<!DOCTYPE HTML> <html lang="en"> <head> <meta charset="utf8"> <meta content="width=device-width, initial-scale=1.0"> <meta content="starry sky"> <meta content="the starry sky"> <title>旋转的星空(the starry sky)</title> <style> body { margin: 0; padding: 0; overflow: hidden; } #canvas { position: absolute; left: 0; } #starCanvasDiv { background-color: white; } </style> </head> <body> <canvas>Your browser can not support canvas</canvas> <script> var doublePI = Math.PI * 2; var canvas = document.getElementById("canvas"); var ctx = canvas.getContext("2d"); var cx,cy; var starCanvas; var alphaChangeProbability = new AlphaChangeProbability(); //色相 var hue = 217; //星空背景颜色 var bgColor = 'hsl(' + hue + ', 60%, 5%)'; //画布的外切圆半径 var canvasRadius; //每旋转一圈要需要的帧数 var radianStepCount; //星星的总个数 var starCount; //群星 var stars; function onResize() { canvas.width = window.innerWidth; canvas.height = window.innerHeight; cx = canvas.width / 2; cy = canvas.height / 2; canvasRadius = Math.sqrt(Math.pow(canvas.width, 2) + Math.pow(canvas.height, 2)) / 2; radianStepCount = canvasRadius * 100; starCount = parseInt((canvas.width + canvas.height) * 0.5); stars = []; for(var i=0; i<starCount; i++) { stars.push(new Star()); } //初始时要先绘制一层背景,否则第一波星星走过的路径会有画布底料涂抹不均匀的感觉 ctx.globalCompositeOperation = "source-over"; ctx.globalAlpha = 1; ctx.fillStyle = bgColor; ctx.fillRect(0, 0, canvas.width, canvas.height); } function init() { createStarCanvas(); window.addEventListener("resize", onResize); onResize(); loop(); } //在[min, max)范围内随机一个整数 function randomInt(min, max) { if(arguments.length === 1) { max = min; min = 0; } else if(min > max) { var tmp = max; mix = min; min = tmp; } return Math.floor(Math.random() * (max - min) + min); } //透明度改变的概率 function AlphaChangeProbability() { //透明度改变的步长 var alphaStep = 0.05; //透明度增加 var plus = 1; //透明度减少 var minus = 1; //透明度不变 var invariant = 8; //总概率 var totalChance = plus + minus + invariant; //获取随机的透明度改变值 function getRandomChangeValue() { var value = Math.random() * totalChance; if(value < plus) { return alphaStep; } value -= plus; if(value < minus) { return -alphaStep; } return 0; } //随机改变alpha的值 this.randomChangeAlpha = function(alpha) { alpha += getRandomChangeValue(); if(alpha > 1) { alpha = 1; } else if(alpha < 0) { alpha = 0; } return alpha; } } //创建星星图片 function createStarCanvas() { starCanvas = document.createElement("canvas"); var ctx = starCanvas.getContext("2d"); starCanvas.width = 100; starCanvas.height = 100; var cx = starCanvas.width / 2; var cy = starCanvas.height / 2; var radius = Math.max(cx, cy); var gradient = ctx.createRadialGradient(cx, cy, 0, cx, cy, radius); gradient.addColorStop(0.025, '#CCC'); gradient.addColorStop(0.1, 'hsl(' + hue + ', 65%, 35%)'); gradient.addColorStop(0.25, bgColor); gradient.addColorStop(1, "transparent"); ctx.fillStyle = gradient; ctx.beginPath(); ctx.arc(cx, cy, radius, 0, doublePI); ctx.fill(); } //星体对象 var Star = function() { //星体运行的轨道半径 this.orbitRadius = Math.random() * canvasRadius; //星体的半径 this.radius = randomInt(60, this.orbitRadius) / 8; //星体透明度 this.alpha = Math.random(); //相对轨道中心(即画布中心)的角度 this.theta = Math.random() * doublePI; //角速度 this.speed = Math.random() * this.orbitRadius / radianStepCount; //获取当前坐标 this.getPos = function() { return { x: cx + this.orbitRadius * Math.cos(this.theta), y: cy + this.orbitRadius * Math.sin(this.theta) } } } Star.prototype.update = function() { this.alpha = alphaChangeProbability.randomChangeAlpha(this.alpha); this.theta += this.speed; this.pos = this.getPos(); } Star.prototype.draw = function() { ctx.globalAlpha = this.alpha; ctx.drawImage(starCanvas, this.pos.x, this.pos.y, this.radius, this.radius); } function loop() { ctx.globalCompositeOperation = "source-over"; ctx.globalAlpha = 0.5; ctx.fillStyle = bgColor; ctx.fillRect(0, 0, canvas.width, canvas.height); ctx.globalCompositeOperation = "lighter"; for(var i = 0; i < stars.length; i++) { stars[i].update(); stars[i].draw(); } //绘制星体图样 ctx.globalCompositeOperation = "source-over"; ctx.globalAlpha = 1; ctx.fillStyle = "white"; ctx.fillRect(0, 0, starCanvas.width, starCanvas.height); ctx.drawImage(starCanvas, 0, 0, starCanvas.width, starCanvas.height); requestAnimationFrame(loop); } init(); </script> </body> </html>
