get("#348") -- rgb functions

w, h = img.width, img.height

g = 4 -- grid size

r = {}

for gy = 0, h-g, g do

  for gx = 0, w-g, g do

    local min, max = 1, 0

    for y = gy, gy+g-1 do

      for x = gx, gx+g-1 do

        local b = bright(rgb(img.getInt(x, y)))

        min, max = math.min(min, b), math.max(max, b)

      end

    end

    if min < 0.5 and max > 0.5 then

      table.insert(r, gx..", "..gy..", "..gx+g..", "..gy+g)

    end

  end

end

return "Grid: "..table.concat(r, "|")