int num = 200;
float r = 10;
float[] x  = new float[num];
float[] y  = new float[num];
float[] vx = new float[num];
float[] vy = new float[num];
float ay = 0.5;
float ref = 0.95;

void setup() {
  size(300,300);
  ellipseMode(RADIUS);
  background(0);
  frameRate(30);
  
  for( int i = 0 ; i < num ; i++ ) {
    x[i]  = random(r,width-r);
    y[i]  = random(r,height-r);
    vx[i] = random(-5,5);
    vy[i] = random(-5,5);
  }
}

void draw() {
  // fade to black
  noStroke();
  fill(0,30);
  rect(0, 0, width, height);
  
  for( int i = 0 ; i < x.length ; i++ ) {
    stroke(0, 0, 255, 100);
    fill(0, 255, 255, 100);
    ellipse(x[i], y[i], r, r);
  
    x[i]  += vx[i];
    y[i]  += vy[i];
    vy[i] += ay;
  
    if( x[i] < r || width  - r < x[i] ) {
      x[i] = max(r,min(width  - r, x[i]));
      vx[i] = -vx[i];
    }
    if( y[i] < r || height - r < y[i] ) {
      y[i] = max(r,min(height - r, y[i]));
      vy[i] = -ref * vy[i];
    }
  }
}