# far_tutorial_3.cpp

https://github.com/PixarAnimationStudios/OpenSubdiv/blob/master/tutorials/far/tutorial_3/far_tutorial_3.cpp

    //------------------------------------------------------------------------------
    // Tutorial description:
    //
    // Building on tutorial 0, this example shows how to instantiate a simple mesh,
    // refine it uniformly and then interpolate both 'vertex' and 'face-varying'
    // primvar data.
    // The resulting interpolated data is output as an 'obj' file, with the
    // 'face-varying' data recorded in the uv texture layout.
    //

    #include <opensubdiv/far/topologyDescriptor.h>
    #include <opensubdiv/far/primvarRefiner.h>

    #include <cstdio>

    //------------------------------------------------------------------------------
    // Face-varying implementation.
    //
    //
    struct Vertex {

        // Minimal required interface ----------------------
        Vertex() { }

        Vertex(Vertex const & src) {
            _position[0] = src._position[0];
            _position[1] = src._position[1];
            _position[2] = src._position[2];
        }

        void Clear( void * =0 ) {
            _position[0]=_position[1]=_position[2]=0.0f;
        }

        void AddWithWeight(Vertex const & src, float weight) {
            _position[0]+=weight*src._position[0];
            _position[1]+=weight*src._position[1];
            _position[2]+=weight*src._position[2];
        }

        // 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];
    };

    //------------------------------------------------------------------------------
    // Face-varying container implementation.
    //
    // We are using a uv texture layout as a 'face-varying' primtiive variable
    // attribute. Because face-varying data is specified 'per-face-per-vertex',
    // we cannot use the same container that we use for 'vertex' or 'varying'
    // data. We specify a new container, which only carries (u,v) coordinates.
    // Similarly to our 'Vertex' container, we add a minimaliztic interpolation
    // interface with a 'Clear()' and 'AddWithWeight()' methods.
    //
    struct FVarVertexUV {

        // Minimal required interface ----------------------
        void Clear() {
            u=v=0.0f;
        }

        void AddWithWeight(FVarVertexUV const & src, float weight) {
            u += weight * src.u;
            v += weight * src.v;
        }

        // Basic 'uv' layout channel
        float u,v;
    };

    struct FVarVertexColor {

        // Minimal required interface ----------------------
        void Clear() {
            r=g=b=a=0.0f;
        }

        void AddWithWeight(FVarVertexColor const & src, float weight) {
            r += weight * src.r;
            g += weight * src.g;
            b += weight * src.b;
            a += weight * src.a;
        }

        // Basic 'color' layout channel
        float r,g,b,a;
    };

    //------------------------------------------------------------------------------
    // Cube geometry from catmark_cube.h

    // 'vertex' primitive variable data & topology
    static float g_verts[8][3] = {{ -0.5f, -0.5f,  0.5f },
                                  {  0.5f, -0.5f,  0.5f },
                                  { -0.5f,  0.5f,  0.5f },
                                  {  0.5f,  0.5f,  0.5f },
                                  { -0.5f,  0.5f, -0.5f },
                                  {  0.5f,  0.5f, -0.5f },
                                  { -0.5f, -0.5f, -0.5f },
                                  {  0.5f, -0.5f, -0.5f }};
    static int g_nverts = 8,
               g_nfaces = 6;

    static int g_vertsperface[6] = { 4, 4, 4, 4, 4, 4 };

    static int g_vertIndices[24] = { 0, 1, 3, 2,
                                     2, 3, 5, 4,
                                     4, 5, 7, 6,
                                     6, 7, 1, 0,
                                     1, 7, 5, 3,
                                     6, 0, 2, 4  };

    // 'face-varying' primitive variable data & topology for UVs
    static float g_uvs[14][2] = {{ 0.375, 0.00 },
                                 { 0.625, 0.00 },
                                 { 0.375, 0.25 },
                                 { 0.625, 0.25 },
                                 { 0.375, 0.50 },
                                 { 0.625, 0.50 },
                                 { 0.375, 0.75 },
                                 { 0.625, 0.75 },
                                 { 0.375, 1.00 },
                                 { 0.625, 1.00 },
                                 { 0.875, 0.00 },
                                 { 0.875, 0.25 },
                                 { 0.125, 0.00 },
                                 { 0.125, 0.25 }};

    static int g_nuvs = 14;

    static int g_uvIndices[24] = {  0,  1,  3,  2,
                                    2,  3,  5,  4,
                                    4,  5,  7,  6,
                                    6,  7,  9,  8,
                                    1, 10, 11,  3,
                                   12,  0,  2, 13  };

    // 'face-varying' primitive variable data & topology for color
    static float g_colors[24][4] = {{1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 0.0, 0.0, 1.0},
                                    {1.0, 0.0, 0.0, 1.0},
                                    {1.0, 0.0, 0.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0},
                                    {1.0, 1.0, 1.0, 1.0}};

    static int g_ncolors = 24;

    static int g_colorIndices[24] = { 0,  3,  9,  6,
                                      7, 10, 15, 12,
                                     13, 16, 21, 18,
                                     19, 22,  4,  1,
                                      5, 23, 17, 11,
                                     20,  2,  8, 14 };

    using namespace OpenSubdiv;

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

        int maxlevel = 3;

        typedef Far::TopologyDescriptor Descriptor;

        Sdc::SchemeType type = OpenSubdiv::Sdc::SCHEME_CATMARK;

        Sdc::Options options;
        options.SetVtxBoundaryInterpolation(Sdc::Options::VTX_BOUNDARY_EDGE_ONLY);
        options.SetFVarLinearInterpolation(Sdc::Options::FVAR_LINEAR_NONE);

        // Populate a topology descriptor with our raw data
        Descriptor desc;
        desc.numVertices  = g_nverts;
        desc.numFaces     = g_nfaces;
        desc.numVertsPerFace = g_vertsperface;
        desc.vertIndicesPerFace  = g_vertIndices;

        int channelUV = 0;
        int channelColor = 1;

        // Create a face-varying channel descriptor
        Descriptor::FVarChannel channels[2];
        channels[channelUV].numValues = g_nuvs;
        channels[channelUV].valueIndices = g_uvIndices;
        channels[channelColor].numValues = g_ncolors;
        channels[channelColor].valueIndices = g_colorIndices;

        // Add the channel topology to the main descriptor
        desc.numFVarChannels = 2;
        desc.fvarChannels = channels;

        // Instantiate a FarTopologyRefiner from the descriptor
        Far::TopologyRefiner * refiner =
            Far::TopologyRefinerFactory<Descriptor>::Create(desc,
                Far::TopologyRefinerFactory<Descriptor>::Options(type, options));

        // Uniformly refine the topolgy up to 'maxlevel'
        // note: fullTopologyInLastLevel must be true to work with face-varying data
        {
            Far::TopologyRefiner::UniformOptions refineOptions(maxlevel);
            refineOptions.fullTopologyInLastLevel = true;
            refiner->RefineUniform(refineOptions);
        }

        // Allocate and initialize the 'vertex' primvar data (see tutorial 2 for
        // more details).
        std::vector<Vertex> vbuffer(refiner->GetNumVerticesTotal());
        Vertex * verts = &vbuffer[0];

        for (int i=0; i<g_nverts; ++i) {
            verts[i].SetPosition(g_verts[i][0], g_verts[i][1], g_verts[i][2]);
        }

        // Allocate and initialize the first channel of 'face-varying' primvar data (UVs)
        std::vector<FVarVertexUV> fvBufferUV(refiner->GetNumFVarValuesTotal(channelUV));
        FVarVertexUV * fvVertsUV = &fvBufferUV[0];
        for (int i=0; i<g_nuvs; ++i) {
            fvVertsUV[i].u = g_uvs[i][0];
            fvVertsUV[i].v = g_uvs[i][1];
        }

        // Allocate & interpolate the 'face-varying' primvar data (colors)
        std::vector<FVarVertexColor> fvBufferColor(refiner->GetNumFVarValuesTotal(channelColor));
        FVarVertexColor * fvVertsColor = &fvBufferColor[0];
        for (int i=0; i<g_ncolors; ++i) {
            fvVertsColor[i].r = g_colors[i][0];
            fvVertsColor[i].g = g_colors[i][1];
            fvVertsColor[i].b = g_colors[i][2];
            fvVertsColor[i].a = g_colors[i][3];
        }

        // Interpolate both vertex and face-varying primvar data
        Far::PrimvarRefiner primvarRefiner(*refiner);

        Vertex *     srcVert = verts;
        FVarVertexUV * srcFVarUV = fvVertsUV;
        FVarVertexColor * srcFVarColor = fvVertsColor;

        for (int level = 1; level <= maxlevel; ++level) {
            Vertex *     dstVert = srcVert + refiner->GetLevel(level-1).GetNumVertices();
            FVarVertexUV * dstFVarUV = srcFVarUV + refiner->GetLevel(level-1).GetNumFVarValues(channelUV);
            FVarVertexColor * dstFVarColor = srcFVarColor + refiner->GetLevel(level-1).GetNumFVarValues(channelColor);

            primvarRefiner.Interpolate(level, srcVert, dstVert);
            primvarRefiner.InterpolateFaceVarying(level, srcFVarUV, dstFVarUV, channelUV);
            primvarRefiner.InterpolateFaceVarying(level, srcFVarColor, dstFVarColor, channelColor);

            srcVert = dstVert;
            srcFVarUV = dstFVarUV;
            srcFVarColor = dstFVarColor;
        }

        { // Output OBJ of the highest level refined -----------

            Far::TopologyLevel const & refLastLevel = refiner->GetLevel(maxlevel);

            int nverts = refLastLevel.GetNumVertices();
            int nuvs   = refLastLevel.GetNumFVarValues(channelUV);
            int ncolors= refLastLevel.GetNumFVarValues(channelColor);
            int nfaces = refLastLevel.GetNumFaces();

            // Print vertex positions
            int firstOfLastVerts = refiner->GetNumVerticesTotal() - nverts;

            for (int vert = 0; vert < nverts; ++vert) {
                float const * pos = verts[firstOfLastVerts + vert].GetPosition();
                printf("v %f %f %f\n", pos[0], pos[1], pos[2]);
            }

            // Print uvs
            int firstOfLastUvs = refiner->GetNumFVarValuesTotal(channelUV) - nuvs;

            for (int fvvert = 0; fvvert < nuvs; ++fvvert) {
                FVarVertexUV const & uv = fvVertsUV[firstOfLastUvs + fvvert];
                printf("vt %f %f\n", uv.u, uv.v);
            }

            // Print colors
            int firstOfLastColors = refiner->GetNumFVarValuesTotal(channelColor) - ncolors;

            for (int fvvert = 0; fvvert < nuvs; ++fvvert) {
                FVarVertexColor const & c = fvVertsColor[firstOfLastColors + fvvert];
                printf("c %f %f %f %f\n", c.r, c.g, c.b, c.a);
            }

            // Print faces
            for (int face = 0; face < nfaces; ++face) {

                Far::ConstIndexArray fverts = refLastLevel.GetFaceVertices(face);
                Far::ConstIndexArray fuvs   = refLastLevel.GetFaceFVarValues(face, channelUV);

                // all refined Catmark faces should be quads
                assert(fverts.size()==4 && fuvs.size()==4);

                printf("f ");
                for (int vert=0; vert<fverts.size(); ++vert) {
                    // OBJ uses 1-based arrays...
                    printf("%d/%d ", fverts[vert]+1, fuvs[vert]+1);
                }
                printf("\n");
            }
        }
    }
    //------------------------------------------------------------------------------

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