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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