get("#348") -- rgb functions
get("#388") -- rect functions

w, h = img.width, img.height
g = 3 -- grid size
maxBlackPixels = g-1
minSize = {20, 20} -- exclude the really small white spaces
gw, gh = math.floor(w/g), math.floor(h/g) -- width & height of grid

function makeGrid()
  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
end

-- stack layout: {{x, y}, ...}
local r
function fill(stack)
  while #stack > 0 do
    local x, y = unpack(stack[#stack])
    stack[#stack] = nil
    
    if not (x < 0 or y < 0 or x >= gw or y >= gh) then
      local idx = y*gw+x+1
      
      if grid[idx] then
        grid[idx] = nil
        local me = newRectangle(x, y, 1, 1)
        if r == nil then
          r = me
        else
          r = mergeRectangles(r, me)
        end
        
        stack[#stack+1] = {x-1, y}
        stack[#stack+1] = {x+1, y}
        stack[#stack+1] = {x, y-1}
        stack[#stack+1] = {x, y+1}
      end
    end
  end
end

function makeSpaces()
  spaces = {}
  for y = 0, gh-1 do
    for x = 0, gw-1 do
      r = nil -- must be global
      fill {{x, y, 1}}
      if r and r.width >= minSize[1] and r.height >= minSize[2] then
        --table.insert(result, (x*g).."/"..(y*g).."-"..c)
        r = scaleRectangle(r, g)
        --r = shrinkRectangle(r, img)
        table.insert(spaces, r)
      end
    end
  end
end

Began life as a copy of #444