeginScatter() to invoke  RixShadingContext::EvalParam() in order to evaluate the relevant bxdf input parameters, and then pass the pointers returned from EvalParam to the Bxdf constructor. Since Bxdfs also generally require geometric data, or built-in variables, such as the shading normal (Nn) and viewing direction (Vn), either BeginScatter() or the Bxdf constructor itself will need to call RixShadingContext::GetBuiltinVar() function for each such built-in variable.

The following code demonstrates a trivial constant Bxdf's implementation of BeginScatter(). Here, the parameter id for the emission color is passed to EvalParam() in order to get a pointer emitColor that is passed to the ConstantBxdf() constructor.

RixBxdf *
PxrConstant::BeginScatter(RixShadingContext const *sCtx,
                          RixBXLobeTraits const &lobesWanted,
                          RixSCShadingMode sm,
                          RtPointer instanceData)
{
    // Get all input data
    RtColorRGB const* emitColor; 
    sCtx->EvalParam(k_emitColor, -1, &emitColor, &m_emitColor, true);
    RixShadingContext::Allocator pool(sCtx);
    void *mem = pool.AllocForBxdf<ConstantBxdf>(1);
    ConstantBxdf *eval = new (mem) ConstantBxdf(sCtx, this, lobesWanted, emitColor);
    return eval;
}


In the following code from PxrDiffuse, we demonstrate how its constructor sets up required geometric information that is later on used for sample generation and evaluation.


    PxrDiffuse(RixShadingContext const *sc, RixBxdfFactory *bx, 
               RixBXLobeTraits const &lobesWanted,
               RtColorRGB const *emit, 
               RtColorRGB const *diff, 
               RtColorRGB const *trans,
               RtNormal3 const *bumpNormal) : 
        RixBxdf(sc, bx), 
        m_lobesWanted(lobesWanted),
        m_emit(emit),
        m_diffuse(diff),
        m_transmission(trans),
        m_bumpNormal(bumpNormal)
    {
        RixBXLobeTraits lobes = s_reflDiffuseLobeTraits | s_albedoLobeTraits;
        if (m_transmission)
            lobes |= s_tranDiffuseLobeTraits;

        m_lobesWanted &= lobes;

        sc->GetBuiltinVar(RixShadingContext::k_P, &m_P);
        if(m_bumpNormal)
            m_Nn = bumpNormal;
        else
            sc->GetBuiltinVar(RixShadingContext::k_Nn, &m_Nn);
        sc->GetBuiltinVar(RixShadingContext::k_Ngn, &m_Ngn);
        sc->GetBuiltinVar(RixShadingContext::k_Tn, &m_Tn);
        sc->GetBuiltinVar(RixShadingContext::k_Vn, &m_Vn);
    }


BeginScatter() is passed an instance data pointer created via CreateInstanceData that can be used to cache and reuse certain calculations common amongst all factory instances of the same Bxdf; for more information, please consult the lightweight instancing discussion in RixShadingPlugin.

BeginScatter() is also passed two parameters that can be used as hints to optimize the calculation. RixBXLobeTraits const &lobesWanted is a description of the Bxdf lobes that the renderer expects to generate or evaluate; this parameter can be used to avoid any computations not necessary for the requested lobes. RixSCShadingMode will take either the value k_RixSCScatterQuery, indicating that the factory should construct a Bxdf for scattering on the surface, or k_RixSCVolumeScatterQuery, indicating that a Bxdf should be constructed for scattering on the inside of a volume. 

RixBxdfFactory::BeginOpacity()

In certain cases, integrators may also call RixBxdfFactory::BeginOpacity() to retrieve a RixOpacity object. BeginOpacity should be implemented in a similar fashion to BeginScatter(), except that will be only be invoked by the renderer in narrower constraints: either for presence and opacity. As such, any inputs to the factory that do not affect presence nor opacity need not be evaluated. Furthermore, the RixSCShadingMode can be examined to further narrow down the inputs; it will take either the value k_RixSCPresenceQuery or k_RixSCOpacityQuery.

RixBxdfFactory::BeginInterior()

Bxdfs that have interesting volumetric interior properties need to implement the BeginInterior() method, as well as an associated RixVolumeIntegrator. Please consult Writing Volume Integrators for more information.

Instance Hints

Bxdfs that require special opacity handling or support interior shading need to indicate their support for these capabilities via an instance hint. Most Bxdfs do not require such, and should simply implement in their factory a trivial implementation of GetInstanceHints() which simply returns k_TriviallyOpaque. Bxdfs that do modulate opacity and/or require interior shading are required to override the GetInstanceHints() method and return the appropriate bits in InstanceHints to the renderer in order to trigger calls to BeginOpacity and BeginInterior.

As a further optimization, Bxdfs that deal with opacity or interiors may choose to change their behavior based upon their instance parameters. For example, they may opt out of opacity entirely if they can prove via inspection of the parameters that the intended result is equivalent to opaque. Bxdfs that choose to do so should "bake" this awareness into their instance data at the time of CreateInstanceData() and inspect this instance data within the GetInstanceHints() implementation.

Note that at the time of CreateInstanceData, like other shading plugins, Bxdfs are unable to inspect the values of pattern network inputs; therefore, in cases these inputs are provided (i.e: RixParameterList::GetParamInfo() returns k_RixSCNetworkValue) the Bxdf may have no choice but to defer inspection of the inputs until BeginOpacity() or BeginInterior(). At that time, those methods may then choose to return NULL instead.



RixBxdf

Once a RixBxdf object is obtained, the integrator may invoke the following methods:

  • RixBxdf::GetEvaluateDomain() to figure out the domain over which the Bxdf evaluate samples;
  • RixBxdf::GenerateSample() to generate samples of the bxdf function, one sample for each point of the shading context;
  • RixBxdf::EvaluateSample() to evaluate the bxdf function, one direction for each point of the shading context;
  • RixBxdf::EvaluateSamplesAtIndex() to evaluate the bxdf function, one-or-many directions for a given point of the shading context;
  • RixBxdf::EmitLocal() to retrieve the bxdf's local emission.

The primary  RixBxdf  entry points operate on a collection of shading points (RixShadingContext) in order to reasonably maximize shading coherency and support SIMD computation. Integrators rely on the  RixBxdf's ability to generate and evaluate samples across the entire collection of points.  Sample evaluation may be performed in an all-points-one-sample variant using  EvaluateSample(), and a 1-point-n-samples variant via  EvaluateSamplesAtIndex(). Generation, however, is constrained to all-points-one-sample. Evaluation typically has different requirements (e.g. for making connections in a bidirectional integrator), whereas generation typically benefits from being performed all points at once.

The RixBxdf methods above are expected to return quantities for each point of the shading context originally given to RixBxdfFactory::BeginScatter(), excepting RixBxdf::EvaluateSamplesAtIndex(), that operates on a single point of the shading context, evaluating the bxdf function for one or many directions.

In order to maintain physical correctness, bxdfs are expected to conserve energy and obey the Helmholtz reciprocity principle. Care should be taken so that RixBxdf::GenerateSample(),  RixBxdf::EvaluateSample() and RixBxdf::EvaluateSamplesAtIndex() return consistent results. This allows bxdf plugins to be compatible with different rendering techniques such as unidirectional path tracingbidirectional path tracingphoton mapping and vertex connection merging (VCM).

Sample Generation

The GenerateSample() function has the following input parameters: transportTrait, lobesWanted, and random number generator rng.

  • The transportTrait tells the Bxdf the subset of light transport to consider: direct illumination, indirect illumination, or both. 
  • lobesWanted specifies what lobes are requested, for example specular reflection, diffuse transmission, etc. 
  • rng should be called to generate well-stratified samples; such samples typically reduce noise and improve convergence compared to using uniform random samples. 

The GenerateSample() function has the following output parameters (results): lobeSampled, directions, weights, forwardPdfs, reversePdfs, and compTrans.  All results are arrays with one value per sample point.

  • lobeSampled is similar to the input lobesWanted, and specifies which lobe was actually sampled. Bxdfs must respect the transportTrait and lobesWanted request and indicate which class of lobe is associated with each sample by setting lobesSampled for each generated sample. If and only if the bxdf is unable to generate a requested sample, then lobesSampled should be marked as invalid by calling SetValid(false); for example, if the lobesWanted argument requests a specific lobe (e.g., diffuse reflection) that the Bxdf does not support because the Bxdf only supports glossy reflections, then lobesSampled[i].SetValid(false) should be called. However, if it is possible to generate the requested samples, then lobesSampled should not be marked invalid and should instead be set to the sampled lobe for each point in the shading context. 

    When generating a Bxdf sample, you can also run into corner cases (often legitimate cases) where a valid sample cannot be generated. For instance, if the camera ray hits the backside of a single-sided object, a valid sample cannot be generated. Therefore, the lobeSampled for this camera ray should also be marked as invalid in this case by calling SetValid(false).

    If an invalid sample is returned to the integrator, the integrator will terminate the ray and avoid any further computation. Bxdfs that do not scatter light (e.g. PxrConstant) should also mark all samples as invalid.

    There is a subtle difference between an invalid sample and a black sample. If a camera ray hits a diffuse black surface, the sample will have zero contribution to the final image despite being valid. It would make sense for the integrator to terminate the ray because further bounces will not contribute to the final image either, but it is important to splat the black contribution to both the beauty and alpha channel. Failing to do so might result in missing geometry in the rendered image. An invalid sample on the other hand, should not only be terminated but also ignored for splatting purposes.

  • directions is the generated ray direction vectors; these directions must be unit length. 
  • weights is a color per sample indicating that sample's weight. 
  • forwardPdfs should account for light moving from the L to V direction, whereas reversePdfs account for the opposite (from V to L). Bxdfs should always provide both pdf values for integrators to use. Bxdfs that do not scatter light (e.g. PxrConstant) should set both pdfs to zero.
  • compTrans is an optional result which can be used to indicate transmission color; this will be used as alpha in compositing. A bxdf should check that compTrans is not NULL before assigning to it.  

As an example, a purely Lambertian diffuse bxdf should loop over the sample points and for each sample point set the result values as follows: set lobeSampled to diffuse reflection, set the reflection direction to a randomly generated direction chosen with a cosine distribution on the hemisphere defined by the surface normal (using a well-stratified 2D "random" sample value generated by calling the provided random number generator), set the weight to the diffuse surface albedo color times dot(Nn, Ln) divided by pi, set the forward pdf to dot(Nn, Ln) divided by pi, and set the reverse pdf to dot(Nn, Vn). More details can be found in the source code for the PxrDiffuse bxdf.

Sample Evaluation

The parameters to the EvaluateSample() function are very similar to GenerateSample()transportTraitlobesWanted, rnglobesEvaluateddirectionsweightsforwardPdfs and reversePdfs.  The main difference is that directions is an array of inputs that the function should compute weights and pdfs for.

The EvaluateSamplesAtIndex() function is very similar to EvaluateSample(), but is used to evaluate multiple samples at the same surface position and normal, but with different illumination directions (Ln). The inputs are the same as for EvaluateSample(), except that it has two additional inputs: index and number of samples numSamples. index is used to index into built-in variables such as the normal Nn and viewing direction Vn, and numSamples is the number of directions (Ln) to evaluate the bxdf for. The functionality of EvaluateSamplesAtIndex() otherwise is similar to EvaluateSample()and exists in order to make sample evaluation more efficient in certain light transport settings.

Evaluation Domain

Bxdfs can help integrators converge more quickly by providing hints about the domain over which they need to be integrated (the full domain being the entire sphere). This is done by the bxdf implementing a RixBxdf::GetEvaluateDomain() function that returns the appropriate RixBXEvaluateDomain value. For more information, please see Bxdf Evaluation Domain.


RixOpacity

The renderer will invoke the following methods on RixOpacity:

  • RixBxdf::GetPresence() to evaluate the geometry presence.
  • RixBxdf::GetOpacity() to evaluate the opacity color.



Additional Considerations

Bxdf Evaluation Domain

Bxdf Lobes

Non-Opaque Surfaces

Alpha for Compositing