normalization in image processing

Question

What is the correct mean of normalization in image processing? I googled it but i had different definition. I\'ll try to explain in detail each definition.

Norma

Answer 1

Answer by @Imanol is great, i just want to add some examples:

Normalize the input either pixel wise or dataset wise. Three normalization schemes are often seen:

1. Normalizing the pixel values between 0 and 1:

img /= 255.0

2. Normalizing the pixel values between -1 and 1 (as Tensorflow does):

img /= 127.5
img -= 1.0

3. Normalizing according to the dataset mean & standard deviation (as Torch does):

img /= 255.0
mean = [0.485, 0.456, 0.406] # Here it's ImageNet statistics
std = [0.229, 0.224, 0.225]

for i in range(3): # Considering an ordering NCHW (batch, channel, height, width)
    img[i, :, :] -= mean[i]
    img[i, :, :] /= std[i]