动画原理——加速度

书籍名称：HTML5-Animation-with-JavaScript

书籍源码：https://github.com/lamberta/html5-animation

这一章节没有仔细讲一是因为和上一章节很相似，只是速率换成加速度。

二是因为初中学的加速度大家都懂得。

1.在某一方向的方向的加速度

06-acceleration-1.html

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Acceleration 1</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(),
          vx = 0,
          ax = 0.1;

      ball.x = 50;
      ball.y = 100;

      (function drawFrame () {
        window.requestAnimationFrame(drawFrame, canvas);
        context.clearRect(0, 0, canvas.width, canvas.height);

        vx += ax;
        ball.x += vx;
        ball.draw(context);
      }());
    };
    </script>
  </body>
</html>

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