# far_tutorial_2.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/master/tutorials/far/tutorial_2/far_tutorial_2.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // Building on tutorial 0, this example shows how to instantiate a simple mesh,
    // refine it uniformly and then interpolate additional sets of primvar data.
    //

    #include <opensubdiv/far/topologyDescriptor.h>
    #include <opensubdiv/far/primvarRefiner.h>

    #include <cstdio>

    //------------------------------------------------------------------------------
    // Vertex container implementation.
    //
    // We are adding a per-vertex color attribute to our primvar data.  While they
    // are separate properties and exist in separate buffers (as when read from an
    // Alembic file) they are both of the form float[3] and so we can use the same
    // underlying type.
    //
    // While color and position may be the same, we'll make the color a "varying"
    // primvar, e.g. it is constrained to being linearly interpolated between
    // vertices, rather than smoothly like position and other vertex data.
    //
    struct Point3 {

        // Minimal required interface ----------------------
        Point3() { }

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

        void AddWithWeight(Point3 const & src, float weight) {
            _point[0]+=weight*src._point[0];
            _point[1]+=weight*src._point[1];
            _point[2]+=weight*src._point[2];
        }

        // Public interface ------------------------------------
        void SetPoint(float x, float y, float z) {
            _point[0]=x;
            _point[1]=y;
            _point[2]=z;
        }

        const float * GetPoint() const {
            return _point;
        }

    private:
        float _point[3];
    };

    typedef Point3 VertexPosition;
    typedef Point3 VertexColor;

    //------------------------------------------------------------------------------
    // Cube geometry from catmark_cube.h
    static float g_verts[8][3] = {{ -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[8][3] = {{ 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 * createFarTopologyRefiner();

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

        int maxlevel = 5;

        Far::TopologyRefiner * refiner = createFarTopologyRefiner();

        // Uniformly refine the topolgy up to 'maxlevel'
        refiner->RefineUniform(Far::TopologyRefiner::UniformOptions(maxlevel));

        // Allocate buffers for vertex primvar data.
        //
        // We assume we received the coarse data for the mesh in separate buffers
        // from some other source, e.g. an Alembic file.  Meanwhile, we want buffers
        // for the last/finest subdivision level to persist.  We have no interest
        // in the intermediate levels.
        //
        // Determine the sizes for our needs:
        int nCoarseVerts = g_nverts;
        int nFineVerts   = refiner->GetLevel(maxlevel).GetNumVertices();
        int nTotalVerts  = refiner->GetNumVerticesTotal();
        int nTempVerts   = nTotalVerts - nCoarseVerts - nFineVerts;

        // Allocate and intialize the primvar data for the original coarse vertices:
        std::vector<VertexPosition> coarsePosBuffer(nCoarseVerts);
        std::vector<VertexColor>    coarseClrBuffer(nCoarseVerts);

        for (int i = 0; i < nCoarseVerts; ++i) {
            coarsePosBuffer[i].SetPoint(g_verts[i][0], g_verts[i][1], g_verts[i][2]);
            coarseClrBuffer[i].SetPoint(g_colors[i][0], g_colors[i][1], g_colors[i][2]);
        }

        // Allocate intermediate and final storage to be populated:
        std::vector<VertexPosition> tempPosBuffer(nTempVerts);
        std::vector<VertexPosition> finePosBuffer(nFineVerts);

        std::vector<VertexColor> tempClrBuffer(nTempVerts);
        std::vector<VertexColor> fineClrBuffer(nFineVerts);

        // Interpolate all primvar data -- separate buffers can be populated on
        // separate threads if desired:
        VertexPosition * srcPos = &coarsePosBuffer[0];
        VertexPosition * dstPos = &tempPosBuffer[0];

        VertexColor * srcClr = &coarseClrBuffer[0];
        VertexColor * dstClr = &tempClrBuffer[0];

        Far::PrimvarRefiner primvarRefiner(*refiner);

        for (int level = 1; level < maxlevel; ++level) {
            primvarRefiner.Interpolate(       level, srcPos, dstPos);
            primvarRefiner.InterpolateVarying(level, srcClr, dstClr);

            srcPos = dstPos, dstPos += refiner->GetLevel(level).GetNumVertices();
            srcClr = dstClr, dstClr += refiner->GetLevel(level).GetNumVertices();
        }

        // Interpolate the last level into the separate buffers for our final data:
        primvarRefiner.Interpolate(       maxlevel, srcPos, finePosBuffer);
        primvarRefiner.InterpolateVarying(maxlevel, srcClr, fineClrBuffer);

        { // Visualization with Maya : print a MEL script that generates colored
          // particles at the location of the refined vertices (don't forget to
          // turn shading on in the viewport to see the colors)

            int nverts = nFineVerts;

            // Output particle positions
            printf("particle ");
            for (int vert = 0; vert < nverts; ++vert) {
                float const * pos = finePosBuffer[vert].GetPoint();
                printf("-p %f %f %f\n", pos[0], pos[1], pos[2]);
            }
            printf(";\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 primvar data
            printf("setAttr \"particleShape1.rgbPP\" -type \"vectorArray\" %d ", nverts);
            for (int vert = 0; vert < nverts; ++vert) {
                float const * color = fineClrBuffer[vert].GetPoint();
                printf("%f %f %f\n", color[0], color[1], color[2]);
            }
            printf(";\n");
        }
    }

    //------------------------------------------------------------------------------
    // Creates Far::TopologyRefiner from raw geometry
    //
    // see far_tutorial_0 for more details
    //
    static Far::TopologyRefiner *
    createFarTopologyRefiner() {

        // 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;
    }
    //------------------------------------------------------------------------------

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