var padding = 20

​

// Size of grid cells (cellSize x cellSize).

var cellSize = 40

​

// Probabiity of drawing an inner rectangle.

var chance = 0.8

​

function setup() {

  createCanvas(400, 400)

  rectMode(CENTER)

  noLoop()

}

​

function draw() {

  background(255)

  for (var y = padding; y < height - 2 * padding; y += cellSize) {

    for (var x = padding; x < width - 2 * padding; x += cellSize) {

      drawCell(x + cellSize/2, y + cellSize/2)

    }

  }

}

​

function drawCell(x, y) {

  push()

  translate(x, y)

  for (var size = 5; size <= cellSize; size += 5) {

    if (random() < chance) {

      noFill()

      rect(0, 0, size, size)  

    }

  }

  pop()

}