int w = 20;
int h = 20;
int index = 10;

void setup(){
  size(500,300);
  background(0);
  strokeWeight(5);
  stroke(255);
  smooth();
}

void draw() {
  int x1 = w* index;
  int x2 = x1+w;
  int y1= h*10;
  int y2 = h*11;
 if (random(2) < 1) {
   line (x2,y1,x1,y2);
 }else{
   line(x1,y1,x2,y2);
 }
 index++;
 if (index== width/w) {
  //PImage(width, height, format, factor)
   PImage p = get( 0, h, width, h*10);
   background (0);
   set(0,0,p);
   index=0;

 }

}