classroom-instruments/public/roll-call/js/star.js

function getStar() {
  var canvas = document.createElement('canvas'),
    ctx = canvas.getContext('2d'),
    w = (canvas.width = window.innerWidth * 2),
    h = (canvas.height = window.innerHeight * 2),
    hue = 217,
    stars = [],
    count = 0,
    maxStars = 350; //星星数量

  //放背景图

  var canvas2 = document.createElement('canvas'),
    ctx2 = canvas2.getContext('2d');
  canvas2.width = 100;
  canvas2.height = 100;
  var half = canvas2.width / 2,
    gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
  gradient2.addColorStop(0.025, '#CCC');
  gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
  gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
  gradient2.addColorStop(1, 'transparent');

  ctx2.fillStyle = gradient2;
  ctx2.beginPath();
  ctx2.arc(half, half, half, 0, Math.PI * 2);
  ctx2.fill();

  // End cache

  function random(min, max) {
    if (arguments.length < 2) {
      max = min;
      min = 0;
    }

    if (min > max) {
      var hold = max;
      max = min;
      min = hold;
    }

    return Math.floor(Math.random() * (max - min + 1)) + min;
  }

  function maxOrbit(x, y) {
    var max = Math.max(x, y),
      diameter = Math.round(Math.sqrt(max * max + max * max));
    return diameter / 2;
    //星星移动范围，值越大范围越小，
  }

  var Star = function () {
    this.orbitRadius = random(maxOrbit(w, h));
    this.radius = random(60, this.orbitRadius) / 8;
    //星星大小
    this.orbitX = w / 2;
    this.orbitY = h / 2;
    this.timePassed = random(0, maxStars);
    this.speed = random(this.orbitRadius) / 150000;
    //星星移动速度
    this.alpha = random(2, 10) / 10;

    count++;
    stars[count] = this;
  };

  Star.prototype.draw = function () {
    var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
      y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
      twinkle = random(10);

    if (twinkle === 1 && this.alpha > 0) {
      this.alpha -= 0.05;
    } else if (twinkle === 2 && this.alpha < 1) {
      this.alpha += 0.05;
    }

    ctx.globalAlpha = this.alpha;
    ctx.drawImage(
      canvas2,
      x - this.radius / 2,
      y - this.radius / 2,
      this.radius,
      this.radius
    );
    this.timePassed += this.speed;
  };

  for (var i = 0; i < maxStars; i++) {
    new Star();
  }

  function animation() {
    ctx.globalCompositeOperation = 'source-over';
    ctx.globalAlpha = 0.5; //尾巴
    ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 2)';
    ctx.fillRect(0, 0, w, h);

    ctx.globalCompositeOperation = 'lighter';
    for (var i = 1, l = stars.length; i < l; i++) {
      stars[i].draw();
    }

    window.requestAnimationFrame(animation);
  }

  var canvasBoxs = document.createElement('div');
  var images = document.createElement('img');
  images.src = './img/starBg.jpg';
  images.style.opacity = '0.4';
  images.style.width = '100%';
  images.style.height = '1280px';
  images.style.zIndex = '22';
  images.style.position = 'absolute';
  images.style.background = '#000';
  canvasBoxs.appendChild(images);
  canvas.style.zIndex = '23';
  canvasBoxs.appendChild(canvas);
  starBG = new THREE.CSS3DObject(canvasBoxs);
  starBG.position.x = 0;
  starBG.position.y = 0;
  starBG.position.z = -5000;
  var scal = (window.innerWidth + 10000) / window.innerWidth / 2;
  starBG.scale.set(scal, scal, scal);
  scene.add(starBG);
  animation();
}