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# Polygon Tessellation Tutorial

In this tutorial you will learn how to use the vl::Tessellator class to tessellate a concave or self intersecting polygon into a set of triangles that can be rendered with Visualization Library.

[From `App_Tessellator.hpp`]

    class App_Tessellator: public BaseDemo
    {
    public:
      App_Tessellator() {}

      void initEvent()
      {
        // Basic initialization
        BaseDemo::initEvent();

        // Filled effect
        vl::ref<vl::Effect> filled_fx = new vl::Effect;

        // Wireframe effect
        vl::ref<vl::Effect> wireframe_fx = new vl::Effect;
        wireframe_fx->shader()->gocPolygonMode()->set(vl::PM_LINE,vl::PM_LINE);

        // Add empty Actors
        mStar1 = sceneManager()->tree()->addActor( new vl::Actor(NULL, filled_fx.get(), new vl::Transform) );
        mStar2 = sceneManager()->tree()->addActor( new vl::Actor(NULL, wireframe_fx.get(), new vl::Transform) );
        vl::VisualizationLibrary::rendering()->as<vl::Rendering>()->transform()->addChild(mStar1->transform());
        vl::VisualizationLibrary::rendering()->as<vl::Rendering>()->transform()->addChild(mStar2->transform());
      }

      void run()
      {
        // Animation: compute rotation matrix to rotate the small star, 45°/sec rotation.
        vl::dmat4 m = vl::dmat4::rotation( vl::Time::currentTime()*45, 0,0,1 );
        // Filled star on the left, wireframe star on the right
        mStar1->transform()->setLocalMatrix( vl::mat4::translation(-4,0,0) );
        mStar2->transform()->setLocalMatrix( vl::mat4::translation(+4,0,0) );

        // Concave and self-intersecting polygons cannot be directly rendered by OpenGL, for this
        // reason in order to render them we have to tessellate them first, i.e. we have to decompose
        // them into a set of triangles using the Tessellator class.

        // The Tessellator class takes as input a set of contours defining a complex polygon and
        // outputs a series of triangles that can be rendered by OpenGL and Visualization Library
        vl::Tessellator tess;

        // Setup tessellation options, see also gluTessProperty() documentation.
        tess.setWindingRule(vl::TW_TESS_WINDING_ODD); // default
        tess.setTessNormal(vl::fvec3(0,0,1)); // default is vl::fvec3(0,0,0)
        tess.setBoundaryOnly(false); // default
        tess.setTolerance(0.0); // default

        // Outline #1 - generate 5-points star (small)
        int size = 2;
        tess.contours().push_back(5);
        for(int i=0; i<5; ++i)
        {
          float t = (float)i/5.0f*vl::fPi*2.0f*2.0f + vl::fPi/2.0f;
          // Fill the first contour
          tess.contourVerts().push_back( m * vl::dvec3(cos(t)*size,sin(t)*size,0.0f) );
        }

        // Outline #2 - generate 5-points star (big)
        size = 4;
        tess.contours().push_back(5);
        for(int i=0; i<5; ++i)
        {
          float t = (float)i/5.0f*vl::fPi*2.0f*2.0f + vl::fPi/2.0f;
          // Fill the second contour
          tess.contourVerts().push_back( vl::dvec3(cos(t)*size,sin(t)*size,0.0f) );
        }

        // Tessellate the two contours into a single polygon
        tess.setTessellateIntoSinglePolygon(true); // default
        tess.tessellate();

        /*
        You can also tessellate each contour separately selecting setTessellateIntoSinglePolygon(false).
        This way each contour will be processed separately and will generate its own set of triangles, this
        is useful when you want to tessellate a large number of polygons with a single tessellate() call.
        */

        // Create a new Geometry and vertex array with the tessellated triangles
        vl::ref<vl::Geometry> tess_poly = new vl::Geometry;
        vl::ref<vl::ArrayFVec3> vert_array = new vl::ArrayFVec3;
        tess_poly->setVertexArray(vert_array.get());
        // Fill the vertex array with the tessellated triangles
        *vert_array = tess.tessellatedTris();
        // Add the primitive description
        tess_poly->primitives()->push_back( new vl::DrawArrays(vl::PT_TRIANGLES,0,vert_array->size()) );

        // Bind the created Geometry to the star Actor
        mStar1->lod(0) = tess_poly.get();
        mStar2->lod(0) = tess_poly.get();
      }

    protected:
      // the Actor defining our complex star
      vl::Actor* mStar1;
      vl::Actor* mStar2;
    };

    // Have fun!

Visualization Library v2009.08 Reference Documentation
Copyright 2005-2009 Michele Bosi. All rights reserved.
Updated on Sun Nov 8 14:28:53 2009.
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