星空旋转特效

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;
}

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

export default class Star {
  constructor(ctx, canvas2, w, h, maxStars) {
    this.ctx = ctx;
    this.canvas2 = canvas2;

    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);
    // 转动速度
    this.speed = random(this.orbitRadius) / 500000;
    // 透明度-闪烁效果
    this.alpha = random(2, 10) / 10;
  }

  draw() {
    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;
    }

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

import 'jquery';
import Star from './star';

const hue = 217;
const stars = [];
const maxStars = 1300; // 星星数量

export default class Stars {
  constructor(canvas) {
    this.ctx = canvas.getContext('2d');
    this.w = canvas.width = canvas.clientWidth;
    this.h = canvas.height = canvas.clientHeight;

    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, '#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();

    for (let i = 0; i < maxStars; i++) {
      stars[i] = new Star(this.ctx, canvas2, this.w, this.h, maxStars);
    }
  }

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

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

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

import Stars from 'utils/star/index.js'

const canvas = document.getElementById('canvas');
const stars = new Stars(canvas);
stars.animation();

#canvas {
    position: absolute;
    left: 0;
    top: 0;
    z-index: -1;
    display: inline-block;
    width: 100%;
    height: 100%;
  }