class ProjectileLayer extends Layer {
  ParticleSystem system;
  float MASS = 1.0f;
  float VELOCITY_Y = -10.0f;
  float MUZZLE_VELOCITY = -1.0f;
  int THRESHOLD_Z = -10;
  int PROJECTILE_SIZE = 20;
  float GRAVITY = 1.5f;

  ProjectileLayer(PApplet parent) {
    super(parent, P3D);
  }

  void setup() {
    system = new ParticleSystem(GRAVITY, 0.001f);
    colorMode(HSB);
  }


  void draw() {
    background(0, 0);
    noStroke();
    lights();
    if (mousePressed) {
      fire();
    }
    system.tick();
    int i = 0;
    while(i<system.numberOfParticles()) {
      Particle p = system.getParticle(i++);
      Integer c = (Integer)colors.get(p);
      if ((p.position().z() < THRESHOLD_Z)) {
        splat(p.position().x(), p.position().y(), THRESHOLD_Z, c.intValue());
        colors.remove(p);
        system.removeParticle(p);
      } 
      else {
        pushMatrix();
        translate(p.position().x(), p.position().y(), p.position().z());
        float size = abs(PROJECTILE_SIZE / p.position().z());
        fill(c.intValue());
        sphere(size);
        popMatrix();
      }
    }
  }

  void fire() {
    Particle p = system.makeParticle(MASS, mouseX, mouseY, -1);
    float vx = random(-1.0f, 1.0f);
    float vy = random(-11.0f, -9.0f);
    p.velocity().set(vx, vy, MUZZLE_VELOCITY);
    Integer c = new Integer(color(random(0, 255), 255, 255));
    colors.put(p, c);
  }
}