• Fast changes in the system that happen even without user input.
• Non-animated apps - reactive applications.
• Requires a form of infinite loop - a Timer.
• Usually achieved by a set of functions or class methods that describe the changes to each object in a small unit of time or delta t - called update or move.

• Two types of animation:
  
  
• glutIdleFunc(display);
  Calls the function (display) at regular intervals.
  
  
• glutTimerFunc(msecs, anim, value);
  void anim(int value);
  Triggers an event calling the function anim after the given number of ms and passes it the value as argument.

void idleAnim()
{
    for (each object)
        object.update();

    display();
    
}

Hard to control the animation speed - platform dependent.

Generic Timer Callback

void anim(int value)
{
    if (value) {
        for (each object)
            object.update();            
        display();
    
        glutTimerFunc(msecs, anim, newValue);
    }
}  

• OpenGL can use two rendering buffers.
• One is displayed while the other one is being rendered.
• Speeds up animations.
• In init:
  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | ...);
• In display, at the very end:
  glutSwapBuffers();

• In Flash / ActionScript:
• var myTimer:Timer = new Timer(200); // ms
• myTimer.addEventListener(TimerEvent.TIMER, callback);
• myTimer.start(); ...
• myTimer.stop();
• In JavaScript: setTimeout(), setInterval().
• In Unity:
  Update() defined in every class;
  FixedUpdate() - called before updating the physics engine.

• Two intervals [x1, x1'] and [x2, x2'] collide if
• x1 <= x2 <= x1'   or   x2 <= x1 <= x2'
• ((x2-x1)*(x2-x1') < 0 && (x1-x2)*(x1-x2') < 0) if you don't know that x1 <= x1' and x2 <= x2'.
• Two boxes collide if they collide both over x and over y.
• Two circles collide if the distance between their centers is smaller than the sum of the radii.