canvas制作星空旋转效果

 canvas制作星空旋转效果

/*
* @params:config:{
  id: String, canvas自己的id
  class: String, canvas外框的class，用来获取宽高
  color: '#306468', // rgb(34, 103, 96) 星星颜色Hsl值
  hue: 1,星星颜色Hsl
  backgroundColor: '#101524', 背景色
  maxStars: 1000, 星星数量
  time: 30000， 转动时长
}
* */
export function stars(config) {
  const canvas = document.getElementById(config.id);
  const box = document.getElementsByClassName(config.class);
  const ctx = canvas.getContext('2d');
  const w = canvas.width = box[0] ? box[0].offsetWidth : 300;
  const h = canvas.height = box[0] ? box[0].offsetHeight : 300;

  const hue = config.hue;
  const stars = [];
  let count = 0;
  const maxStars = config.maxStars;
  const canvas2 = document.createElement('canvas');
  const ctx2 = canvas2.getContext('2d');
  canvas2.width = 100;
  canvas2.height = 100;
  const half = canvas2.width / 2;
  const gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
  gradient2.addColorStop(0.025, config.color);
  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();

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

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

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

  function maxOrbit(x, y) {
    const max = Math.max(x, y);
    const diameter = Math.round(Math.sqrt(max * max + max * max));
    return diameter / 2;
  }

  const Star = function() {
    this.orbitRadius = random(maxOrbit(w, h));
    this.radius = random(60, this.orbitRadius) / 12;
    this.orbitX = w / 2;
    this.orbitY = h / 2;
    this.timePassed = random(0, maxStars);
    this.speed = random(this.orbitRadius) / config.time;
    this.alpha = random(2, 10) / 10;

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

  Star.prototype.draw = function() {
    const x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX;
    const y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY;
    const 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 (let i = 0; i < maxStars; i++) {
    new Star();
  }

  function animation() {
    ctx.globalCompositeOperation = 'source-over';
    ctx.globalAlpha = 0.8;
    ctx.fillStyle = config.backgroundColor;
    ctx.fillRect(0, 0, w, h);

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

    window.requestAnimationFrame(animation);
  }
  animation();
}