c - VLFeat: vlmodulo_2pi_f function -

i trying understand inner workings of vlfeat sift algorithm , notice statement when computing image gradient:

 *grad++ = vl_mod_2pi_f   (vl_fast_atan2_f (gy, gx) + 2*vl_pi); 

i wondering if expression not same vl_fast_atan2_f (gy, gx) adding 2 pi , modulo of expression on rhs should evaluate atan2(gy, gx)?

vl_fast_atan2_f approximated (and faster) version of atan2 (see this doc more details).

still returns results in [-pi, pi] adding 2.pi , take modulus (vl_mod_2pi_f) rescales result [0, 2.pi] how gradient angle represented.