# bfr_tutorial_2_1.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/release/tutorials/bfr/tutorial_2_1/bfr_tutorial_2_1.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 <cmath>
    #include <algorithm>

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

    using namespace OpenSubdiv;

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

    public:
        Args(int argc, char * argv[]) :
            inputObjFile(),
            outputObjFile(),
            schemeType(Sdc::SCHEME_CATMARK),
            tessInterval(0.0f),
            tessRateMax(10),
            useHullFlag(false),
            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], "-length")) {
                    if (++i < argc) tessInterval = (float) atof(argv[i]);
                } else if (!strcmp(argv[i], "-max")) {
                    if (++i < argc) tessRateMax = atoi(argv[i]);
                } else if (!strcmp(argv[i], "-hull")) {
                    useHullFlag = true;
                } else if (!strcmp(argv[i], "-quads")) {
                    tessQuadsFlag = true;
                } else {
                    fprintf(stderr,
                        "Warning: Unrecognized argument '%s' ignored\n", argv[i]);
                }
            }
        }

    private:
        Args() { }
    };

    //
    //  Local trivial functions for simple edge length calculations and the
    //  determination of associated tessellation rates:
    //
    inline float
    EdgeLength(float const * v0, float const * v1) {

        float dv[3];
        dv[0] = std::abs(v0[0] - v1[0]);
        dv[1] = std::abs(v0[1] - v1[1]);
        dv[2] = std::abs(v0[2] - v1[2]);
        return std::sqrt(dv[0]*dv[0] + dv[1]*dv[1] + dv[2]*dv[2]);
    }

    float
    FindLongestEdge(Far::TopologyRefiner const & mesh,
                    std::vector<float>   const & vertPos, int pointSize) {

        float maxLength = 0.0f;

        int numEdges = mesh.GetLevel(0).GetNumEdges();
        for (int i = 0; i < numEdges; ++i) {
            Far::ConstIndexArray edgeVerts = mesh.GetLevel(0).GetEdgeVertices(i);

            float edgeLength = EdgeLength(&vertPos[edgeVerts[0] * pointSize],
                                          &vertPos[edgeVerts[1] * pointSize]);

            maxLength = std::max(maxLength, edgeLength);
        }
        return maxLength;
    }

    void
    GetEdgeTessRates(std::vector<float> const & vertPos, int pointSize,
                     Args               const & options,
                     int                      * edgeRates) {

        int numEdges = (int) vertPos.size() / pointSize;
        for (int i = 0; i < numEdges; ++i) {
            int j = (i + 1) % numEdges;

            float edgeLength = EdgeLength(&vertPos[i * pointSize],
                                          &vertPos[j * pointSize]);

            edgeRates[i] = 1 + (int)(edgeLength / options.tessInterval);
            edgeRates[i] = std::min(edgeRates[i], options.tessRateMax);
        }
    }

    //
    //  The main tessellation function:  given a mesh and vertex positions,
    //  tessellate each face -- writing results in Obj format.
    //
    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<int>   faceTessRates;

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

        //
        //  Assign Tessellation Options applied for all faces.  Tessellations
        //  allow the creation of either 3- or 4-sided faces -- both of which
        //  are supported here via a command line option:
        //
        //  Remember that the use of non-uniform tessellation rates can lead
        //  to triangles being generated in 4-sided facets along boundaries
        //  (quad-preservation does not generate all quads). Such triangles
        //  are indicated by the use of an invalid/negative index in the fourth
        //  position.
        //
        int const tessFacetSize = 3 + options.tessQuadsFlag;

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

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

        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;
            }

            //
            //  Prepare the Surface patch points first as it may be evaluated
            //  to determine suitable edge-rates for Tessellation:
            //
            int pointSize = 3;

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

            faceSurface.PreparePatchPoints(meshVertexPositions.data(), pointSize,
                                           facePatchPoints.data(), pointSize);

            //
            //  For each of the N edges of the face, a tessellation rate is
            //  determined to initialize a non-uniform Tessellation pattern.
            //
            //  Many metrics are possible -- some based on the geometry itself
            //  (size, curvature), others dependent on viewpoint (screen space
            //  size, center of view, etc.) and many more. Simple techniques
            //  are chosen here for illustration and can easily be replaced.
            //
            //  Here two methods are shown using lengths between the corners of
            //  the face -- the first using the vertex positions of the face and
            //  the second using points evaluated at the corners of its limit
            //  surface. Use of the control hull is more efficient (avoiding the
            //  evaluation) but may prove less effective in some cases (though
            //  both estimates have their limitations).
            //
            int N = faceSurface.GetFaceSize();

            //  Use the output array temporarily to hold the N positions:
            outPos.resize(N * pointSize);

            if (options.useHullFlag) {
                Far::ConstIndexArray verts =
                        meshTopology.GetLevel(0).GetFaceVertices(faceIndex);

                for (int i = 0, j = 0; i < N; ++i, j += pointSize) {
                    float const * vPos = &meshVertexPositions[verts[i] * pointSize];
                    outPos[j  ] = vPos[0];
                    outPos[j+1] = vPos[1];
                    outPos[j+2] = vPos[2];
                }
            } else {
                Bfr::Parameterization faceParam = faceSurface.GetParameterization();

                for (int i = 0, j = 0; i < N; ++i, j += pointSize) {
                    float uv[2];
                    faceParam.GetVertexCoord(i, uv);
                    faceSurface.Evaluate(uv, facePatchPoints.data(), pointSize,
                                         &outPos[j]);
                }
            }

            faceTessRates.resize(N);
            GetEdgeTessRates(outPos, pointSize, options, faceTessRates.data());

            //
            //  Declare a non-uniform Tessellation using the rates for each
            //  edge and identify coordinates of the points to evaluate:
            //
            //  Additional interior rates can be optionally provided (2 for
            //  quads, 1 for others) but will be inferred in their absence.
            //
            Bfr::Tessellation tessPattern(faceSurface.GetParameterization(),
                                          N, faceTessRates.data(), tessOptions);

            int numOutCoords = tessPattern.GetNumCoords();

            outCoords.resize(numOutCoords * 2);

            tessPattern.GetCoords(outCoords.data());

            //
            //  Resize the output arrays and evaluate:
            //
            outPos.resize(numOutCoords * pointSize);
            outDu.resize(numOutCoords * pointSize);
            outDv.resize(numOutCoords * pointSize);

            for (int i = 0, j = 0; i < numOutCoords; ++i, j += pointSize) {
                faceSurface.Evaluate(&outCoords[i*2],
                                     facePatchPoints.data(), pointSize,
                                     &outPos[j], &outDu[j], &outDv[j]);
            }

            //
            //  Identify the faces of the Tessellation:
            //
            //  Note the need to offset vertex indices for the output faces --
            //  using the number of vertices generated prior to this face. One
            //  of several Tessellation methods to transform the facet indices
            //  simply translates all indices by the desired offset.
            //
            //  Remember also that triangles may be generated in 4-sided facets
            //  along boundaries and should be detected accordingly.
            //
            int objVertexIndexOffset = objWriter.GetNumVertices();

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

            tessPattern.TransformFacetCoordIndices(outFacets.data(),
                                                   objVertexIndexOffset);

            //
            //  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;
        }

        //
        //  If no interval length was specified, set one by finding the longest
        //  edge of the mesh and dividing it by the maximum tessellation rate:
        //
        if (args.tessInterval <= 0.0f) {
            args.tessInterval = FindLongestEdge(*meshTopology, meshVtxPositions, 3)
                              / (float) args.tessRateMax;
        }

        tessellateToObj(*meshTopology, meshVtxPositions, args);

        delete meshTopology;
        return EXIT_SUCCESS;
    }

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

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