Introduction

RixShadingPlugin is a base class characterizing the requirements of a RenderMan renderer from shading plugins: RixBxdfFactory, RixDisplacementFactory, RixDisplayFilter, RixIntegratorFactory, RixLightFactory, RixLightFilter, RixPattern, RixProjectionFactory, and RixSampleFilter. These are plugins that implement services for the renderer.

This class provides entry points that are executed with a mix of various frequencies:

• once per rendering session
• once per render (in an interactive session, there can be multiple renders)
• once per batch of points to be shaded (i.e. RixShadingContext)
• multiple times per batch of points to be shaded

and various granularity:

• once per plugin
• once per instance of the shading plugin (as defined in the next section)
• multiple times per instance of the shading plugin

Plugin vs. Plugin instance vs. Closures

Here, the term instance is unfortunately overloaded, and it is important to differentiate between: an instance of a shading plugin as defined by the unique set of parameters given to the material definition, and a C++ instance which involves an actual memory allocation, construction, and destruction of an object.

Each use of a shading plugin with a different set of parameters will be considered as separate plugin instance. A render using two PxrTexture patterns (each using a different texture file parameter), will trigger the initialization of the (unique) PxrTexture plugin, and the initialization of two instances of the PxrTexture plugin (independently of the internal C++ representation chosen).

Finally, some plugin types will create short-lived objects referred to as closures. These transient objects are created for a given plugin instance, for a given batch of points. They allow the pre-computation of data that can be re-used during multiple computations associated with the the given batch of points.

Plugin Instance C++ representation

Depending on the plugin type, plugin instances may correspond to different internal C++ structures.

void RixShadingPlugin::CreateInstanceData(InstanceData*)

Some shading plugin types can create private instance data using the CreateInstanceData() method. Instance data would typically be computed from the unique evocation parameters, supplied to CreateInstanceData() via the RixParameterList class. This occurs when the shading plugin instance is created for the first time from those parameters (e.g. at the time a material definition is created (i.e. very early on in a render)).

Using these parameters, plugins may bake a cached understanding of their behavior, requirements, or even precomputed results into a private representation that the renderer will automatically track with the instance and supply back to the plugin methods. This allows the plugin to cache and therefore avoid repeated computations when processing each batch of points. In this case, the instance data is essentially a C++ structure representing one instance of the plugin.

If the shading plugin does create instance data, it should be stored in InstanceData::data. If the data requires non-trivial deletion, InstanceData::freefunc should be set to a function that the renderer will invoke when the plugin instance will no longer be needed. A trivial implementation of CreateInstanceData() produces no instance data and returns without modifying any member of the provided InstanceData structure.

Any instance data that is created will be automatically returned to the shading plugin methods by the renderer when the compute methods are invoked. The implementation of the compute methods is now free to use this instance data to reduce the cost of processing the current batch of points.

For example, the RixPattern plugin type uses this mechanism:

• RixPattern::CreateInstanceData(..., RixParameterList const*, InstanceData*)
• RixPattern::ComputeOutputParams(RixShadingContext const*, ..., RtPointer data)

The other plugin types using this representation are: RixBxdf, RixDisplacement, RixDisplayFilter, RixLightFilter and  RixSampleFilter. RixBxdf and RixDisplacement additionally make use of closures.

XXX* RixYYYPlugin()::CreateXXX()

Some other shading plugin types use a different mechanism instead of CreateInstanceData(). Generally, the RixShadingPlugin sub-class associated with the plugin type will expose a CreateXXX() method, used to build a new C++ object. In this case, the C++ object is a direct representation of the plugin instance. Generally, the render will then call methods on the newly minted C++ object, instead of using RixShadingPlugin methods and passing an instance data pointer.

For example, the RixProjection plugin type uses this mechanism:

• RixProjection* RixProjectionFactory::CreateProjection(..., RixParameterList const*)
• RixProjection::Project(RixProjectionContext&)

The other plugin types using this representation are: RixIntegrator and RixLightFactory.

Closures

Generally, the RixShadingPlugin subclass (e.g. RixPattern) will expose only a few compute methods (e.g. RixPattern::ComputeOutputParams()) that is executed once per batch of points (RixShadingContext), per plugin instance. When a specific class is used to represent the plugin instance (e.g. RixProjection), it will usually expose the compute methods itself (e.g. RixProjection::Project()).

However some plugins types require particular computations to happen multiple times for a given plugin instance, for a given batch of points. In this case, a closure object will be created. Note that the closures are built after the plugin instance has been created. Since any instance data (created by CreateInstanceData()) is be automatically returned to the shading plugin methods by the renderer, the plugin is free to use this instance data to reduce the cost of creating the associated closure.

Bxdf plugins are the best example of this:

• RixBxdfFactory::CreateInstanceData() fill the InstanceData::data with a representation of the bxdf instance, and this will provided to the closure creation method below.
  
• RixBxdf RixBxdfFactory::BeginScatter(RixShadingContext*, ..., RtPointer data) returns a closure for the provided batch of points and plugin instance
• RixBxdf::GenerateSamples() and RixBxdf::EvaluateSamples() may be called repeatedly on this closure. These methods are able to share and re-use any precomputation that may have happened in the RixBxdf closure constructor.

Parameter Table

All shading plugins are expected to return a description of their input and output parameters via the GetParamTable() method. This returns a pointer to an array of RixSCParamInfo, containing one entry for each input and output parameter, as well as an extra empty entry to mark the end of the table. This parameter table is used by the renderer to ensure proper type checking and validate the connections of upstream and downstream nodes. As such, each entry in the table should set a name, a type (RixSCType enumeration), detail (varying vs uniform, RixSCDetail enumeration), and access (input vs output, RixSCAccess enumeration). These declarations also need to be kept in sync with the associated .args file.

For an example of usage, consider a pattern plugin which returns a color. The resultC output parameter is a color, so it is defined in the parameter table as:

RixSCParamInfo("resultC", k_RixSCColor, k_RixSCOutput)

A float input parameter named density can be defined as:

RixSCParamInfo("density", k_RixSCFloat)

While a float[16] input parameter named placementMatrix can be defined as:

RixSCParamInfo("placementMatrix", k_RixSCFloat, k_RixSCInput, 16)

The full implementation of GetParamTable() for this plugin would look something like this:

RixSCParamInfo const *
MyPattern::GetParamTable()
{
    static RixSCParamInfo s_ptable[] =
    {
        // outputs
        RixSCParamInfo("resultC", k_RixSCColor, k_RixSCOutput),
        // inputs
        RixSCParamInfo("density", k_RixSCFloat),
        RixSCParamInfo("placementMatrix", k_RixSCFloat, k_RixSCInput, 16),
        RixSCParamInfo(), // end of table
    };
    return &s_ptable[0];
}

The ordinal position of a parameter in the parameter table is the integer paramId used to evaluate parameter inputs using the RixShadingContext::EvalParam method. Because these need to be kept in sync, it is recommended that you create a parameter enumeration (a private enum type) to keep track of the order that your parameters were created in the table. The enumeration can be used later on when calling RixShadingContext::EvalParam  in the body of the shader. Following the three parameter table entries above:

enum paramId
{
    k_resultC=0, // output
    k_density,
    k_placementMatrix,
    k_numParams
};

Dynamic Parameters

A plugin can create its parameter table dynamically based on the parameters provided to each instance of the plugin. This dynamically created table is created using the CreateInstanceData() method, and should be saved in the paramtable member of the InstanceData. If the associated memory need to be freed, if should be taken care of in the freefunc() routine. Generally, static interfaces should be preferred over dynamic interfaces due to their extra memory expense. If the paramtable member remains null, all instances will share the parameter table returned by GetParamTable(). In order to prevent the renderer from filtering out dynamic parameters as bad inputs, a plugin that is using a dynamically created table should have a k_RixSCAnyType entry in its plugin parameter table.

Initialization and Synchronization

Plugin initialization

In order to initialize the plugin, the renderer will call Init() once. Even if the plugin is evoked multiple times during the render with different arguments, Init() will still be called only once during the lifetime of the render. The RixContext parameter can be used by the plugin to request any RixInterfaces services provided by the renderer. Any expensive memory allocations or operations that can be reused during the lifetime of the plugin can be performed in this routine. Upon successful initialization, this routine should return a zero status.

Finalize() is the companion to Init(), called at the end of rendering with the expectation that any data allocated within the Init() implementation will be released.

Plugin instance initialization

Depending on the paradigm used by the plugin type, CreateInstanceData() or CreateXXX() will be called once per plugin instance:

• CreateInstanceData() allows the plugin instance to create private data (stored in InstanceData::data), that will then be provided back to the various plugin methods the renderer will call during rendering.
• CreateXXX() returns a C++ object that should store its own representation of the plugin instance. The renderer will call methods on this object during rendering (instead of calling plugin methods).

The renderer provides the unique evocation parameters that triggered the creation of the plugin instance through the the RixParameterList class.

RixParameterList allows for the evaluation of the plugin instance parameters via the EvalParam() method. To aid in this, it allows for the querying via RixParameterList::GetParamInfo() of whether the parameters have been unset (and are therefore at their default value), set as a uniform value, or are part of a network  connection, i.e. the parameter is computed by an upstream node in the shading graph. A network connection is understood to be a varying quantity, and its value cannot be evaluated at the time that CreateInstanceData is evoked; this is why EvalParam() will return k_RixSCInvalidDetail if the parameter is a network connection. Otherwise, EvalParam() can be used to get an understanding of the uniform, non-varying parameters that are passed to the shading instance, and these can be used to perform any precomputations as needed.

Plugin Synchronization

The Synchronize() routine allows the plugin to respond to synchronization signals delivered by the renderer. This call may happen multiple times during a render session and/or during a given render.

The renderer may provide additional information to the plugin via the input parameter RixParameterList. These signals include:

• k_RixSCRenderBegin: The renderer is being initialized.
• k_RixSCRenderEnd: The renderer is about to end.
• k_RixSCInstanceEdit: Currently unused.
• k_RixSCCancel: Currently unused.
• k_RixSCCheckpointRecover: A signal that the renderer is about to restart rendering from a checkpoint. The parameter list will contain a single constant integer "increment" which contains the increment value from which the renderer will restart.
• k_RixSCCheckpointWrite: A signal that the renderer is about to write a checkpoint. The parameter list will contain two values: a constant integer "increment" indicating the increment value the renderer will write, and a constant string "reason" which contains one of three values: "checkpoint", "exiting", or "finished", indicating why the renderer is writing the checkpoint.
• k_RixSCIncrementBarrier: A signal that the rendering of an new increment is about to begin. This signal will only be received if the integrator has set wantsIncrementBarrier to true in the RixIntegratorEnvironment. The parameter list will contain a single constant integer "increment" which contains the increment value the renderer is about to render.

Plugin instance synchronization

Similarly to Synchronize(), the SynchronizeInstanceData() method allows the plugin instance to update its state when a render starts.

There are some subtle differences with Synchronize() though:

• there is no synchronization message, it is always assumed to be k_RixSCRenderBegin
• the plugin instance must explicitly subscribe to this mechanism, by appropriately setting InstanceData::synchronizeHints duringCreateInstanceData()

If the plugin type doesn't use CreateInstanceData() but CreateXXX(), the created objects will sometimes expose an Edit() or Synchronize() method.

Closures synchronization

Closures are transient objects with a very short lifetime, and are re-created with such a frequency that there is no need for synchronization mechanism.

Interactive rendering sessions

Because Init() and CreateInstanceData() are only called once during a rendering session, they are unable to capture edits that may have happened after the plugin instance has been initialized. In some cases, this includes edits that may have happened before the render starts, yielding counter-intuitive behaviors.

It is therefore strongly recommended for these two methods to only rely on data that was explicitly provided (e.g. the plugin instance parameter list). In particular, special care should be taken not to query options or anything related to the render state (displays, integrator environment, lpe-related quantities, etc...) for the following reasons:

• options may be edited after these methods have been called
• displays and render state will most likely be undefined when these methods are called, and may be subsequently edited

In order to support general edits, and to be future-proof, the Synchronize() and SynchronizeInstanceData() should be used instead to query options and render state.

Subtleties

• RixProjection (the type of objects created by RixProjectionFactory::CreateProjection() to represent projection plugin instances) doesn't expose a Edit() or Synchronize() method. Instead, RixProjection::RenderBegin() will be called, allowing for the plugin to set the RixIntegratorEnvironment::deepMetric. There is currently no way RixProjection plugins to opt-in / opt-out of the RixProjection::RenderBegin() call.
• RixLight (the type of objects created by RixLightFactory::CreateLight() to represent light plugin instances) currently uses a mix of CreateInstanceData() and CreateLight() paradigms. It is strongly recommended to keep CreateInstanceData() empty and put the majority of the implementation in CreateLight().
• RixIntegrator ((the type of objects created by RixIntegratorFactory::CreateIntegrator() to represent integrator plugin instances) exposes Synchronize(). This method uses a slightly different signature, including a synchronization message, and it is currently not possible to opt-in/opt-out of the synchronization calls.

Known limitations

• RixBxdf::GetInstanceHints() is called after CreateInstanceData() but before SynchronizeInstanceData(). As a consequence, if a bxdf instance's hint depends on a global option or on the render state, edits will have no effect. This will lead to counter-intuitive behaviors, and it is strongly recommended for bxdf instance hints to only depend on the shader parameters provided to CreateInstanceData()
• RixPattern::Bake2dOutput() and RixPattern::Bake3dOutput() are called after CreateInstanceData() but before SynchronizeInstanceData(). The same limitations as the one described in the item above apply.
• RixLight::Edit() and RixLight::SynchronizeInstanceData() are currently not called.

Misc

Overview

The following table summarizes the structure of the various RixShadingPlugin, in relation to initialization and synchronization. All plugins use the RixShadingPlugin::Init() and RixShadingPlugin::Finalize() calls.

Examples

PxrDiffuse

First, note that while the bxdf plugin is published and exposed as 'PxrDiffuse', it is comprised of two classes:

• PxrDiffuseFactory, that inherits from RixBxdfFactory, a subclass of RixShadingPlugin. This is the class that manages instance data and associated closures.
• PxrDiffuse, that inherits from RixBxdf. This is the type used by the bxdf closures.

As an example of usage of instance data, consider the PxrDiffuse bxdf. Although it is a fairly trivial bxdf, it does handle presence and opacity, and the renderer passes instance data to the RixBxdfFactory::GetInstanceHints() interface in order to get an understanding of the requirements for presence and opacity. In the following code, PxrDiffuseFactory checks its own presence parameter to see if it is a connection, knowing that its Args file only allows presence to be set to a default value (and therefore is trivially fully opaque) or is connected (and therefore requires the renderer to perform presence calculations). If it is connected, then it sets the instance data to be the same InstanceHints bitfield that is requested by the renderer from RixBxdf::GetInstanceHints.

    plist->GetParamInfo(k_presence, &typ, &cnx1);
    if(cnx1 == k_RixSCNetworkValue)
    {
        if (cachePresence == 0)
        {
            req |= k_ComputesPresence;
        }
        else
        {
            req |= k_ComputesPresence | k_PresenceCanBeCached;
        }
    }

PxrDirt

For a more complicated example of instance data usage, consider the PxrDirt pattern. Its instance data routine caches the values of many uniform parameters and reuses them in PxrDirt::ComputeOutputParams(), knowing that its .args file prohibits those parameters from being set to network connections.

    Data *data = static_cast<Data*>(instanceData->data);

    data->numSamples = 4;
    data->distribution = k_distributionCosine;
    data->cosineSpread = 1.0f;
    data->falloff = 0.0f;
    data->maxDistance = 0.0f;
    data->direction = k_directionOutside;
    data->raySpread = 1.0f;

    params->EvalParam(k_numSamples, 0, &data->numSamples);
    data->numSamples = RixMax(1, data->numSamples);
    params->EvalParam(k_distribution, 0, &data->distribution);
    params->EvalParam(k_cosineSpread, 0, &data->cosineSpread);
    params->EvalParam(k_falloff, 0, &data->falloff);
    params->EvalParam(k_maxDistance, 0, &data->maxDistance);
    params->EvalParam(k_direction, 0, &data->direction);

Installation

RenderMan will search for shading plugins on demand, under the rixplugin searchpath. Custom shading plugins can be installed in a directory that can either be appended to the /rixpluginpath settings in Rendermn.ini; or the directory can be appended to the rixplugin search path which is emitted by the bridge. 

Creating an .Args File

If you would like RenderMan for Maya or RenderMan for Katana to recognize your plugin and provide a user interface for changing input parameters and connecting output parameters to other nodes, then you will need to create an args file for your shading plugin. The args file defines the input and output parameters in XML so that tools like RMS or Katana can easily read them, discover their type, default values, and other information used while creating the user interface for the pattern node. Please consult the Args File Reference for more information.