最近给公司网站banner做了流星特效，踩了一些坑，在这里mark下。
(function () {
var context;//画布上下文
var boundaryHeight;//画布高，边界值
var boundaryWidth;//画布宽，边界值
var starArr = [];
var meteorArr = [];
var STAR_COUNT = 500;//星星数，常量
var METEOR_COUNT = 4;//流星数，常量
var METEOR_SEPARATE = 300; //流星之间间隔，常量
var meteorCoordinateArr = [];//存所有流星的坐标数组
var playMeteorTimeout;
var playStarsTimeout;
//初始化画布及context
function init(container) {
starArr = [];
meteorArr = [];
var canvas = document.createElement("canvas");
container.appendChild(canvas);
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
boundaryHeight = canvas.height;
boundaryWidth = canvas.width;
//获取context
context = canvas.getContext("2d");
context.fillStyle = "black";
//画星星
for (var i = 0; i < STAR_COUNT; i++) {
var star = new Star();
star.init();
star.draw();
starArr.push(star);
}
//画流星
for (var j = 0; j < METEOR_COUNT; j++) {
var rain = new MeteorRain();
rain.init(j);
rain.draw();
meteorArr.push(rain);
}
playStars();//星星动起来
playMeteor();//流星动起来
}
//创建一个星星对象
var Star = function () {
this.x = boundaryWidth * Math.random();//横坐标
this.y = boundaryHeight * Math.random();//纵坐标
this.color = "";//星星颜色
};
Star.prototype = {
constructor: Star,
//初始化
init: function () {
this.getColor();
},
//产生随机颜色
getColor: function () {
var _randomNum = Math.random();
if (_randomNum < 0.5) {
this.color = "gray";
}
else {
this.color = "white";
}
},
//绘制
draw: function () {
context.beginPath();
//画圆点
context.arc(this.x, this.y, 0.05, 0, 2 * Math.PI);
context.strokeStyle = this.color;
context.stroke();
context.closePath();
}
}
//星星闪起来
function playStars() {
for (var n = 0; n < STAR_COUNT; n++) {
starArr[n].getColor();
starArr[n].draw();
}
clearTimeout(playStarsTimeout);
playStarsTimeout = setTimeout(playStars, 200);
}
//创建一个流星对象
var MeteorRain = function () {
this.x = -1;//流星的横坐标
this.y = -1;//流星的纵坐标
this.length = -1;//流星的长度
this.angle = 30; //倾斜角度
this.width = -1;//流星所占宽度，及矩形的宽度
this.height = -1;//流星所占高度，及矩形的高度
this.speed = 1;//速度
this.offset_x = -1;//横轴移动偏移量
this.offset_y = -1;//纵轴移动偏移量
this.alpha = 1; //透明度
};
MeteorRain.prototype = {
constructor: MeteorRain,
//初始化
init: function (i) {
this.alpha = 1;//透明度
this.angle = 30; //流星倾斜角
this.speed = Math.ceil(Math.random() + 0.5); //流星的速度
var x = Math.random() * 80 + 180;
var cos = Math.cos(this.angle * 3.14 / 180);
var sin = Math.sin(this.angle * 3.14 / 180) ;
this.length = Math.ceil(x);//流星长度
this.width = this.length * cos; //流星所占宽度，及矩形的宽度
this.height = this.length * sin; //流星所占高度，及矩形的高度
this.offset_x = this.speed * cos * 3.5;
this.offset_y = this.speed * sin * 3.5;
this.getPos(i);
},
//计算流星坐标
countPos: function () {
//往左下移动,x减少，y增加
this.x = this.x - this.offset_x;
this.y = this.y + this.offset_y;
},
//获取随机坐标
getPos: function (i) {
_this = this;
function getCoordinate() {
_this.x = Math.random() * boundaryWidth; //x坐标
for (var k = 0; k < meteorCoordinateArr.length; k++) {
if (Math.abs(_this.x - meteorCoordinateArr[k]) < METEOR_SEPARATE) { //这里如果流星之间距离过小，会把其他流星隔断，严重影响效果。
return getCoordinate();
}
}
meteorCoordinateArr[i] = _this.x;
}
getCoordinate();
this.y = 0.2 * boundaryHeight; //y坐标
},
//画流星
draw: function () {
context.save();
context.beginPath();
context.lineWidth = 2.5; //宽度
context.globalAlpha = this.alpha; //设置透明度
//创建横向渐变颜色,起点坐标至终点坐标
var line = context.createLinearGradient(this.x, this.y, this.x + this.width, this.y - this.height);
//分段设置颜色
line.addColorStop(0, "rgba(255, 255, 255, 1)");
line.addColorStop(1, "rgba(255, 255,255 , 0)");
if (this.alpha < 0 ) {
this.alpha = -this.alpha;
}
//填充
context.strokeStyle = line;
//起点
context.moveTo(this.x, this.y);
//终点
context.lineTo(this.x + this.width, this.y - this.height);
context.closePath();
context.stroke();
context.restore();
},
move: function () {
var x = this.x + this.width - this.offset_x;
var y = this.y - this.height;
this.alpha -= 0.002;
//重新计算位置，往左下移动
this.countPos();
if (this.alpha <= 0) {
this.alpha = 0;
}
else if(this.alpha > 1) {
this.alpha = 1;
}
//画一个矩形去清空流星
context.clearRect(this.x - this.offset_x, y, this.width + this.offset_x, this.height);
//重绘
this.draw();
}
}
//流星动起来
function playMeteor() {
for (var n = 0; n < METEOR_COUNT; n++) {
var rain = meteorArr[n];
rain.move();//移动
if (rain.y > boundaryHeight + 100) {//超出界限后重来
context.clearRect(rain.x, rain.y - rain.height, rain.width, rain.height);
meteorCoordinate[n] = 0;//清空坐标数组具体流星的坐标
meteorArr[n] = new MeteorRain(n);
meteorArr[n].init(n);
}
}
clearTimeout(playMeteorTimeout);
playMeteorTimeout = setTimeout(playMeteor, 5);
}
//初始化
init(document.getElementsByTagName("body")[0]);
}()); 