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Introduction

This documentation is intended to instruct developers in the authoring of custom patterns. Developers should also consult the RixPattern.h header file for complete details.

A RixPattern plugin is used to connect textures and procedurally generated patterns to RiBxdf parameters, or to other patterns to create a shading graph. There are numerous pattern plugins included with the RenderMan software, but if none of the included plugins generate the pattern you want, then this guide will help you write your own pattern plugin. Source code for many of the RenderMan pattern plugins can be found in the PixarRenderMan-Examples-VERSION/plugins/pattern/ directory which is installed as part of the separate examples package.

For pattern generation using the Open Shading Language (OSL), see the PxrOSL plugin documentation on Working with PxrOSL.

Implementing the RixPattern Interface

RixPattern.h defines the interface that all pattern plugins must implement. RixPattern is a subclass of RixShadingPlugin, and therefore shares the same initializationsynchronization, and parameter table logic as other shading plugins. Because a RixPattern is expected to be a lightweight object that may be created many times over the course of the render, RixPattern is expected to take advantage of the lightweight instancing services provided by RixShadingPluginTherefore to start developing your own pattern, you can #include "RixPattern.h" and make sure your pattern class implements the required methods inherited from the RixShadingPlugin interface: Init(), Finalize()Synchronize(), GetParamTable(), and CreateInstanceData().

The RIX_PATTERNCREATE() method is called by the renderer to create an instance of the pattern plugin. Generally, the implementation of this method should simply return a new allocated copy of your pattern class. Similarly, the RIX_PATTERNDESTROY() method is called by the renderer to delete an instance of the pattern plugin; a typical implementation of this method is to delete the passed in pattern pointer:


RIX_PATTERNCREATE
{
return new MyPattern();
}
    RIX_PATTERNDESTROY
{
delete ((MyPattern*)pattern);
}


ComputeOutputParams

ComputeOutputParams() is the heart of a pattern plugin: it evaluates the input parameters, and computes the pattern output. It is called once per graph execution, and all outputs must be computed during this single invocation. The number and type of outputs should match the number and type of outputs declared in the parameter table. The domain of evaluation of this function is a shading context, which is of type RixShadingContext, defined in RixShading.h

To read an input value, use the RixShadingContext::EvalParam() method. The desired input parameter to the pattern is selected by an integer paramId, which is the ordinal position of the parameter in the parameter table. Patterns are expected to know the paramId, the type of the associated parameter, and are expected to pass a pointer to a pointer of the appropriate type. As such, it is suggested that a private parameter enumeration is used to keep track of the order that the parameters are created in the parameter table. For more information, please consult the documentation for RixShadingContext::EvalParam() and RixShadingPlugin::GetParamTable().

After reading input values, output values need to be set up. First, memory buffers for the requested outputs should be allocated using the RixShadingContext memory allocation services. These buffers should then be bound to the requested OutputSpec outputs parameter passed to ComputeOutputParams(), and the type and detail information about those outputs filled in as well. This information should match the declarations from the parameter table. The following code is boilerplate code that can be used to set up the memory allocations for the output parameters. It reads the plugin's parameter table, loops through and allocates the appropriate buffers, and sets the detail and type assuming that the output is always a varying color or float (typical of most patterns).

    // Find the number of outputs
RixSCParamInfo const* paramTable = GetParamTable();
int numOutputs = -1;
while (paramTable[++numOutputs].access == k_RixSCOutput) {}
    // Allocate and bind our outputs
RixShadingContext::Allocator pool(sctx);
OutputSpec* out = pool.AllocForPattern<OutputSpec>(numOutputs);
*outputs = out;
*noutputs = numOutputs;
    // looping through the different output ids
for (int i = 0; i < numOutputs; ++i)
{
        out[i].paramId = i;
out[i].detail = k_RixSCInvalidDetail;
out[i].value = NULL;
        type = paramTable[i].type; // we know this

sctx->GetParamInfo(i, &type, &cinfo);
if(cinfo == k_RixSCNetworkValue)
{
            if( type == k_RixSCColor )
{
out[i].detail = k_RixSCVarying;
out[i].value = pool.AllocForPattern<RtColorRGB>(sctx->numPts);
}
else if( type == k_RixSCFloat )
{
out[i].detail = k_RixSCVarying;
out[i].value = pool.AllocForPattern<RtFloat>(sctx->numPts);
}
}
}

Finally, the pattern can now actually compute the values that go into the output buffers. This is typically done by using the inputs and looping through the number of shaded points RixShadingContext::numPts to compute some values that are stored in the allocated output buffers.


for (int i=0; i<sctx->numPts; i++)
{
    // Compute some output values based on your input. Here we assume
    // outColor is the memory buffer allocated for an output parameter,
    // and inputColor and inputFloat are two inputs that were returned from
    // EvalParam.
    if (style == 1)
    {
        outColor[i] = inputColor[i] * inputFloat[i];
    }
}

In the simple example above, we assume the style variable was a uniform RtInt input value, so there is only one value for all the points in the shading context. Meanwhile, the inputColor and inputFloat  variable were varying instead of uniform, so they are pointers to an array of RtColorRGB values and array of RtFloat values respectively, one for each shaded point in the shading context.

The ComputeOutputParams() method should return 0 if no error occurred while calculating the output, otherwise it should return a non-zero integer value.

Testing Your Pattern Plugin

After you have implemented the code for your pattern plugin, you can build it using the commands listed in the Compiling Plugins page. The next step is to test your plugin. To test it, you'll need to make sure prman can find your plugin in the standardrixpluginpath list of directories, which is defined in $RMANTREE/etc/rendermn.ini as:

/standardrixpluginpath          ${RMANTREE}/lib/RIS/pattern:${RMANTREE}/lib/RIS/bxdf:${RMANTREE}/lib/RIS/integrator:${RMANTREE}/lib/RIS/projection

You can add a rendermn.ini file to your HOME directory and modify the standardrixpluginpath value to contain the directory where your pattern plugin is located.

Then you can try to render this RIB file after you have replaced "PxrCustomPattern" with the name of your pattern plugin and connect your pattern's output parameter to one of the input parameters of the PxrDiffuse Bxdf:

Display "patternTest" "framebuffer" "rgba"
Quantize "rgba" 255 0 255 0
Format 128 128 1
Projection "perspective" "fov" [45]
Hider "raytrace" "string integrationmode" ["path"]
Integrator "PxrPathTracer" "integrator"

WorldBegin

    AttributeBegin
        Attribute "identifier" "name" ["sphere1"]
        Translate 0 0 2.75

        Pattern "PxrCustomPattern" "customPattern"

        Bxdf "PxrDiffuse" "smooth"
            "reference color diffuseColor" "customPattern:outColor"
        Sphere 1.0 -1.0 1.0 360.0
    AttributeEnd

WorldEnd

Creating a Pattern args File

If you would like RenderMan for Maya or Katana to recognize your pattern 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 pattern. 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.

The args file format is described in the Katana Technical Guide and there are also example args files for the RPS pattern plugins in the/lib/RIS/pattern/Args/ directory of your RenderMan Pro Server installation.