# hbr_tutorial_2.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/release/tutorials/hbr/tutorial_2/hbr_tutorial_2.cpp

    #include <cassert>
    #include <cstdio>

    //------------------------------------------------------------------------------
    //
    // For this tutorial, we have to flesh out the Vertex class further. Note that now
    // the copy constructor, Clear() and AddwithWeight() methods have been
    // implemented to interpolate our float3 position data.
    //
    // This vertex specialization pattern leaves client-code free to implement
    // arbitrary vertex primvar data schemes (or none at all to conserve efficiency)
    //
    struct Vertex {

        // Hbr minimal required interface ----------------------
        Vertex() { }

        Vertex(int /*i*/) { }

        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];
        }

        void AddVaryingWithWeight(Vertex const &, float) { }

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

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

    private:
        float _position[3];
    };

    typedef OpenSubdiv::HbrMesh<Vertex>      Hmesh;
    typedef OpenSubdiv::HbrFace<Vertex>      Hface;
    typedef OpenSubdiv::HbrVertex<Vertex>    Hvertex;
    typedef OpenSubdiv::HbrHalfedge<Vertex>  Hhalfedge;

    Hmesh * createMesh();

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

        Hmesh * hmesh = createMesh();

        int maxlevel=2,    // 2 levels of subdivision
            firstface=0,   // marker to the first face index of level 2
            firstvertex=0; // marker to the first vertex index of level 2

        // Refine the mesh to 'maxlevel'
        for (int level=0; level<maxlevel; ++level) {

            // Total number of faces in the mesh, across all levels
            //
            // Note: this function iterates over the list of faces and can be slow
            int nfaces = hmesh->GetNumFaces();

            if (level==(maxlevel-1)) {
                // Save our vertex marker
                firstvertex = hmesh->GetNumVertices();
            }

            // Iterate over the faces of the current level of subdivision
            for (int face=firstface; face<nfaces; ++face) {

                Hface * f = hmesh->GetFace(face);

                // Note: hole tags would have to be dealt with here.
                f->Refine();
            }

            // Save our face index marker for the next level
            firstface = nfaces;
        }

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

            // Print vertex positions
            int nverts = hmesh->GetNumVertices();
            for (int vert=firstvertex; vert<nverts; ++vert) {
                float const * pos = hmesh->GetVertex(vert)->GetData().GetPosition();
                printf("v %f %f %f\n", pos[0], pos[1], pos[2]);
            }

            // Print faces
            for (int face=firstface; face<hmesh->GetNumFaces(); ++face) {

                Hface * f = hmesh->GetFace(face);

                assert(f->GetNumVertices()==4 );

                printf("f ");
                for (int vert=0; vert<4; ++vert) {

                    // OBJ uses 1-based arrays
                    printf("%d ", f->GetVertex(vert)->GetID() - firstvertex + 1);
                }
                printf("\n");
            }
        }

    }

    //------------------------------------------------------------------------------
    // Creates an Hbr mesh
    //
    // see hbr_tutorial_0 and hbr_tutorial_1 for more details
    //
    Hmesh *
    createMesh() {

        // Pyramid geometry from catmark_pyramid.h
        static float verts[5][3] = {{ 0.0f,  0.0f,  2.0f},
                                    { 0.0f, -2.0f,  0.0f},
                                    { 2.0f,  0.0f,  0.0f},
                                    { 0.0f,  2.0f,  0.0f},
                                    {-2.0f,  0.0f,  0.0f}};

        static int nverts = 5,
                   nfaces = 5;

        static int facenverts[5] = { 3, 3, 3, 3, 4 };

        static int faceverts[16] = { 0, 1, 2,
                                     0, 2, 3,
                                     0, 3, 4,
                                     0, 4, 1,
                                     4, 3, 2, 1 };

        OpenSubdiv::HbrCatmarkSubdivision<Vertex> * catmark =
            new OpenSubdiv::HbrCatmarkSubdivision<Vertex>();

        Hmesh * hmesh = new Hmesh(catmark);

        // Populate the vertices
        Vertex v;
        for (int i=0; i<nverts; ++i) {
            v.SetPosition(verts[i][0], verts[i][1], verts[i][2]);
            hmesh->NewVertex(i, v);
        }

        // Create the topology
        int * fv = faceverts;
        for (int i=0; i<nfaces; ++i) {

            int nv = facenverts[i];

            bool valid = true;

            for(int j=0;j<nv;j++) {

                Hvertex const * origin      = hmesh->GetVertex(fv[j]),
                              * destination = hmesh->GetVertex(fv[(j+1)%nv]);
                Hhalfedge const * opposite = destination->GetEdge(origin);

                // Make sure that the vertices exist in the mesh
                if (origin==NULL || destination==NULL) {
                    printf(" An edge was specified that connected a nonexistent vertex\n");
                    valid=false;
                    break;
                }

                // Check for a degenerate edge
                if (origin == destination) {
                    printf(" An edge was specified that connected a vertex to itself\n");
                    valid=false;
                    break;
                }

                // Check that no more than 2 faces are adjacent to the edge
                if (opposite && opposite->GetOpposite() ) {
                    printf(" A non-manifold edge incident to more than 2 faces was found\n");
                    valid=false;
                    break;
                }

                // Check that the edge is unique and oriented properly
                if (origin->GetEdge(destination)) {
                    printf(" An edge connecting two vertices was specified more than once."
                           " It's likely that an incident face was flipped\n");
                    valid=false;
                    break;
                }
            }

            if (valid) {
                hmesh->NewFace(nv, fv, 0);
            } else {
                printf(" Skipped face %d\n", i);
            }

            fv+=nv;
        }

        hmesh->SetInterpolateBoundaryMethod(Hmesh::k_InterpolateBoundaryEdgeOnly);

        hmesh->Finish();

        return hmesh;
    }

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

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