int n = 30;

Circle[] c = new Circle[n];

void setup() {
  size(320, 200);
  //size(640, 480);
  for(int i = 0; i < n; i++) {
    c[i] = new Circle();
  }
  //ellipseMode(CENTER_DIAMETER);
  frameRate(30);
  smooth();
  noFill();
}


void draw() {
  for(int i = 0; i < n; i++) {
    c[i].update();
  }
  background(255);
  for(int i = 0; i < n; i++) {
    c[i].show();
  }
}


class Circle {
  float x, y, r, maxR;
  color c;
  
  Circle() {
    x = random(width/4, width/2 + width/4);
    y = random(height/4, height/2 + height/4);
    maxR = random(10, 25);
    r = random(maxR);
  }
  
  void update() {
    r += 0.6;
    if(r > maxR) {
      r = 1;
      maxR = random(10, 25);
    //x = random(width/4, width/2 + width/4);
    //y = random(height/4, height/2 + height/4);    }
    x = random(width);
    y = random(height); }
  }
  
  void show() {
    c = color(93, 135, 255, 255 - (r/maxR)*255);
    stroke(c);
    ellipse(x, y, r*2, r*2);
  }
}