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# The cameraLight example

This example shows how to position a GL_LIGHT on the camera.

A spot light position and direction are set so that they always match the camera's current setup.

This is done using the `camera->position()` and `camera->viewDirection()` methods which return values expressed in the world coordinates system.

## cameraLight.h

    #include <QGLViewer/qglviewer.h>

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

    private :
      virtual void drawSpiral();
    };

## cameraLight.cpp

    #include "cameraLight.h"

    using namespace std;
    using namespace qglviewer;

    void Viewer::draw()
    {
    	// Place light at camera position
    	const Vec cameraPos = camera()->position();
    	const GLfloat pos[4] = {cameraPos[0], cameraPos[1], cameraPos[2], 1.0};
    	glLightfv(GL_LIGHT1, GL_POSITION, pos);

    	// Orientate light along view direction
    	glLightfv(GL_LIGHT1, GL_SPOT_DIRECTION, camera()->viewDirection());

    	drawSpiral();
    }

    // Draws a spiral
    void Viewer::drawSpiral()
    {
    	const float nbSteps = 1000.0;

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

    void Viewer::init()
    {
    	// Light setup
    	glDisable(GL_LIGHT0);
    	glEnable(GL_LIGHT1);

    	// Light default parameters
    	const GLfloat light_ambient[4]  = {1.0, 1.0, 1.0, 1.0};
    	const GLfloat light_specular[4] = {1.0, 1.0, 1.0, 1.0};
    	const GLfloat light_diffuse[4]  = {1.0, 1.0, 1.0, 1.0};

    	glLightf( GL_LIGHT1, GL_SPOT_EXPONENT, 3.0);
    	glLightf( GL_LIGHT1, GL_SPOT_CUTOFF,   10.0);
    	glLightf( GL_LIGHT1, GL_CONSTANT_ATTENUATION,  0.1f);
    	glLightf( GL_LIGHT1, GL_LINEAR_ATTENUATION,    0.3f);
    	glLightf( GL_LIGHT1, GL_QUADRATIC_ATTENUATION, 0.3f);
    	glLightfv(GL_LIGHT1, GL_AMBIENT,  light_ambient);
    	glLightfv(GL_LIGHT1, GL_SPECULAR, light_specular);
    	glLightfv(GL_LIGHT1, GL_DIFFUSE,  light_diffuse);

    	// Restore previous viewer state.
    	restoreStateFromFile();

    	// Opens help window
    	help();
    }

    QString Viewer::helpString() const
    {
    	QString text("<h2>C a m e r a L i g h t</h2>");
    	text += "See the <b>Mouse</b> tab and the documentation web pages for details.<br><br>";
    	text += "Press <b>Escape</b> to exit the viewer.";
    	return text;
    }

## main.cpp

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

    int main(int argc, char** argv)
    {
      // Read command lines arguments.
      QApplication application(argc,argv);

      // Instantiate the viewer.
      Viewer viewer;

      viewer.setWindowTitle("cameraLight");

      // Make the viewer window visible on screen.
      viewer.show();

      // Run main loop.
      return application.exec();
    }

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