PSystem ps;
Particle a, b, c, d;

void setup() {
  size(400,400);
  ps = (PSystem)loadPlugin("PSystem");

  a = new Particle(width/2 - 50, 0, 0);
  a.fix();
  ps.addParticle(a);

  b = new Particle(width/2 + 50, 0, 0);
  ps.addParticle(b);
  
  c = new Particle(width/2, 70, 0);
  ps.addParticle(c);
  
  d = new Particle(width/2, 70, 0);
  ps.addParticle(d);
  
  ps.addSpring(a, b);
  ps.addSpring(b, c);
  ps.addSpring(c, d);
  ps.addSpring(d, a);
  
  b.fix(0, 30);
  d.fix(1, 30);
  
  ps.drawForces = false;
}

void loop() {

  a.setPos(mouseX, mouseY, 0);
  background(255);
  stroke(100, 100, 100, 100);
  line(30, 0, 30, height);
  line(0, 30, width, 30);
  stroke(200, 100, 100, 150);
  fill(255, 0, 0);
  ps.draw();
}