# far_tutorial_4_2.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/release/tutorials/far/tutorial_4_2/far_tutorial_4_2.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // This tutorial shows how to create and manipulate both 'vertex' and 'varying'
    // Far::StencilTable to interpolate 2 primvar data buffers: vertex positions and
    // vertex colors.
    //

    #include <opensubdiv/far/topologyDescriptor.h>
    #include <opensubdiv/far/stencilTable.h>
    #include <opensubdiv/far/stencilTableFactory.h>

    #include <cstdio>
    #include <cstring>

    //------------------------------------------------------------------------------
    // Vertex container implementation.
    //
    struct Vertex {

        // Minimal required interface ----------------------
        Vertex() { }

        Vertex(Vertex const & src) {
            _data[0] = src._data[0];
            _data[1] = src._data[1];
            _data[2] = src._data[2];
        }

        void Clear( void * =0 ) {
            _data[0]=_data[1]=_data[2]=0.0f;
        }

        void AddWithWeight(Vertex const & src, float weight) {
            _data[0]+=weight*src._data[0];
            _data[1]+=weight*src._data[1];
            _data[2]+=weight*src._data[2];
        }

        // Public interface ------------------------------------
        float const * GetData() const {
            return _data;
        }

    private:
        float _data[3];
    };

    //------------------------------------------------------------------------------
    // Cube geometry from catmark_cube.h

    static float g_verts[24] = {-0.5f, -0.5f,  0.5f,
                                 0.5f, -0.5f,  0.5f,
                                -0.5f,  0.5f,  0.5f,
                                 0.5f,  0.5f,  0.5f,
                                -0.5f,  0.5f, -0.5f,
                                 0.5f,  0.5f, -0.5f,
                                -0.5f, -0.5f, -0.5f,
                                 0.5f, -0.5f, -0.5f };

    // Per-vertex RGB color data
    static float g_colors[24] = { 1.0f, 0.0f, 0.5f,
                                  0.0f, 1.0f, 0.0f,
                                  0.0f, 0.0f, 1.0f,
                                  1.0f, 1.0f, 1.0f,
                                  1.0f, 1.0f, 0.0f,
                                  0.0f, 1.0f, 1.0f,
                                  1.0f, 0.0f, 1.0f,
                                  0.0f, 0.0f, 0.0f };

    static int g_nverts = 8,
               g_nfaces = 6;

    static int g_vertsperface[6] = { 4, 4, 4, 4, 4, 4 };

    static int g_vertIndices[24] = { 0, 1, 3, 2,
                                     2, 3, 5, 4,
                                     4, 5, 7, 6,
                                     6, 7, 1, 0,
                                     1, 7, 5, 3,
                                     6, 0, 2, 4  };

    using namespace OpenSubdiv;

    static Far::TopologyRefiner * createTopologyRefiner();

    //------------------------------------------------------------------------------
    int main(int, char **) {

        // Generate a Far::TopologyRefiner (see tutorial_1_1 for details).
        Far::TopologyRefiner * refiner = createTopologyRefiner();

        // Uniformly refine the topology up to 'maxlevel'.
        int maxlevel = 4;
        refiner->RefineUniform(Far::TopologyRefiner::UniformOptions(maxlevel));

        int nverts = refiner->GetLevel(maxlevel).GetNumVertices();

        // Use the Far::StencilTable factory to create discrete stencil table
        Far::StencilTableFactory::Options options;
        options.generateIntermediateLevels=false; // only the highest refinement level.
        options.generateOffsets=true;

        //
        // Vertex primvar data
        //

            // Create stencils table for 'vertex' interpolation
            options.interpolationMode=Far::StencilTableFactory::INTERPOLATE_VERTEX;

            Far::StencilTable const * vertexStencils =
                Far::StencilTableFactory::Create(*refiner, options);
            assert(nverts==vertexStencils->GetNumStencils());

            // Allocate vertex primvar buffer (1 stencil for each vertex)
            std::vector<Vertex> vertexBuffer(vertexStencils->GetNumStencils());

            // Use the cube vertex positions as 'vertex' primvar data
            Vertex * vertexCVs = reinterpret_cast<Vertex *>(g_verts);

        //
        // Varying primvar data
        //

            // Create stencils table for 'varying' interpolation
            options.interpolationMode=Far::StencilTableFactory::INTERPOLATE_VARYING;

            Far::StencilTable const * varyingStencils =
                Far::StencilTableFactory::Create(*refiner, options);
            assert(nverts==varyingStencils->GetNumStencils());

            // Allocate varying primvar buffer (1 stencil for each vertex)
            std::vector<Vertex> varyingBuffer(varyingStencils->GetNumStencils());

            // Use per-vertex array of RGB colors as 'varying' primvar data
            Vertex * varyingCVs = reinterpret_cast<Vertex *>(g_colors);

        delete refiner;

        //
        // Apply stencils (in frame loop)
        //

        { // This section would be applied every frame after control vertices have
          // been moved.

            // Apply stencils on the control vertex data to update the primvar data
            // of the refined vertices.

            vertexStencils->UpdateValues(vertexCVs, &vertexBuffer[0]);

            varyingStencils->UpdateValues(varyingCVs, &varyingBuffer[0]);
        }

        { // Visualization with Maya : print a MEL script that generates particles
          // at the location of the refined vertices

            printf("particle ");
            for (int vert=0; vert<(int)nverts; ++vert) {
                float const * pos = vertexBuffer[vert].GetData();
                printf("-p %f %f %f\n", pos[0], pos[1], pos[2]);
            }
            printf("-c 1;\n");

            // Set particle point size (20 -- very large)
            printf("addAttr -is true -ln \"pointSize\" -at long -dv 20 particleShape1;\n");

            // Add per-particle color attribute ('rgbPP')
            printf("addAttr -ln \"rgbPP\" -dt vectorArray particleShape1;\n");

            // Set per-particle color values from our 'varying' primvar data
            printf("setAttr \"particleShape1.rgbPP\" -type \"vectorArray\" %d ", nverts);
            for (int vert=0; vert<nverts; ++vert) {
                float const * color = varyingBuffer[vert].GetData();
                printf("%f %f %f\n", color[0], color[1], color[2]);
            }
            printf(";\n");
        }

        delete vertexStencils;
        delete varyingStencils;
    }

    //------------------------------------------------------------------------------
    static Far::TopologyRefiner *
    createTopologyRefiner() {

        // Populate a topology descriptor with our raw data.

        typedef Far::TopologyDescriptor Descriptor;

        Sdc::SchemeType type = OpenSubdiv::Sdc::SCHEME_CATMARK;

        Sdc::Options options;
        options.SetVtxBoundaryInterpolation(Sdc::Options::VTX_BOUNDARY_EDGE_ONLY);

        Descriptor desc;
        desc.numVertices = g_nverts;
        desc.numFaces = g_nfaces;
        desc.numVertsPerFace = g_vertsperface;
        desc.vertIndicesPerFace = g_vertIndices;

        // Instantiate a Far::TopologyRefiner from the descriptor.
        Far::TopologyRefiner * refiner =
            Far::TopologyRefinerFactory<Descriptor>::Create(desc,
                Far::TopologyRefinerFactory<Descriptor>::Options(type, options));

        return refiner;
    }

    //------------------------------------------------------------------------------