function col2hex(num)

  local hexstr = '0123456789abcdef'

  local s = ''

  for i=1, 6 do

    local mod = math.fmod(num, 16)

    s = string.sub(hexstr, mod+1, mod+1) .. s

    num = math.floor(num / 16)

  end

  return s

end

function colortostring(c)

  if c == 0xFFFFFF then return "white" end

  return col2hex(c)

end

w, h = img.width, img.height

ofs = 0

while ofs <= w/2 and ofs <= h/2 do

  colors = {}

  for y=ofs, h-ofs-1 do

    colors[img.getInt(ofs, y)] = 1

    colors[img.getInt(w-ofs-1, y)] = 1

  end

  for x=ofs, w-ofs-1 do

    colors[img.getInt(x, ofs)] = 1

    colors[img.getInt(x, h-ofs-1)] = 1

  end

  list, n = {}, 0

  for color, _ in pairs(colors) do

    n=n+1

    list[n] = color

  end

  if #list > 1 then

    return ofs == 0 and "no border found" or "border thickness: "..ofs

  elseif lastcolor == nil then

    lastcolor = list[1]

  elseif lastcolor ~= list[1] then

    return ofs == 0 and "no border found" or "border thickness: "..ofs

  end

  ofs = ofs+1

end

--return "unicolor "..colortostring(lastcolor)

return "unicolor "..lastcolor