Let me clarify, if the img has three channels, you should have three number for mean, just for example, img is RGB, mean is [0.5, 0.5, 0.5], the normalize result is R * 0.5, G * 0.5, B * 0.5. If img is gray type that only one channel, so mean should be [0.5], the normalize result is R * 0.5