I am currently working on implementation of HoG (histogram of oriented gradients). At first I must calculate the gradients on image in both directions (x and y). I will be use gradient filter mask mx=[-1 0 1] in x direction. For y direction I found two possibilites;
my=-mx' and my=mx' in the literature. Can anybody help me to understand when to use any of them?

Thank you for your answer in advance.