function blend(a, b, c)
  return b*c+a*(1-c)
end

return function()
  w, h = w or 200, h or 200 -- keep default width/height if supplied
  
  sw = math.random(10, 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
  
  mx = math.floor(x1+sw/2+0.5)
  for y=y1, y1+sh-1 do
    ww = math.floor(blend(1, sw/2, (y-y1)/sh)+0.5)
    for x=mx-ww, mx+ww-1 do
      pixels[y*w+x+1] = 0
    end
  end
end

Began life as a copy of #488