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 b = bit32.band(i, 255)
    sum = sum + b
  end
end

return "average brightness (B channel): "..(sum/(255*w*h))