w, h = 100, 100

img = {getInt = function (x,y)

  return x > 2 and x < 98 and y > 2 and y < 98 and -1 or

0

end}

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