动画原理——摩擦力
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小，在动画程序中，我们可以简单的使每一帧的速度都变小。类似如下代码
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>
3.将摩擦力运用到飞船程序中
<!doctype html> <html> <head> <meta charset="utf-8"> <title>Ship Sim Friction</title> <link rel="stylesheet" href="../include/style.css"> <style> #canvas { background-color: #000000; } </style> </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> <aside>Press left and right arrow keys to rotate ship, up to add thrust.</aside> <script src="../include/utils.js"></script> <script src="./classes/ship.js"></script> <script> window.onload = function () { var canvas = document.getElementById('canvas'), context = canvas.getContext('2d'), ship = new Ship(), vr = 0, vx = 0, vy = 0, thrust = 0, friction = 0.97; ship.x = canvas.width / 2; ship.y = canvas.height / 2; window.addEventListener('keydown', function (event) { switch (event.keyCode) { case 37: //left vr = -3; break; case 39: //right vr = 3; break; case 38: //up thrust = 0.05; ship.showFlame = true; break; } }, false); window.addEventListener('keyup', function () { vr = 0; thrust = 0; ship.showFlame = false; }, false); (function drawFrame () { window.requestAnimationFrame(drawFrame, canvas); context.clearRect(0, 0, canvas.width, canvas.height); ship.rotation += vr * Math.PI / 180; var angle = ship.rotation, ax = Math.cos(angle) * thrust, ay = Math.sin(angle) * thrust, left = 0, right = canvas.width, top = 0, bottom = canvas.height; vx += ax; vy += ay; vx *= friction; vy *= friction; ship.x += vx; ship.y += vy; //screen wrapping if (ship.x - ship.width / 2 > right) { ship.x = left - ship.width / 2; } else if (ship.x + ship.width / 2 < left) { ship.x = right + ship.width / 2; } if (ship.y - ship.height / 2 > bottom) { ship.y = top - ship.height / 2; } else if (ship.y < top - ship.height / 2) { ship.y = bottom + ship.height / 2; } ship.draw(context); }()); }; </script> </body> </html>
