int particleNum = 3000;
int halfWidth;
int halfHeight;
Particle[] particles = new Particle[particleNum];

void setup(){
  size(500, 500, P2D);
  for (int i = 0; i<particleNum; i++){
    particles[i] = new Particle();
  }
  halfWidth = width / 2;
  halfHeight = height / 2;
}

void draw(){
  fill(0,0,0,20);
  rect(0,0,width, height);
  loadPixels();
  for (int i = 0; i<particleNum; i++){
    particles[i].run();
  }
  updatePixels();
}

class Particle {
  float x;
  float y;
  float vx;
  float vy;
  
  Particle(){
    x = width / 2;
    y = width / 2;
    vx = random(-2, 2);
    vy = random(-2, 2);
  }
  
  void run(){
    x += vx;
    y += vy;
    float dx = halfWidth - x;
    float dy = halfHeight - y;
    if (sqrt((dx * dx) + (dy * dy)) > 160){
       float ang = atan2(dy, dx) / PI * 180;
       vx = cos(ang);
       vy = sin(ang);
    }
    pixels[(int)x + width * (int)y] = 0xFFFFFFFF;
  }
}