return function()
  w, h = w or 200, h or 200 -- keep default width/height if supplied
  
  sw = math.random(5, w-15)
  sh = math.random(5, h-15)
  x1 = math.random(5, w-sw-5)
  y1 = math.random(5, h-sh-5)

  pixels = {}
  for i=1, w*h do pixels[i] = -1 end
  
  for y=y1, y1+sh-1 do
    for x=x1, x1+sw-1 do
      pixels[y*w+x+1] = 0
    end
  end
end