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书籍名称：HTML5-Animation-with-JavaScript
书籍源码：https://github.com/lamberta/html5-animation1
1.有摩擦力的情况
摩擦力在物体发生相对运动时产生，是运动方向的反向加速度。
代码表示意思如下
if (speed > friction) { speed -= friction; } else { speed = 0; }
06-friction-1.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Friction 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 = Math.random() * 10 - 5,
vy = Math.random() * 10 - 5,
friction = 0.1;
ball.x = canvas.width / 2;
ball.y = canvas.height / 2;
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
var speed = Math.sqrt(vx * vx + vy * vy),
angle = Math.atan2(vy, vx);
if (speed > friction) {
speed -= friction;
} else {
speed = 0;
}
vx = Math.cos(angle) * speed;
vy = Math.sin(angle) * speed;
ball.x += vx;
ball.y += vy;
ball.draw(context);
}());
};
</script>
</body>
</html>
2.简单的方法
摩擦力实际就是速度逐渐变相，在动画程序中，我们可以简单的使每一帧的速度都变小。类似。
vx *= 0.9;
vy *= 0.9;
06-friction-2.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Friction 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(),
vx = Math.random() * 10 - 5,
vy = Math.random() * 10 - 5,
friction = 0.95;
ball.x = canvas.width / 2;
ball.y = canvas.height / 2;
(function drawFrame () {
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
vx *= friction;
vy *= friction;
ball.x += vx;
ball.y += vy;
ball.draw(context);
}());
};
</script>
</body>
</html>
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原文地址：http://www.cnblogs.com/winderby/p/4255448.html
