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

function map(list, f)
  local tbl2 = {}
  for _, x in ipairs(list) do
    table.insert(tbl2, f(x))
  end
  return tbl2
end

function segment2string(s)
  local x1, x2 = unpack(s)
  return x1.."-"..x2
end

w, h = img.width, img.height
brightnessThreshold = 0.9

function isEmpty(x)
  for y=0, h-1 do
    if bright(rgb(img.getInt(x, y))) <= brightnessThreshold then
      return false
    end
  end
  return true
end

lastEmpty = true
segments = {}

for x=0, w-1 do
  empty = isEmpty(x)
  if empty and not lastEmpty then
    table.insert(segments, {start, x})
  elseif not empty and lastEmpty then
    start = x
  end
  lastEmpty = empty
end

if not lastEmpty then
  table.insert(segments, {start, w})
end

if #segments >= 2 then -- don't bother with only 1 segment
  return "Horizontal segments: "..table.concat(map(segments, segment2string), ", ")
end