Particle Simulation.java

import java.util.Calendar;

class Node extends PVector {
  PVector velocity = new PVector();
  float minX=5, minY=5, maxX=width-5,maxY=height-5;
  float damping = 0.000001;
  int r,g,b;
  int trailLength = 5;
  
  
  Node (float theX, float theY){
    x = theX;
    y = theY;
    r = round(random(0,255));
    g = round(random(0,255));
    b = round(random(0,255));
  }
  
  void display(){
    for (int i = 0; i < trailLength; i++){
       fill(r,g,b,0 + 30*i);
       ellipse(x + 2*i*velocity.x, y + 2*i*velocity.y, 10, 10);
    }
  }
  
  void update(){
    
    x += velocity.x;
    y += velocity.y;
    
    if (x < minX) {
      x = minX - (x - minX);
      velocity.x = -velocity.x;
    }
    
    if (x > maxX) {
      x = maxX - (x - maxX);
      velocity.x = -velocity.x;
    }
    
    if (y < minY) {
      y = minY - (y - minY);
      velocity.y = -velocity.y;
    }
    
    if (y > maxY) {
      y = maxY - (y - maxY);
      velocity.y = -velocity.y;
    }
    
    //velocity.x *= (1 - damping);
    //velocity.y *= (1 - damping);
  }
}

int nodeCount = 100;
Node[] myNodes = new Node[nodeCount];

// image output
boolean saveOneFrame = false;

void setup() {
  // other stuff here
  noStroke();
  size(600,600);
  background(255,255,255);
  for (int i = 0; i < nodeCount; i++){
    myNodes[i] = new Node(random(width), random(height));
    myNodes[i].velocity.x = random(-3,3);
    myNodes[i].velocity.y = random(-3,3);
    //myNodes[i].damping = 0.01;
  }
}

void draw(){
  background(255,255,255);
  for (int i = 0; i < nodeCount; i++){
   myNodes[i].update();
   myNodes[i].display();
  }
  
  if (saveOneFrame == true) {
    saveFrame("ParticleSimulation"+timestamp()+".png");
    saveOneFrame = false;
  }
}

void keyPressed() {
  if (key=='s' || key=='S') {
    saveOneFrame = true;
  }
}

String timestamp() {
  return String.format("%1$ty%1$tm%1$td_%1$tH%1$tM%1$tS", Calendar.getInstance());
}