Curious about the constant used in the normalize function

In layers.py, there is a normalize function that has a constant of 127.5:

def normalize(layer):
return layer/127.5 - 1.

I'm a little confused as to where the 127.5 comes from. It's a very specific question of course, but i'm interested in extending the regularization loss function with other types of transforms outside of the identity mapping used in the paper. If you have any tips or pointers in modifying that I'd love to hear. Great work and thanks for doing this!