examples/ch10/02-rotate-2.html

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Rotate 2</title>
    <link rel="stylesheet" href="../include/style.css">
  </head>
  <body>
    <header>
      Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
    </header>
    <canvas id="canvas" width="400" height="400"></canvas>

    <script src="../include/utils.js"></script>
    <script src="./classes/ball.js"></script>
    <script>
    window.onload = function () {
      var canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          ball = new Ball(),
          vr = 0.05,
          cos = Math.cos(vr),
          sin = Math.sin(vr),
          centerX = canvas.width / 2,
          centerY = canvas.height / 2;
      
      ball.x = Math.random() * canvas.width;
      ball.y = Math.random() * canvas.height;

      (function drawFrame () {
        window.requestAnimationFrame(drawFrame, canvas);
        context.clearRect(0, 0, canvas.width, canvas.height);
        
        var x1 = ball.x - centerX,
            y1 = ball.y - centerY,
            x2 = cos * x1 - sin * y1,
            y2 = cos * y1 + sin * x1;
        
        ball.x = centerX + x2;
        ball.y = centerY + y2;
        
        ball.draw(context);
      }());
    };
    </script>
  </body>
</html>