where 1 and 2 are the azimuth and elevation angles for
the definition of the filtering sphere, α(p, 1, 2) is the
angle α(p, 1, 2) = arccos(p · ν
p ) between the position vector at a point p in the frequency domain and a
unit vector at point on the unit sphere defined as ν =
(sin1cos2,sin1sin2, cos1), and τα is the standard
deviation (τα = 0.5 in this paper) of the Gaussian spreading
function in the angular direction and describes the angular
selectivity of the filter.