# bfr_tutorial_2_2.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/release/tutorials/bfr/tutorial_2_2/bfr_tutorial_2_2.cpp

    #include <opensubdiv/far/topologyRefiner.h>
    #include <opensubdiv/bfr/refinerSurfaceFactory.h>
    #include <opensubdiv/bfr/surface.h>
    #include <opensubdiv/bfr/tessellation.h>

    #include <vector>
    #include <string>
    #include <cstring>
    #include <cstdio>
    #include <cassert>

    //  Local headers with support for this tutorial in "namespace tutorial"
    #include "./meshLoader.h"
    #include "./objWriter.h"

    using namespace OpenSubdiv;

    using Far::Index;
    using Far::IndexArray;
    using Far::ConstIndexArray;

    //
    //  Simple command line arguments to provide input and run-time options:
    //
    class Args {
    public:
        std::string     inputObjFile;
        std::string     outputObjFile;
        Sdc::SchemeType schemeType;
        int             tessUniformRate;
        bool            tessQuadsFlag;

    public:
        Args(int argc, char * argv[]) :
            inputObjFile(),
            outputObjFile(),
            schemeType(Sdc::SCHEME_CATMARK),
            tessUniformRate(5),
            tessQuadsFlag(false) {

            for (int i = 1; i < argc; ++i) {
                if (strstr(argv[i], ".obj")) {
                    if (inputObjFile.empty()) {
                        inputObjFile = std::string(argv[i]);
                    } else {
                        fprintf(stderr,
                            "Warning: Extra Obj file '%s' ignored\n", argv[i]);
                    }
                } else if (!strcmp(argv[i], "-o")) {
                    if (++i < argc) outputObjFile = std::string(argv[i]);
                } else if (!strcmp(argv[i], "-bilinear")) {
                    schemeType = Sdc::SCHEME_BILINEAR;
                } else if (!strcmp(argv[i], "-catmark")) {
                    schemeType = Sdc::SCHEME_CATMARK;
                } else if (!strcmp(argv[i], "-loop")) {
                    schemeType = Sdc::SCHEME_LOOP;
                } else if (!strcmp(argv[i], "-res")) {
                    if (++i < argc) tessUniformRate = atoi(argv[i]);
                } else if (!strcmp(argv[i], "-quads")) {
                    tessQuadsFlag = true;
                } else {
                    fprintf(stderr,
                        "Warning: Unrecognized argument '%s' ignored\n", argv[i]);
                }
            }
        }

    private:
        Args() { }
    };

    //
    //  Simple local structs supporting shared points for vertices and edges:
    //
    namespace {
        struct SharedVertex {
            SharedVertex() : pointIndex(-1) { }

            bool IsSet() const { return pointIndex >= 0; }
            void Set(int index) { pointIndex = index; }

            int pointIndex;
        };

        struct SharedEdge {
            SharedEdge() : pointIndex(-1), numPoints(0) { }

            bool IsSet() const { return pointIndex >= 0; }
            void Set(int index, int n) { pointIndex = index, numPoints = n; }

            int pointIndex;
            int numPoints;
        };
    } // end namespace

    //
    //  The main tessellation function:  given a mesh and vertex positions,
    //  tessellate each face -- writing results in Obj format.
    //
    //  This tessellation function differs from earlier tutorials in that it
    //  computes and reuses shared points at vertices and edges of the mesh.
    //  There are several ways to compute these shared points, and which is
    //  best depends on context.
    //
    //  Dealing with shared data poses complications for threading in general,
    //  so computing all points for the vertices and edges up front may be
    //  preferred -- despite the fact that faces will be visited more than once
    //  (first when generating potentially shared vertex or edge points, and
    //  later when generating any interior points). The loops for vertices and
    //  edges can be threaded and the indexing of the shared points is simpler.
    //
    //  For the single-threaded case here, the faces are each processed in
    //  order and any shared points will be computed and used as needed. So
    //  each face is visited once (and so each Surface initialized once) but
    //  the bookkeeping to deal with indices of shared points becomes more
    //  complicated.
    //
    void
    tessellateToObj(Far::TopologyRefiner const & meshTopology,
                    std::vector<float>   const & meshVertexPositions,
                    Args                 const & options) {

        //
        //  Use simpler local type names for the Surface and its factory:
        //
        typedef Bfr::RefinerSurfaceFactory<> SurfaceFactory;
        typedef Bfr::Surface<float>          Surface;

        //
        //  Initialize the SurfaceFactory for the given base mesh (very low
        //  cost in terms of both time and space) and tessellate each face
        //  independently (i.e. no shared vertices):
        //
        //  Note that the SurfaceFactory is not thread-safe by default due to
        //  use of an internal cache.  Creating a separate instance of the
        //  SurfaceFactory for each thread is one way to safely parallelize
        //  this loop.  Another (preferred) is to assign a thread-safe cache
        //  to the single instance.
        //
        //  First declare any evaluation options when initializing (though
        //  none are used in this simple case):
        //
        SurfaceFactory::Options surfaceOptions;

        SurfaceFactory meshSurfaceFactory(meshTopology, surfaceOptions);

        //
        //  The Surface to be constructed and evaluated for each face -- as
        //  well as the intermediate and output data associated with it -- can
        //  be declared in the scope local to each face. But since dynamic
        //  memory is involved with these variables, it is preferred to declare
        //  them outside that loop to preserve and reuse that dynamic memory.
        //
        Surface faceSurface;

        std::vector<float> facePatchPoints;

        std::vector<float> outCoords;
        std::vector<float> outPos, outDu, outDv;
        std::vector<int>   outFacets;

        //
        //  Assign Tessellation Options applied for all faces.  Tessellations
        //  allow the creating of either 3- or 4-sided faces -- both of which
        //  are supported here via a command line option:
        //
        int const tessFacetSize = 3 + options.tessQuadsFlag;

        Bfr::Tessellation::Options tessOptions;
        tessOptions.SetFacetSize(tessFacetSize);
        tessOptions.PreserveQuads(options.tessQuadsFlag);

        //
        //  Declare vectors to identify shared tessellation points at vertices
        //  and edges and their indices around the boundary of a face:
        //
        Far::TopologyLevel const & baseLevel = meshTopology.GetLevel(0);

        std::vector<SharedVertex> sharedVerts(baseLevel.GetNumVertices());
        std::vector<SharedEdge>   sharedEdges(baseLevel.GetNumEdges());

        std::vector<int> tessBoundaryIndices;

        //
        //  Process each face, writing the output of each in Obj format:
        //
        tutorial::ObjWriter objWriter(options.outputObjFile);

        int numMeshPointsEvaluated = 0;

        int numFaces = meshSurfaceFactory.GetNumFaces();
        for (int faceIndex = 0; faceIndex < numFaces; ++faceIndex) {
            //
            //  Initialize the Surface for this face -- if valid (skipping
            //  holes and boundary faces in some rare cases):
            //
            if (!meshSurfaceFactory.InitVertexSurface(faceIndex, &faceSurface)) {
                continue;
            }

            //
            //  Declare a simple uniform Tessellation for the Parameterization
            //  of this face and identify coordinates of the points to evaluate:
            //
            Bfr::Tessellation tessPattern(faceSurface.GetParameterization(),
                                          options.tessUniformRate, tessOptions);

            int numOutCoords = tessPattern.GetNumCoords();

            outCoords.resize(numOutCoords * 2);

            tessPattern.GetCoords(outCoords.data());

            //
            //  Prepare the patch points for the Surface, then use them to
            //  evaluate output points for all identified coordinates:
            //
            //  Resize patch point and output arrays:
            int pointSize = 3;

            facePatchPoints.resize(faceSurface.GetNumPatchPoints() * pointSize);

            outPos.resize(numOutCoords * pointSize);
            outDu.resize(numOutCoords * pointSize);
            outDv.resize(numOutCoords * pointSize);

            //  Populate the patch point array:
            faceSurface.PreparePatchPoints(meshVertexPositions.data(), pointSize,
                                           facePatchPoints.data(), pointSize);

            //
            //  Evaluate the sample points of the Tessellation:
            //
            //  First traverse the boundary of the face to determine whether
            //  to evaluate or share points on vertices and edges of the face.
            //  Both pre-existing and new boundary points are identified by
            //  index in an array for later use.  The interior points are all
            //  trivially computed after the boundary is dealt with.
            //
            //  Identify the boundary and interior coords and initialize the
            //  index array for the potentially shared boundary points:
            //
            int numBoundaryCoords = tessPattern.GetNumBoundaryCoords();
            int numInteriorCoords = numOutCoords - numBoundaryCoords;

            float const * tessBoundaryCoords = &outCoords[0];
            float const * tessInteriorCoords = &outCoords[numBoundaryCoords*2];

            ConstIndexArray fVerts = baseLevel.GetFaceVertices(faceIndex);
            ConstIndexArray fEdges = baseLevel.GetFaceEdges(faceIndex);

            tessBoundaryIndices.resize(numBoundaryCoords);

            //
            //  Walk around the face, inspecting each vertex and outgoing edge,
            //  and populating the index array of boundary points:
            //
            float * patchPointData = facePatchPoints.data();

            int boundaryIndex = 0;
            int numFacePointsEvaluated = 0;
            for (int i = 0; i < fVerts.size(); ++i) {
                Index vertIndex = fVerts[i];
                Index edgeIndex = fEdges[i];
                int   edgeRate  = options.tessUniformRate;

                //
                //  Evaluate/assign or retrieve the shared point for the vertex:
                //
                SharedVertex & sharedVertex = sharedVerts[vertIndex];
                if (!sharedVertex.IsSet()) {
                    //  Identify indices of the new shared point in both the
                    //  mesh and face and increment their inventory:
                    int indexInMesh = numMeshPointsEvaluated++;
                    int indexInFace = numFacePointsEvaluated++;

                    sharedVertex.Set(indexInMesh);

                    //  Evaluate new shared point and assign index to boundary:
                    float const * uv = &tessBoundaryCoords[boundaryIndex*2];

                    int pIndex = indexInFace * pointSize;
                    faceSurface.Evaluate(uv, patchPointData, pointSize,
                            &outPos[pIndex], &outDu[pIndex], &outDv[pIndex]);

                    tessBoundaryIndices[boundaryIndex++] = indexInMesh;
                } else {
                    //  Assign shared vertex point index to boundary:
                    tessBoundaryIndices[boundaryIndex++] = sharedVertex.pointIndex;
                }

                //
                //  Evaluate/assign or retrieve all shared points for the edge:
                //
                //  To keep this simple, assume the edge is manifold. So the
                //  second face sharing the edge has that edge in the opposite
                //  direction in its boundary relative to the first face --
                //  making it necessary to reverse the order of shared points
                //  for the boundary of the second face.
                //
                //  To support a non-manifold edge, all subsequent faces that
                //  share the assigned shared edge must determine if their
                //  orientation of that edge is reversed relative to the first
                //  face for which the shared edge points were evaluated. So a
                //  little more book-keeping and/or inspection is required.
                //
                if (edgeRate > 1) {
                    int pointsPerEdge = edgeRate - 1;

                    SharedEdge & sharedEdge = sharedEdges[edgeIndex];
                    if (!sharedEdge.IsSet()) {
                        //  Identify indices of the new shared points in both the
                        //  mesh and face and increment their inventory:
                        int nextInMesh = numMeshPointsEvaluated;
                        int nextInFace = numFacePointsEvaluated;

                        numFacePointsEvaluated += pointsPerEdge;
                        numMeshPointsEvaluated += pointsPerEdge;

                        sharedEdge.Set(nextInMesh, pointsPerEdge);

                        //  Evaluate shared points and assign indices to boundary:
                        float const * uv = &tessBoundaryCoords[boundaryIndex*2];

                        for (int j = 0; j < pointsPerEdge; ++j, uv += 2) {
                            int pIndex = (nextInFace++) * pointSize;
                            faceSurface.Evaluate(uv, patchPointData, pointSize,
                                &outPos[pIndex], &outDu[pIndex], &outDv[pIndex]);

                            tessBoundaryIndices[boundaryIndex++] = nextInMesh++;
                        }
                    } else {
                        //  See note above on simplification for manifold edges
                        assert(!baseLevel.IsEdgeNonManifold(edgeIndex));

                        //  Assign shared points to boundary in reverse order:
                        int nextInMesh = sharedEdge.pointIndex + pointsPerEdge - 1;
                        for (int j = 0; j < pointsPerEdge; ++j) {
                            tessBoundaryIndices[boundaryIndex++] = nextInMesh--;
                        }
                    }
                }
            }

            //
            //  Evaluate any interior points unique to this face -- appending
            //  them to those shared points computed above for the boundary:
            //
            if (numInteriorCoords) {
                float const * uv = tessInteriorCoords;

                int iLast = numFacePointsEvaluated + numInteriorCoords;
                for (int i = numFacePointsEvaluated; i < iLast; ++i, uv += 2) {
                    int pIndex = i * pointSize;
                    faceSurface.Evaluate(uv, patchPointData, pointSize,
                             &outPos[pIndex], &outDu[pIndex], &outDv[pIndex]);
                }
                numFacePointsEvaluated += numInteriorCoords;
                numMeshPointsEvaluated += numInteriorCoords;
            }

            //
            //  Remember to trim/resize the arrays storing evaluation results
            //  for new points to reflect the size actually populated.
            //
            outPos.resize(numFacePointsEvaluated * pointSize);
            outDu.resize(numFacePointsEvaluated * pointSize);
            outDv.resize(numFacePointsEvaluated * pointSize);

            //
            //  Identify the faces of the Tessellation:
            //
            //  Note that the coordinate indices used by the facets are local
            //  to the face (i.e. they range from [0..N-1], where N is the
            //  number of coordinates in the pattern) and so need to be offset
            //  when writing to Obj format.
            //
            //  For more advanced use, the coordinates associated with the
            //  boundary and interior of the pattern are distinguishable so
            //  that those on the boundary can be easily remapped to refer to
            //  shared edge or corner points, while those in the interior can
            //  be separately offset or similarly remapped.
            //
            //  So transform the indices of the facets here as needed using
            //  the indices of shared boundary points assembled above and a
            //  suitable offset for the new interior points added:
            //
            int tessInteriorOffset = numMeshPointsEvaluated - numOutCoords;

            int numFacets = tessPattern.GetNumFacets();
            outFacets.resize(numFacets * tessFacetSize);
            tessPattern.GetFacets(outFacets.data());

            tessPattern.TransformFacetCoordIndices(outFacets.data(),
                            tessBoundaryIndices.data(), tessInteriorOffset);

            //
            //  Write the evaluated points and faces connecting them as Obj:
            //
            objWriter.WriteGroupName("baseFace_", faceIndex);

            objWriter.WriteVertexPositions(outPos);
            objWriter.WriteVertexNormals(outDu, outDv);

            objWriter.WriteFaces(outFacets, tessFacetSize, true, false);
        }
    }

    //
    //  Load command line arguments, specified or default geometry and process:
    //
    int
    main(int argc, char * argv[]) {

        Args args(argc, argv);

        Far::TopologyRefiner * meshTopology = 0;
        std::vector<float>     meshVtxPositions;
        std::vector<float>     meshFVarUVs;

        meshTopology = tutorial::createTopologyRefiner(
                args.inputObjFile, args.schemeType, meshVtxPositions, meshFVarUVs);
        if (meshTopology == 0) {
            return EXIT_FAILURE;
        }

        tessellateToObj(*meshTopology, meshVtxPositions, args);

        delete meshTopology;
        return EXIT_SUCCESS;
    }

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

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