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# The drawLight example

The `drawLight()` function is a light debugging tool.

drawLight() takes as an argument the GL index of the lamp : GL_LIGHT0, GL_LIGHT1... and displays a symbolic representation of the light. This function is usefull for debugging your light setup.

## drawLight.h

    #include <QGLViewer/qglviewer.h>

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

    private:
      qglviewer::ManipulatedFrame* light1;
      qglviewer::ManipulatedFrame* light2;
    };

## drawLight.cpp

    #include "drawLight.h"
    #include <math.h>

    using namespace std;
    using namespace qglviewer;

    void Viewer::draw()
    {
      float pos[4] = {1.0, 0.5, 1.0, 0.0};
      // Directionnal light
      glLightfv(GL_LIGHT0, GL_POSITION, pos);

      pos[3] = 1.0;
      // Spot light
      light1->getPosition(pos[0], pos[1], pos[2]);
      glLightfv(GL_LIGHT1, GL_POSITION, pos);
      glLightfv(GL_LIGHT1, GL_SPOT_DIRECTION, light1->inverseTransformOf(Vec(0,0,1)));

      // Point light
      light2->getPosition(pos[0], pos[1], pos[2]);
      glLightfv(GL_LIGHT2, GL_POSITION, pos);

      // Draws the spiral
      const float nbSteps = 200.0;
      glBegin(GL_QUAD_STRIP);
      for (float i=0; i<nbSteps; ++i)
        {
          float ratio = i/nbSteps;
          float angle = 21.0*ratio;
          float c = cos(angle);
          float s = sin(angle);
          float r1 = 1.0 - 0.8*ratio;
          float r2 = 0.8 - 0.8*ratio;
          float alt = ratio - 0.5;
          const float nor = .5;
          const float up = sqrt(1.0-nor*nor);
          glColor3f(1-ratio, 0.2f , ratio);
          glNormal3f(nor*c, up, nor*s);
          glVertex3f(r1*c, alt, r1*s);
          glVertex3f(r2*c, alt+0.05, r2*s);
        }
      glEnd();

      drawLight(GL_LIGHT0);

      if (light1->grabsMouse())
        drawLight(GL_LIGHT1, 1.2f);
      else
        drawLight(GL_LIGHT1);

      if (light2->grabsMouse())
        drawLight(GL_LIGHT2, 1.2f);
      else
        drawLight(GL_LIGHT2);
    }

    void Viewer::init()
    {
      glMatrixMode(GL_MODELVIEW);
      glLoadIdentity();

      // Light0 is the default ambient light
      glEnable(GL_LIGHT0);

      // Light1 is a spot light
      glEnable(GL_LIGHT1);
      const GLfloat light_ambient[4]  = {0.8f, 0.2f, 0.2f, 1.0};
      const GLfloat light_diffuse[4]  = {1.0, 0.4f, 0.4f, 1.0};
      const GLfloat light_specular[4] = {1.0, 0.0, 0.0, 1.0};

      glLightf( GL_LIGHT1, GL_SPOT_EXPONENT,  3.0);
      glLightf( GL_LIGHT1, GL_SPOT_CUTOFF,    20.0);
      glLightf( GL_LIGHT1, GL_CONSTANT_ATTENUATION, 0.5);
      glLightf( GL_LIGHT1, GL_LINEAR_ATTENUATION, 1.0);
      glLightf( GL_LIGHT1, GL_QUADRATIC_ATTENUATION, 1.5);
      glLightfv(GL_LIGHT1, GL_AMBIENT,  light_ambient);
      glLightfv(GL_LIGHT1, GL_SPECULAR, light_specular);
      glLightfv(GL_LIGHT1, GL_DIFFUSE,  light_diffuse);

      // Light2 is a classical directionnal light
      glEnable(GL_LIGHT2);
      const GLfloat light_ambient2[4]  = {0.2f, 0.2f, 2.0, 1.0};
      const GLfloat light_diffuse2[4]  = {0.8f, 0.8f, 1.0, 1.0};
      const GLfloat light_specular2[4] = {0.0, 0.0, 1.0, 1.0};

      glLightfv(GL_LIGHT2, GL_AMBIENT,  light_ambient2);
      glLightfv(GL_LIGHT2, GL_SPECULAR, light_specular2);
      glLightfv(GL_LIGHT2, GL_DIFFUSE,  light_diffuse2);

      light1 = new ManipulatedFrame();
      light2 = new ManipulatedFrame();
      setMouseTracking(true);

      light1->setPosition(0.5, 0.5, 0);
      // Align z axis with -position direction : look at scene center
      light1->setOrientation(Quaternion(Vec(0,0,1), -light1->position()));

      light2->setPosition(-0.5, 0.5, 0);

      restoreStateFromFile();
      help();
    }

    QString Viewer::helpString() const
    {
      QString text("<h2>D r a w L i g h t</h2>");
      text += "The <i>drawLight()</i> function displays a representation of the OpenGL lights ";
      text += "of your scene. This is convenient for debugging your light setup.<br><br>";
      text += "This scene features a directionnal ligth (arrow), a spot light (cone) and a point ";
      text += "light source (sphere). The representation color, position and shape matches the light setup.<br><br>";
      text += "Hover over the point light or the spot light to manipulate it using the mouse (right ";
      text += "button translates and left button rotates).";
      return text;
    }

## main.cpp

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

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

      Viewer viewer;

    #if QT_VERSION < 0x040000
      application.setMainWidget(&viewer);
    #else
      viewer.setWindowTitle("drawLight");
    #endif

      viewer.show();

      return application.exec();
    }

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Last modified on Sunday, February 28, 2010.