ArrayList pieces;
PImage maskImage;

int totalPieces = 9;
int counter = 0;
int counterThreshold = 1000;

class Piece{
  PImage img;
  float x, y;
  float xDir, yDir;
  Piece(String path)
  {
    img = loadImage(path);
    x = random(1)*width;
    y = random(1)*height;
    xDir = random(1)*2+0.5;         // change this to a fix number like xDir = 3; will fix the speed

    if(random(1)>0.5)
      yDir = random(1);
    else
      yDir = random(1)*(-1); 
  }
}

void setup(){
 
  size(800,600);
  background(255);
 
  maskImage = loadImage("mask.png");
 
  pieces = new ArrayList();
  for(int i=0; i<totalPieces*100; i++){
    String imgPath = "p"+Integer.toString( i%totalPieces+1 )+".png";
    pieces.add(new Piece(imgPath));
  }
}

void draw(){
 
  background(255);
 
  for(int i=0; i<pieces.size(); i++){
    Piece piece = (Piece)pieces.get(i);
    piece.x = piece.x + piece.xDir;
    image(piece.img, piece.x, piece.y);
  }
  image(maskImage,0,0);
 
  if(counter>counterThreshold){
    for(int j=0; j<pieces.size(); j++){
      Piece piece = (Piece)pieces.get(j);
      piece.x = 50;
    }
    counter = 0;
  }
  counter++;
 
}