get("#348") -- rgb functions

w, h = img.width, img.height

g = 3 -- grid size

threshold = 0xA5/255

r, rn = {}, 0

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)

        if min < threshold and max > threshold then

          rn=rn+1 r[rn] = gx..", "..gy..", "..gx+g..", "..gy+g

          goto next

        end

      end

    end

    ::next::

  end

end

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