js实现旋转的星空效果

时间:2021-04-02 人气:0
<!DOCTYPE HTML>
<html lang="en">
 <head>
 <meta charset="utf8">
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <meta name="keywords" content="starry sky">
 <meta name="description" 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 id="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>
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