get("#372") -- getContrast

_, _, min, max = getContrast(img)

-- copy image and invert

w, h = img.width, img.height
pixels = {}
for y = 0, h-1 do
  for x = 0, w-1 do
    b = bright(rgb(img.getInt(x, y)))
    n = math.max(0, math.min(255, math.floor((b-min)/(max-min)*255+0.5)))
    pixels[y*w+x+1] = n*0x010101
  end
end