# far_tutorial_4_3.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/release/tutorials/far/tutorial_4_3/far_tutorial_4_3.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // This tutorial shows how to create and manipulate table of cascading stencils.
    //
    // We initialize a Far::TopologyRefiner with a cube and apply uniform
    // refinement. We then use a Far::StencilTableFactory to generate a stencil
    // table. We set the factory Options to not factorize intermediate levels,
    // thus giving a table of "cascading" stencils.
    //
    // We then apply the stencils to the vertex position primvar data, and insert
    // a hierarchical edit at level 1. This edit is smoothed by the application
    // of the subsequent stencil cascades.
    //
    // The results are dumped into an OBJ file that shows the intermediate levels
    // of refinement of the original cube.
    //

    #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;
        }

        float * GetPosition() {
            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 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));

        // Use the Far::StencilTable factory to create cascading stencil table
        // note: we want stencils for each refinement level
        //       "cascade" mode is achieved by setting "factorizeIntermediateLevels"
        //       to false
        Far::StencilTableFactory::Options options;
        options.generateIntermediateLevels=true;
        options.factorizeIntermediateLevels=false;
        options.generateOffsets=true;

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

        std::vector<Vertex> vertexBuffer(refiner->GetNumVerticesTotal()-g_nverts);

        Vertex * destVerts = &vertexBuffer[0];

        int start = 0, end = 0; // stencil batches for each level of subdivision
        for (int level=0; level<maxlevel; ++level) {

            int nverts = refiner->GetLevel(level+1).GetNumVertices();

            Vertex const * srcVerts = reinterpret_cast<Vertex *>(g_verts);
            if (level>0) {
                 srcVerts = &vertexBuffer[start];
            }

            start = end;
            end += nverts;

            stencilTable->UpdateValues(srcVerts, destVerts, start, end);

            // apply 2 hierarchical edits on level 1 vertices
            if (level==1) {
                float * pos = destVerts[start+5].GetPosition();
                pos[1] += 0.5f;

                pos = destVerts[start+20].GetPosition();
                pos[0] += 0.25f;
            }
        }

        { // Output OBJ of the highest level refined -----------

            Vertex * verts = &vertexBuffer[0];

            // Print vertex positions
            for (int level=1, firstvert=0; level<=maxlevel; ++level) {

                Far::TopologyLevel const & refLevel = refiner->GetLevel(level);

                printf("g level_%d\n", level);

                int nverts = refLevel.GetNumVertices();
                for (int vert=0; vert<nverts; ++vert) {
                    float const * pos = verts[vert].GetPosition();
                    printf("v %f %f %f\n", pos[0], pos[1], pos[2]);
                }
                verts += nverts;

                // Print faces
                for (int face=0; face<refLevel.GetNumFaces(); ++face) {

                    Far::ConstIndexArray fverts = refLevel.GetFaceVertices(face);

                    // all refined Catmark faces should be quads
                    assert(fverts.size()==4);

                    printf("f ");
                    for (int vert=0; vert<fverts.size(); ++vert) {
                        printf("%d ", fverts[vert]+firstvert+1); // OBJ uses 1-based arrays...
                    }
                    printf("\n");
                }
                firstvert+=nverts;
            }
        }

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

    }

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