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使用JavaScript和Canvas实现下雪动画效果

2015-07-14 15:46 876 查看

该下雪动画效果使用了HTML5中Canvas画布实现，其中涉及了物理学中曲线运动的相关知识与运算。

index.html

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta content="width=device-width,initial-scale=1.0,maximum-scale=1.0,user-scalable=no" name="viewport">
<meta content="yes" name="apple-mobile-web-app-capable">
<meta content="black" name="apple-mobile-web-app-status-bar-style">
<meta content="telephone=no" name="format-detection">
<meta content="email=no" name="format-detection">
<title>Snow</title>
<link rel="stylesheet" href="css/main.css">
</head>
<body>
<canvas id="canvas"></canvas>
<script src="js/snow.js"></script>
<script>
window.addEventListener('load', function(){
this.snow = new Snow();
// 初始化snow对象并开始下雪动画
snow.init().start();
});
</script>
</body>
</html>

main.css

html, body{
width: 100%;
height: 100%;
overflow: hidden;
margin: 0;
padding: 0;
background-color: #000;
font-family: 微软雅黑, 华文细黑, 黑体;
}

Snow.js

(function(exports, undefined){
'use strict';
var document = exports.document;
function Snow(){
this.colors = ['#fff'];
this.balls = [];
this.windDirection = -1;
this.ballRadius = 3;
this.ballsPerFrame = 2;
this.timeInterval = 40;
this.windDirectionChangedInterval = 5000;
this.accumulativeTime = 0;
return this;
};
exports.Snow = Snow;
Snow.prototype = {
init: function(args){
for(var p in args){
this[p] = args[p];
}
this.canvas = this.canvas || document.querySelector('#canvas');
this.context = this.context || this.canvas.getContext('2d');
this.canvasWidth = this.canvasWidth || document.body.offsetWidth || document.body.clientWidth;
this.canvasHeight = this.canvasHeight || document.body.offsetHeight || document.body.clientHeight;
this.canvas.width = this.canvasWidth;
this.canvas.height = this.canvasHeight;
return this;
},
start: function(){
this.timer = this.timer || setTimeout(this.frame.bind(this), this.timeInterval);
return this;
},
frame: function(){
this.accumulativeTime += this.timeInterval;
(this.accumulativeTime % this.windDirectionChangedInterval < this.timeInterval) && (this.windDirection *= -1);
this.render.call(this);
this.update.call(this);
this.timer = null;
this.timer = setTimeout(this.frame.bind(this), this.timeInterval);
},
update: function(){
this.addBalls.call(this);
this.updateBalls.call(this);
},
updateBalls: function(){
var balls = this.balls,
len = balls.length,
i = 0,
cnt = 0;
for(;i<len;i++){
balls[i].x += balls[i].vx * this.windDirection;
balls[i].y += balls[i].vy;
balls[i].vy += balls[i].g * balls[i].t;
balls[i].t += this.timeInterval;
if(balls[i].y - this.ballRadius < this.canvasHeight){
balls[cnt++] = balls[i];
}
}
while(len>cnt){
balls.pop();
len--;
}
},
addBalls: function(){
var ball,
i = 0,
len = this.ballsPerFrame,
_this = this;
for(;i<len;i++){
ball = {
x: Math.pow(-1, Math.ceil(Math.random() * 1000)) * Math.floor(Math.random() * _this.canvasWidth * 1.5),
y: Math.floor(Math.random() * this.ballRadius) * -1,
g: 0.00005,
vx: 1 + Math.floor(Math.random() * 2),
vy: 2 + Math.floor(Math.random() * 5),
t: 0,
color: _this.colors[Math.floor(Math.random() * _this.colors.length)]
}
this.balls.push(ball);
}
},
render: function(){
var cxt = this.context,
i = 0,
len = this.balls.length;
cxt.clearRect(0, 0, this.canvasWidth, this.canvasHeight);
for(;i<len;i++){
cxt.fillStyle = this.balls[i].color;
cxt.beginPath();
cxt.arc(this.balls[i].x, this.balls[i].y, this.ballRadius, 0, 2 * Math.PI, true);
cxt.closePath();
cxt.fill();
}
},
pause: function(){
clearTimeout(this.timer);
this.timer = null;
},
resume: function(){
this.start.call(this);
},
clear: function(){
clearTimeout(this.timer);
this.timer = null;
this.context.clearRect(0, 0, this.canvasWidth, this.canvasHeight);
}
}
})(window);

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