RixPatterns
Accessing Built-in Variables

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Links:
    RixPattern Plugins: Cutter
    CutrSideMask.args
    CutrSideMask.cpp RfM 22.0

Overview

This page demonstrates how built-in variables can be accessed from a custom RixPattern. The code shown in listinga 1 and 2 are for a node called CutrSideMask. CutrSideMask outputs a color that depends on the value of the dot product of the surface normal and the (camera) viewing vector.

Figure 1

By recognizing the difference between the front and the rear of a surface the node can be used to achieve moderately interesting effects. For example, figure 1 shows a texture map applied to the outward facing surfaces of a nurbs patch and a nurbs hemisphere.

Figure 2

The shading network and the attribute editor for the CutrSideMask and pxrTexture nodes are shown in figures 3 and 4.

Figure 3

Figure 4

Notes on the Code

The basic code for CutrSideMask.cpp was generated from the .args file shown below. Refer to the tutorial,
    RixPattern Plugins: Cutter

Listing 1 (CutrSideMask.args)

<args format="1.0">
    <shaderType>
        <tag value="pattern"/>
    </shaderType>
    <page name="Parameters" open="True">
        <param name="input_outsideRGB" label="Exterior Color" type="color" default="1 0.4 0.4" connectable="True" widget="color">
            <tags>
                <tag value="color"/>
            </tags>
        </param>
        <param name="input_insideRGB"  label="Interior Color" type="color" default="0.5 1 0.5" connectable="True" widget="color">
            <tags>
                <tag value="color"/>
            </tags>
        </param>
    </page>
    <output name="resultRGB">
        <tags>
            <tag value ="color"/>
        </tags>
    </output>
    <rfmdata nodeid="5" classification="rendernode/RenderMan/pattern"/>
</args>

Once the .args script had been created and it UI had been previewed a .cpp document was generated by Cutter. Most of the code generated by Cutter does not require editing, however, lines (112 to 133) were deleted,

    ----snip----snip----snip----snip----snip----snip----
    // Access the primitive variables that will be needed for the 
    // calculation of the output values. For example, 'st' texture
    // values.
    RtFloat2 const *st, st_default(0, 0);
    sctx->GetPrimVar("st", st_default, &st);    
    
    // Assign values to each point.
    for(int n = 0; n < sctx->numPts; n++) {
        // For example, assign values based on the value of 's'...
        if(st[n].x > 0.5) {
            resultC[n].r = input_outsideRGB->r;
            resultC[n].g = input_outsideRGB->g;
            resultC[n].b = input_outsideRGB->b;
            }
        else
            {
            resultC[n].r = 1;
            resultC[n].g = 1;
            resultC[n].b = 1;
            }
    ----snip----snip----snip----snip----snip----snip----

and replaced by the following code.

    // Access the primitive variables that will be needed for the 
    // calculation of the output values. 
    RtNormal3 const *Ngn;   // normalized geometric normal
    RtVector3 const *Vn;    // normalized view vector
    sctx->GetBuiltinVar(RixShadingContext::k_Ngn, &Ngn);
    sctx->GetBuiltinVar(RixShadingContext::k_Vn, &Vn);    
    
    // Assign values to each point.
    for(int i = 0; i < sctx->numPts; i++) {
        // Calculate the dot product and use it to determine if the
        // point being shaded is facing the camera. 
        float NdotV = Dot(Ngn[i], Vn[i]);
        if (NdotV > 0.f) {
            resultRGB[i] = input_outsideRGB[i];
            }
        else
            {
            resultRGB[i] = input_insideRGB[i];
            }
        }

No other edits were made. The final code for plugin can be viewed here.