get("#348") -- rgb functions
get("#388") -- rect functions
get("#410") -- shrinkRectangle
get("#417") -- stringRepImage

w, h = img.width, img.height
g = 4 -- grid size
gw, gh = math.floor(w/g), math.floor(h/g) -- widht & height of grid

grid = {}
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 < 0.5 and max > 0.5 then
          grid[(gy/g)*gw+(gx/g)+1] = true
          goto next
        end
      end
    end
    ::next::
  end
end

function fill(x, y, r)
  if x < 0 or y < 0 or x >= gw or y >= gh then return r end
  local idx = y*gw+x+1
  if not grid[idx] then return r end
  grid[idx] = nil
  local me = newRectangle(x, y, 1, 1)
  if r == nil then
    r = me
  else
    r = mergeRectangles(r, me)
  end
  r = fill(x-1, y, r)
  r = fill(x+1, y, r)
  r = fill(x, y-1, r)
  r = fill(x, y+1, r)
  return r
end

result = {}
for y = 0, gh-1 do
  for x = 0, gw-1 do
    local r = fill(x, y, nil)
    if r then
      --table.insert(result, (x*g).."/"..(y*g).."-"..c)
      r = scaleRectangle(r, g)
      r = shrinkRectangle(r, img)
      
      local cropped = {width=r.width, height=r.height,
        getInt = function(x, y) return img.getInt(x+r.x, y+r.y) end}
      local hash = stringRepImage(cropped)
  
      local line = recttostring(r).." -> "..hash
      print(line)
      table.insert(result, line)
    end
  end
end

return "Floodfill string rep:\n"..table.concat(result, "\n")

Began life as a copy of #416