# far_tutorial_4.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/master/tutorials/far/tutorial_4/far_tutorial_4.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // This tutorial shows how to create and manipulate FarStencilTable. We use the
    // factorized stencils to interpolate vertex primvar data buffers.
    //

    #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) {
            _position[0] = src._position[0];
            _position[1] = src._position[1];
            _position[2] = src._position[2];
        }

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

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

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

        float const * GetPosition() const {
            return _position;
        }

    private:
        float _position[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 };

    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 some FarTopologyRefiner (see far_tutorial_0 for details).
        Far::TopologyRefiner * refiner = createTopologyRefiner();

        // Uniformly refine the topology up to 'maxlevel'.
        int maxlevel = 3;
        refiner->RefineUniform(Far::TopologyRefiner::UniformOptions(maxlevel));

        // Use the FarStencilTable factory to create discrete stencil table
        // note: we only want stencils for the highest refinement level.
        Far::StencilTableFactory::Options options;
        options.generateIntermediateLevels=false;
        options.generateOffsets=true;

        Far::StencilTable const * stencilTable =
            Far::StencilTableFactory::Create(*refiner, options);

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

        // Quick & dirty re-cast of the primvar data from our cube
        // (this is where you would drive shape deformations every frame)
        Vertex * controlValues = reinterpret_cast<Vertex *>(g_verts);

        { // 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.
            stencilTable->UpdateValues(controlValues, &vertexBuffer[0]);
        }

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

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

        delete refiner;
        delete stencilTable;
    }

    //------------------------------------------------------------------------------
    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 FarTopologyRefiner from the descriptor.
        return Far::TopologyRefinerFactory<Descriptor>::Create(desc,
                    Far::TopologyRefinerFactory<Descriptor>::Options(type, options));

    }

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

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