本节笔记中引用的js文件具体参见前几节笔记。
速度实例中主要理解速度向量概念。
自此节笔记开始，代码段：
if (!window.requestAnimationFrame) {
window.requestAnimationFrame = (window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || window.oRequestAnimationFrame || window.msRequestAnimationFrame || function(callback){
return window.setTimeout(callback,1000/60);
})
};写入utils.js文件里。
速度实例1：文件名：velocity1.html。
<!DOCTYPE html>
<html>
<head>
<meta charset = "utf-8">
<title>速度实例1</title>
<style>
#canvas{background-color: #99cc33;}
</style>
</head>
<body>
<canvas id="canvas" width="400" height="400"></canvas>
<script type="text/javascript" src="ball.js"></script>
<script type="text/javascript" src="utils.js"></script>
<script type="text/javascript">
window.onload = function(){
var canvas = document.getElementById('canvas'),
context = canvas.getContext("2d"),
ball = new Ball(),
angle = 45,
speed = 1;
ball.x = 50;
ball.y = 100;
(function drawFrame(){
window.requestAnimationFrame(drawFrame,canvas);
context.clearRect(0,0,canvas.width,canvas.height);
var radius = angle*Math.PI/180,
vx = Math.cos(radius)*speed,
vy = Math.sin(radius)*speed;
ball.x += vx;
ball.y += vy;
ball.draw(context);
}());
}
</script>
</body>
</html>效果图：
文件名：velocity2.html。
<!DOCTYPE html>
<html>
<head>
<meta charset = "utf-8">
<title>速度实例2</title>
<style>
#canvas{background-color: #99cc33;}
</style>
</head>
<body>
<canvas id="canvas" width="400" height="400"></canvas>
<script type="text/javascript" src="arrow.js"></script>
<script type="text/javascript" src="utils.js"></script>
<script type="text/javascript">
window.onload = function(){
var canvas = document.getElementById('canvas'),
context = canvas.getContext("2d"),
mouse = utils.captureMouse(canvas),
arrow = new Arrow(),
speed = 3;
(function drawFrame(){
window.requestAnimationFrame(drawFrame,canvas);
context.clearRect(0,0,canvas.width,canvas.height);
var dx = mouse.x - arrow.x,
dy = mouse.y - arrow.y,
angle = Math.atan2(dy,dx),
vx = Math.cos(angle)*speed,
vy = Math.sin(angle)*speed;
arrow.rotation = angle;
arrow.x += vx;
arrow.y += vy;
arrow.draw(context);
}());
}
</script>
</body>
</html>效果图：
文件名：velocity3.html。
<!DOCTYPE html>
<html>
<head>
<meta charset = "utf-8">
<title>速度实例3</title>
<style>
#canvas{background-color: #99cc33;}
</style>
</head>
<body>
<canvas id="canvas" width="400" height="400"></canvas>
<script type="text/javascript" src="arrow.js"></script>
<script type="text/javascript" src="utils.js"></script>
<script type="text/javascript">
window.onload = function(){
var canvas = document.getElementById('canvas'),
context = canvas.getContext("2d"),
arrow = new Arrow(),
vr = 3;
arrow.x = canvas.width/2;
arrow.y = canvas.height/2;
(function drawFrame(){
window.requestAnimationFrame(drawFrame,canvas);
context.clearRect(0,0,canvas.width,canvas.height);
arrow.rotation += vr*Math.PI/180;
arrow.draw(context);
}());
}
</script>
</body>
</html>效果图：
参见《HTML5+Javascript动画基础》。