libQGLViewer cameraLight example

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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] = {(float)cameraPos[0], (float)cameraPos[1],
                              (float)cameraPos[2], 1.0f};
      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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