# hbr_tutorial_1.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/master/tutorials/hbr/tutorial_1/hbr_tutorial_1.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // This tutorial shows how to safely create Hbr meshes from arbitrary topology.
    // Because Hbr is a half-edge data structure, it cannot represeent non-manifold
    // topology. Ensuring that the geometry used is manifold is a requirement to use
    // Hbr safely. This tutorial presents some simple tests to detect inappropriate
    // topology.
    //

    #include <opensubdiv/hbr/mesh.h>
    #include <opensubdiv/hbr/catmark.h>

    #include <cstdio>

    //------------------------------------------------------------------------------
    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 ) { }

        void AddWithWeight(Vertex const &, float ) { }

        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;

    //------------------------------------------------------------------------------
    // Non-manifold geometry from catmark_fan.h
    //
    //                     o
    //                    /|
    //                   / |
    //                  /  |
    //                 /   |
    //                o    |
    //                | f2 |
    //                |    |
    //       o--------+----o------------o
    //      /         |   /            /
    //     /          |  /            /
    //    /    f0     | /     f1     /
    //   /            |/            /
    //  o------------ o------------o
    //
    // The shared edge of a fan is adjacent to 3 faces, and therefore non-manifold.
    //
    static float verts[8][3] = {{-1.0,  0.0, -1.0},
                                {-1.0,  0.0,  0.0},
                                { 0.0,  0.0,  0.0},
                                { 0.0,  0.0, -1.0},
                                { 1.0,  0.0,  0.0},
                                { 1.0,  0.0, -1.0},
                                { 0.0,  1.0,  0.0},
                                { 0.0,  1.0, -1.0}};

    static int nverts = 8,
               nfaces = 3;

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

    static int faceverts[12] = { 0, 1, 2, 3,
                                 3, 2, 4, 5,
                                 3, 2, 6, 7 };

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

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

        Hmesh * hmesh = new Hmesh(catmark);

        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();

        printf("Created a fan with %d faces and %d vertices.\n",
            hmesh->GetNumFaces(), hmesh->GetNumVertices());

        delete hmesh;
        delete catmark;
    }

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

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