function round(x) return math.floor(x+0.5) end

w, h = img.width, img.height

local sum = 0
for y=0, h-1 do
  for x=0, w-1 do
    local i = img.getInt(x, y)
    local r = bit32.band(bit32.rshift(i, 16), 255)
    local g = bit32.band(bit32.rshift(i, 8), 255)
    local b = bit32.band(i, 255)
    sum = sum+r+g+b
  end
end

return "Average brightness: "..round(sum/(255*w*h*3)*100).."%"

Began life as a copy of #301