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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?

enter image description here

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