I have learned that HOG angular bins have values from 0 to 180 since the algorithm uses unsigned gradients. I am failing to understand why.
As far as I know gradient orientation is calculated as
angle = atan(S_y / S_x)
where S_x and S_y are image gradients in x and y directions respectively.
Provided that S_y and S_x are non-negative, the ratio will also never be negative. Hence, the angle should be in range from 0 to 90 degrees (see atan graph below). What am I missing?