# bfr_tutorial_1_5.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/release/tutorials/bfr/tutorial_1_5/bfr_tutorial_1_5.cpp

    //------------------------------------------------------------------------------
    //  Tutorial description:
    //
    //      This tutorial is an alternative to an earlier tutorial that showed
    //      uniform tessellation. This version differs by evaluating the points
    //      of the tessellation using limit stencils instead of the standard
    //      Surface evaluation methods.
    //
    //      Limit stencils factor the evaluation into a set of coefficients for
    //      each control point affecting the Surface.
    //

    #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>

    //  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;
        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() { }
    };

    //
    //  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> faceControlPoints;

        std::vector<float> limitStencils;

        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);

        //
        //  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;
            }

            //
            //  Resize stencils and control point arrays based on the number
            //  of control points for the Surface:
            //
            int numControlPoints = faceSurface.GetNumControlPoints();

            limitStencils.resize(3 * numControlPoints);

            float * pStencil  = limitStencils.data();
            float * duStencil = limitStencils.data() + numControlPoints;
            float * dvStencil = limitStencils.data() + numControlPoints * 2;

            //
            //  Limit stencils can be applied using the control points in a
            //  local array or directy from the mesh. Both are shown here, so
            //  if using the local array, resize and populate it:
            //
            bool gatherControlPoints = true;
            if (gatherControlPoints) {
                faceControlPoints.resize(numControlPoints * 3);

                faceSurface.GatherControlPoints(meshVertexPositions.data(), 3,
                                                faceControlPoints.data(), 3);
            }

            //
            //  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());

            //
            //  Evaluate and apply stencils to compute points of the tessellation:
            //
            outPos.resize(numOutCoords * 3);
            outDu.resize(numOutCoords * 3);
            outDv.resize(numOutCoords * 3);

            for (int i = 0; i < numOutCoords; ++i) {
                float const * uv = outCoords.data() + i * 2;

                faceSurface.EvaluateStencil(uv, pStencil, duStencil, dvStencil);

                float * p  = outPos.data() + i * 3;
                float * du = outDu.data()  + i * 3;
                float * dv = outDv.data()  + i * 3;

                if (gatherControlPoints) {
                    float const * controlPoints = faceControlPoints.data();

                    faceSurface.ApplyStencil(pStencil,  controlPoints, 3, p);
                    faceSurface.ApplyStencil(duStencil, controlPoints, 3, du);
                    faceSurface.ApplyStencil(dvStencil, controlPoints, 3, dv);
                } else {
                    float const * meshPoints = meshVertexPositions.data();

                    faceSurface.ApplyStencilFromMesh(pStencil,  meshPoints, 3, p);
                    faceSurface.ApplyStencilFromMesh(duStencil, meshPoints, 3, du);
                    faceSurface.ApplyStencilFromMesh(dvStencil, meshPoints, 3, dv);
                }
            }

            //
            //  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.
            //
            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;
        }

        tessellateToObj(*meshTopology, meshVtxPositions, args);

        delete meshTopology;
        return EXIT_SUCCESS;
    }

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

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