// Ported from the book

​

var w = 16;

var h = 16;

var index = 0;

​

function setup() {

  createCanvas(640, 384);

  background('#0000ff');

  strokeWeight(3);

  stroke(224);

}

​

function draw() {

  var x1 = w*index;

  var x2 = x1 + w;

  var y1 = h*23;

  var y2 = h*24;

  if (random(2) < 1) {

    line(x2, y1, x1, y2);

  } 

  else {

    line(x1, y1, x2, y2);

  }

  index++;

  if (index == width/w) {

    var p = get(0, h, width, h*23);

    background('#0000ff');

    set(0, 0, p);

    index = 0;

  }

}