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 pixeldiff (u, v)

  local r1, g1, b1 = rgb(u)

  local r2, g2, b2 = rgb(v)

  return bright(math.abs(r1-r2), math.abs(g1-g2), math.abs(b1-b2))

end

w, h = img.width, img.height

hw, hh = math.floor(w/2), math.floor(h/2)

sum = 0

for y=0, hh-1 do

  for x=0, hw-1 do

    u = img.getInt(x,y)

    v = img. getInt(x, h-1-y)

    sum = sum+pixeldiff(u, v)

  end

end

sym = 1-sum/(hw*hh)

percent = sym*100

return "Vertical symmetry: "..math.floor(percent+0.5).."%"