get("#348") -- rgb functions
get("#388") -- rect functions
get("#410") -- shrinkRectangle
get("#380") -- hashImage

w, h = img.width, img.height
g = 3 -- grid size
maxBlackPixels = g-1
gw, gh = math.floor(w/g), math.floor(h/g) -- width & height of grid

grid = {}
for gy = 0, h-g, g do
  for gx = 0, w-g, g do
    local black = 0
    for y = gy, gy+g-1 do
      for x = gx, gx+g-1 do
        local b = bright(rgb(img.getInt(x, y)))
        if b < 0.94 then
          black = black+1
          if black > maxBlackPixels then
            goto next
          end
        end
      end
    end
    grid[(gy/g)*gw+(gx/g)+1] = true
    ::next::
  end
end

result = {}
for y = 0, gh-1 do
  for x = 0, gw-1 do
    if grid[y*gw+x+1] then
      r = newRectangle(x, y, 1, 1) 
      r = scaleRectangle(r, g)
      table.insert(result, recttostring(r))
    end
  end
end

return "White Grid: "..table.concat(result, "|")

Began life as a copy of #444