libQGLViewer animation example

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# The animation example

The `animate()` function illustrated by a water particle simulation.

When animation is activated (the Return key toggles animation), the `animate()` and then the `draw()` functions are called in an infinite loop.

You can tune the frequency of your animation (default is 25Hz) using `setAnimationInterval()`. The frame rate will then be fixed, provided that your animation loop function is fast enough.

## animation.h

    #include <QGLViewer/qglviewer.h>

    class Particle {
    public:
      Particle();

      void init();
      void draw();
      void animate();

    private:
      qglviewer::Vec speed_, pos_;
      int age_, ageMax_;
    };

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

    private:
      int nbPart_;
      Particle *particle_;
    };

## animation.cpp

    #include "animation.h"
    #include <math.h>
    #include <stdlib.h> // RAND_MAX

    using namespace qglviewer;
    using namespace std;

    ///////////////////////   V i e w e r  ///////////////////////
    void Viewer::init() {
      restoreStateFromFile();
      glDisable(GL_LIGHTING);
      nbPart_ = 2000;
      particle_ = new Particle[nbPart_];
      glPointSize(3.0);
      setGridIsDrawn();
      help();
      startAnimation();
    }

    void Viewer::draw() {
      glBegin(GL_POINTS);
      for (int i = 0; i < nbPart_; i++)
        particle_[i].draw();
      glEnd();
    }

    void Viewer::animate() {
      for (int i = 0; i < nbPart_; i++)
        particle_[i].animate();
    }

    QString Viewer::helpString() const {
      QString text("<h2>A n i m a t i o n</h2>");
      text += "Use the <i>animate()</i> function to implement the animation part "
              "of your ";
      text += "application. Once the animation is started, <i>animate()</i> and "
              "<i>draw()</i> ";
      text += "are called in an infinite loop, at a frequency that can be "
              "fixed.<br><br>";
      text += "Press <b>Return</b> to start/stop the animation.";
      return text;
    }

    ///////////////////////   P a r t i c l e   ///////////////////////////////

    Particle::Particle() { init(); }

    void Particle::animate() {
      speed_.z -= 0.05f;
      pos_ += 0.1f * speed_;

      if (pos_.z < 0.0) {
        speed_.z = -0.8 * speed_.z;
        pos_.z = 0.0;
      }

      if (++age_ == ageMax_)
        init();
    }

    void Particle::draw() {
      glColor3f(age_ / (float)ageMax_, age_ / (float)ageMax_, 1.0);
      glVertex3fv(pos_);
    }

    void Particle::init() {
      pos_ = Vec(0.0, 0.0, 0.0);
      float angle = 2.0 * M_PI * rand() / RAND_MAX;
      float norm = 0.04 * rand() / RAND_MAX;
      speed_ = Vec(norm * cos(angle), norm * sin(angle),
                   rand() / static_cast<float>(RAND_MAX));
      age_ = 0;
      ageMax_ = 50 + static_cast<int>(100.0 * rand() / RAND_MAX);
    }

## main.cpp

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

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

      Viewer viewer;

      viewer.setWindowTitle("animation");

      viewer.show();

      return application.exec();
    }

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