# osd_tutorial_0.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/master/tutorials/osd/tutorial_0/osd_tutorial_0.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // This tutorial demonstrates the manipulation of Osd Evaluator and
    // BufferDescriptor.
    //

    #include <opensubdiv/far/topologyDescriptor.h>
    #include <opensubdiv/far/stencilTableFactory.h>
    #include <opensubdiv/osd/cpuEvaluator.h>
    #include <opensubdiv/osd/cpuVertexBuffer.h>

    #include <cstdio>
    #include <cstring>

    //------------------------------------------------------------------------------
    // 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 const * createTopologyRefiner(int maxlevel);

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

        int maxlevel=2,
            nCoarseVerts=0,
            nRefinedVerts=0;

        //
        // Setup phase
        //
        Far::StencilTable const * stencilTable = NULL;
        { // Setup Far::StencilTable
            Far::TopologyRefiner const * refiner = createTopologyRefiner(maxlevel);

            // Setup a factory to create FarStencilTable (for more details see
            // Far tutorials)
            Far::StencilTableFactory::Options options;
            options.generateOffsets=true;
            options.generateIntermediateLevels=false;

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

            nCoarseVerts = refiner->GetLevel(0).GetNumVertices();
            nRefinedVerts = stencilTable->GetNumStencils();

            // We are done with Far: cleanup table
            delete refiner;
        }

        // Setup a buffer for vertex primvar data:
        Osd::CpuVertexBuffer * vbuffer =
            Osd::CpuVertexBuffer::Create(3, nCoarseVerts + nRefinedVerts);

        //
        // Execution phase (every frame)
        //
        {
            // Pack the control vertex data at the start of the vertex buffer
            // and update every time control data changes
            vbuffer->UpdateData(g_verts, 0, nCoarseVerts);

            Osd::BufferDescriptor srcDesc(0, 3, 3);
            Osd::BufferDescriptor dstDesc(nCoarseVerts*3, 3, 3);

            // Launch the computation
            Osd::CpuEvaluator::EvalStencils(vbuffer, srcDesc,
                                            vbuffer, dstDesc,
                                            stencilTable);
        }

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

            printf("particle ");
            float const * refinedVerts = vbuffer->BindCpuBuffer() + 3*nCoarseVerts;
            for (int i=0; i<nRefinedVerts; ++i) {
                float const * vert = refinedVerts + 3*i;
                printf("-p %f %f %f\n", vert[0], vert[1], vert[2]);
            }
            printf("-c 1;\n");
        }

        delete stencilTable;
        delete vbuffer;
    }

    //------------------------------------------------------------------------------
    static Far::TopologyRefiner const *
    createTopologyRefiner(int maxlevel) {

        // 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
        Far::TopologyRefiner * refiner =
            Far::TopologyRefinerFactory<Descriptor>::Create(desc,
                Far::TopologyRefinerFactory<Descriptor>::Options(type, options));

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

        return refiner;
    }

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

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