I have come across a puzzling issue when using arctan2 in Numpy. Searching for atan2 errors did not answer the question, but someone might know the reason for this.
f = np.arange(0,100)
w = 2*np.pi*f/50
x = np.arctan2(sin(-w*d/2)*cos(w*d/2), cos(w*d/2)*cos(w*d/2))
gives different results to
f = np.arange(0,100)
w = 2*np.pi*f/50
x = np.arctan2(sin(-w*d/2), cos(w*d/2))
The former is out by an offset of $pi$ every period. Looks like a numeric issue but I have not seen any notes on this particular case.
Note that the error seems to be induced by a negative value in the cos() factor. This gets calculated correctly.
H = np.arctan2(sin(-w*d/2)*abs(cos(w*d/2)), cos(w*d/2)*abs(cos(w*d/2)))
atan2( y, x) gives the angle between the x axis and the vector x,y. Therefore atan2( y, x) and atan2( a y, a x) are the same if and only if a>0. For example atan2( y, x) is pi/4 but atan2( -1*y, -1*x) is -3pi/4.
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