Star.js

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，
}

class Star {
  constructor(p, ctx, canvas2) {
    this.maxStars = 1100; //星星数量,默认1100
    this.orbitRadius = random(maxOrbit(p.w, p.h));

    this.ctx = ctx;
    this.canvas2 = canvas2;

    this.radius = random(60, this.orbitRadius) / 20; //星星大小,值越大星星越小,默认8
  
    this.orbitX = p.w / 2;
    this.orbitY = p.h / 2;
    this.timePassed = random(0, this.maxStars);
    this.speed = random(this.orbitRadius) / 400000; //星星移动速度,值越大越慢,默认5W
    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;
    let 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;
  }
}