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# The fastDraw example

The `fastDraw()` function enables interactivity even with large scenes.

This example demonstrates the use of the `fastDraw()` function, which is called when the camera is manipulated. This function can be used to display a simplified version of the scene and is usefull for displaying very complex scene, while keeping an interactive camera motion.

## fastDraw.h

    #include <QGLViewer/qglviewer.h>

    class Viewer : public QGLViewer
    {
    protected:
      virtual void init();
      virtual void draw();
      virtual void fastDraw();
      virtual QString helpString() const;
    };

## fastDraw.cpp

    #include "fastDraw.h"

    using namespace std;
    using namespace qglviewer;

    void Viewer::init()
    {
      // Increase the material shininess, so that the difference between
      // the two versions of the spiral is more visible.
      glMaterialf(GL_FRONT_AND_BACK, GL_SHININESS, 50.0);
      GLfloat specular_color[4] = { 0.8f, 0.8f, 0.8f, 1.0 };
      glMaterialfv(GL_FRONT_AND_BACK, GL_SPECULAR,  specular_color);

      restoreStateFromFile();
      help();
    }

    static void drawSpiral(bool simplified = false)
    {
      int nbSteps = 600;
      int nbSub = 50;
      if (simplified)
        {
          nbSteps = 60;
          nbSub = 2;
        }

      for (float n=0; n<nbSub; ++n)
        {
          glBegin(GL_QUAD_STRIP);
          for (float i=0.0; i<nbSteps; ++i)
    	{
    	  float ratio = i/nbSteps;
    	  float angle = 21.0*ratio;
    	  float radius = 1.0 - 0.5*ratio;

    	  Vec center(radius*cos(angle), ratio-0.5, radius*sin(angle));

    	  for (unsigned short j=0; j<2; ++j)
    	    {
    	      float delta = 3.0*(n+j)/nbSub;
    	      Vec shift(cos(angle)*cos(delta), sin(delta), sin(angle)*cos(delta));

    	      glColor3f(1-ratio, (n+j)/nbSub , ratio);
    	      glNormal3fv(shift);
    	      glVertex3fv(center+0.2f*shift);
    	    }
    	}
          glEnd();
        }
    }

    void Viewer::draw()
    {
      drawSpiral();
    }

    void Viewer::fastDraw()
    {
      drawSpiral(true);
    }

    QString Viewer::helpString() const
    {
      QString text("<h2>F a s t D r a w</h2>");
      text += "The <code>fastDraw()</code> function is called instead of <code>draw()</code> when the camera ";
      text += "is manipulated. Providing such a simplified version of <code>draw()</code> allows for interactive ";
      text += "frame rates when the camera is moved, even for very complex scenes.";
      return text;
    }

## main.cpp

    #include "fastDraw.h"
    #include <qapplication.h>

    int main(int argc, char** argv)
    {
      QApplication application(argc,argv);

      Viewer viewer;

      viewer.setWindowTitle("fastDraw");

      viewer.show();

      return application.exec();
    }

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Last modified on Wednesday, June 17, 2015.