[Numpy-discussion] NumPy Feature Request: Function to wrap angles to range [ 0, 2*pi] or [ -pi, pi ]

[Daniele Nicolodi]

On 24/11/2020 02:49, Nathaniel Smith wrote:
> How would this proposed function compare to using the modulo operator,
> like 'arr % (2*pi)'?

I wrote almost the same word bu word reply, before realizing that taking
the modulo looses the sign. The correct operation is slightly more
complex (untested):

def wrap(alpha):
    return (alpha + np.pi) % 2.0 * np.pi - np.pi

However, I don't think there is much value in adding something so
trivial as a function to numpy: I cannot think to any commonly used
algorithm that requires wrapping the phase, and it is going to be an
infinite source of bikesheeding whether the wrapped range should be
[-pi, pi) or (-pi, pi] or (0, 2*pi] or [0, 2*pi)

Cheers,
Dan


> On Mon, Nov 23, 2020, 16:13 Thomas <thomasbbrunner at gmail.com
> <mailto:thomasbbrunner at gmail.com>> wrote:
> 
>     Hi,
> 
>     I have a proposal for a feature and I hope this is the right place
>     to post this.
> 
>     The idea is to have a function to map any input angle to the range
>     of [ 0, 2*pi ] or [ - pi, pi ].
> 
>     There already is a function called 'unwrap' that does the opposite,
>     so I'd suggest calling this function 'wrap'.
> 
>     Example usage:
>     # wrap to range [ 0, 2*pi ]
>     >>> np.wrap([ -2*pi, -pi, 0, 4*pi ])
>     [0, pi, 0, 2*pi]
> 
>     There is some ambiguity regarding what the solution should be for
>     the extremes. An example would be an input of 4*pi, as both 0 and
>     2*pi would be valid mappings.
> 
>     There has been interest for this topic in the community
>     (see https://stackoverflow.com/questions/15927755/opposite-of-numpy-unwrap
>     <https://stackoverflow.com/questions/15927755/opposite-of-numpy-unwrap>).
> 
>     Similar functions exist for Matlab
>     (see https://de.mathworks.com/help/map/ref/wrapto2pi.html
>     <https://de.mathworks.com/help/map/ref/wrapto2pi.html>). They solved
>     the ambiguity by mapping "positive multiples of 2*pi map to 2*pi and
>     negative multiples of 2*pi map to 0." for the 0 to 2*pi case.
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