找不到单位粒子系统的2D圆

let particles = [];

function setup() {
  createCanvas(400, 400);
  for (let i = 0; i < 100; i++) {
    particles.push(new Particle(width / 2, height / 2));
  }
}

function draw() {
  background(220);
  for (let particle of particles) {
    particle.update();
    particle.display();
  }
}

class Particle {
  constructor(x, y) {
    this.x = x;
    this.y = y;
    this.vx = random(-1, 1);
    this.vy = random(-1, 1);
    this.life = 255;
  }

  update() {
    this.x += this.vx;
    this.y += this.vy;
    this.life -= 2;

    if (this.life <= 0) {
      this.x = width / 2;
      this.y = height / 2;
      this.life = 255;
    }
  }

  display() {
    stroke(0, this.life);
    fill(175, this.life);
    ellipse(this.x, this.y, 10, 10);
  }
}