RixPatterns
CutrSparky

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Code:
    CutrSparky.args
    CutrSparky.cpp

Introduction

This tutorial presents a C++ pattern plugin called CutrSparky that was derived from an OSL shader named sparky. Both the plugin and the shader when used with a Bxdf such as PxrConstant can control the presence and color of a curve to provide the illusion of a particle or spark - figure 1 rollover.

Figure 1
Curve with and without "sparky" shading.

The OSL code for the original sparky shader is shown below.

Listing 1 (sparky.sl)

shader
sparky(
    float age = 0 
        [[
        string label = "Spark Age",
        ]],
    float len = 0.01 
        [[
        string label = "Spark Length",
        ]],
    float rearblur = 0.05
        [[
        string label = "Spark Rear Blur",
        ]],
    float frontblur = 0.0 
        [[
        string label = "Spark Front Blur",
        ]],
    color start = color(1,1,1)
        [[
        string label = "Spark Head Color",
        ]],
    color end = color(0.482,0.776,0.996)
        [[
        string label = "Spark Tail Color",
        ]],
    output color resultRGB = 0,
    output float resultMask = 0)
{
resultMask = smoothstep(age - rearblur, len + age, v) * 
              (1 - smoothstep(age, age + frontblur + len, v));
resultRGB = mix(start, end, resultMask);
}

Althought the code for CutrSparky is considerably more complicated that the OSL code of sparky most of it is responsible for what might be described as "bookkeeping" - referencing the plugin's inputs and managing memory for it's outputs. The most interesting section of C++ code is shown below and for the purposes of comparison it is accompanied by the equivalent OSL code.

C++

    // Surface parameter
    RtFloat const  *V;
    ctx->GetBuiltinVar(RixShadingContext::k_v, &V);
 
    // Assign values to each point.
    for(int i = 0; i < ctx->numPts; i++) {
        resultF[i] = RixSmoothStep(age[i] - rearblur[i], width[i] + age[i], V[i]) * 
                     (1 - RixSmoothStep(age[i], width[i] + age[i] + frontblur[i], V[i]));
        resultRGB[i] = RixMix(start[i], end[i], resultF[i]);
        } 
    return 0;
    }

OSL

    resultMask = smoothstep(age - rearblur, len + age, v) * 
                  (1 - smoothstep(age, age + frontblur + len, v));
    resultRGB = mix(start, end, resultMask);

This video,
shows the result of applying CutrSparky and PxrConstant to 500 static curves.

On frame 1 the "Spark Age" parameter was set to 0.0. On the last frame the value was set to 1.0 - figure 2.

Figure 2

Many OSL functions have Rix... equivalents, such as, smoothstep() and RixSmoothStep(). The Rix "versions" are declared in,

    RenderManProServer-XX.0/include/RixShadingUtils.h

Useful color conversion functions are declared in,

    RenderManProServer-XX.0/include/RixColorUtils.h

For convenience the commonly used Rix functions are accessible from Cutter's popup menu.

Figure 4

Before using the CutrSparky.args file the value of "rfmdata nodeid" should be edited so that it does not conflict with any other node ids specified in the users RenderMan_for_Maya.ini file. Refer to the tutorial "RixPattern Plugins: Cutter" for information about compiling and using custom RixPattern plugins.