Why do pixel normalization?

[Chrgo]

A quite simple question, why normalize pixels with (img - 127.5)/128?
I didn't find any useful information. Can anyone help me? Thanks.

[wy1iu]

It is a standard data processing procedure. I think it is used not only in deep neural nets. Imagine a simple case where you just classify two points in 2D space, if you don't do such normalization, they will just lie in the positive domain, which limits the learning space of the decision boundary and incorporates unnecessary bias.

[johnnysclai]

I think @Chrgo is asking why (img - 127.5)/128 is used, instead of (img - 127.5)/127.5.

[wy1iu]

@johnnysclai I don’t think it really matters. As long as your testing and training are consistent, you will be fine.