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, "|")

Began life as a copy of #408