动画原理——脉动（膨胀缩小）&&无规则运动
书籍名称：HTML5-Animation-with-JavaScript
书籍源码：https://github.com/lamberta/html5-animation
1.脉动是一种半径r来回反复的运动，在canvas中他是对原图形的方法和缩小。可以将sin应用在控制大小的属性scaleX，scaleY。
以下代码在先前的实例中对index.html的进行修改即可。
index.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Pulse</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(),
angle = 0,
centerScale = 1,
range = 0.5,
speed = 0.05;
ball.x = canvas.width / 2;
ball.y = canvas.height / 2;
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
//利用正弦设置放大的倍数
ball.scaleX = ball.scaleY = centerScale + Math.sin(angle) * range;
angle += speed;
ball.draw(context);
}());
};
</script>
</body>
</html>
2.对球体的位置x，y分别用一个sin函数来表示，且角度递增的速度不一样，可以实现无规则运动，代码如下
仍然只是index.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Random</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(),
angleX = 0,
angleY = 0,
range = 50,
centerX = canvas.width / 2,
centerY = canvas.height / 2,
xspeed = 0.07,
yspeed = 0.11;
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
ball.x = centerX + Math.sin(angleX) * range;
ball.y = centerY + Math.sin(angleY) * range;
angleX += xspeed;
angleY += yspeed;
ball.draw(context);
}());
};
</script>
</body>
</html>
