get("#507") -- averageBrightness

ow, oh = img.width, img.height

g = math.min(ow, oh)/16 -- grid size

-- copy image and invert

pixels = {}
w, h = math.floor(ow/g), math.floor(oh/g)
for y = 0, h-1 do
  for x = 0, w-1 do
    x1 = math.floor(x*g+0.5)
    y1 = math.floor(y*g+0.5)
    x2 = math.floor((x+1)*g+0.5)
    y2 = math.floor((y+1)*g+0.5)
  
    b = averageBrightness(img, x1, y1, x2, y2)
    n = math.max(0, math.min(255, math.floor(b*255+0.5)))
    pixels[y*w+x+1] = n*0x010101
  end
end

Began life as a copy of #506