From ds config file,
net-scale-factor=0.007843
offsets=127.5;127.5;127.5
For Unet, if input is RGB, then it will convert to BGR. Then, each pixel value will divide 127.5 and then substract 1. After that, convert HWC to CHW.
In above normalization step,
x/127.5 - 1
equals to (x - 127.5) /127.5 , and also
equals to (x - 127.5) * 0.007843 which implies (x - offsets) * net-scale-factor, mentioned in ds config file.