brightnessThreshold = 0.9

maxSignatureLen = 10

function rgb(i)

  local r = bit32.band(bit32.rshift(i, 16), 255)

    local g = bit32.band(bit32.rshift(i, 8), 255)

    local b = bit32.band(i, 255)

  return r, g, b

end

function bright(r, g, b)

  return (r+g+b)/(255*3)

end

-- from Java class "SegmentCounter"

function countSegments(y, flip)

  local lastPix = 1

  local segments = 0

  for x = 0, (flip and h or w)-1 do

    local pix = bright(rgb(flip and img.getInt(y, x) or img.getInt(x, y)))

    if pix >= brightnessThreshold and lastPix < brightnessThreshold then

      segments = segments+1

    end

    lastPix = pix

  end

  if lastPix < brightnessThreshold then segments = segments+1 end

  return segments

end

function getSegmentSignature(flip)

  local s = ""

  local lastN = 0

  for y = 0, (flip and w or h)-1 do

    local n = math.min(countSegments(y, flip), 9)

    if n ~= lastN then

      if #s >= maxSignatureLen then return nil end

      s = s..n

    end

    lastN = n

  end

  if lastN == 0 and #s > 0 then

    s = s:sub(1, #s-1)

  end

  return s

end