# Boost :

From: more effective thinking in the exceptional C++ programming language (effective.thinking_at_[hidden])
Date: 2007-06-06 09:03:22

Thank you for the involved discussions. Picking random points on the surface
of a sphere seems to be a never-ending topic of discussion since ages!

Thank to the very interesting post of Jeffrey, I discovered a fourth
technique described in the post of George Marsaglia pasted bellow (technique
2). It can be viewed as a combination of rejection sampling and
trigonometric relations. It appears to be a clear winner in the point of
view of performances. So please consider the following updated document in
attachment as applicable.

At the end of his post, George Marsalia announce using a very fast Gaussian
distributed generator of random numbers. This algorithm could be the key to
improve the performances of both the random_on_sphere and the Gaussian
distributions of boost.

Colas Schretter

From: George Marsaglia <geo_at_[hidden]>
Subject: Random points on a sphere.
Date: Tue, 15 Jun 1999 18:03:07 -0400
Newsgroups: sci.math.num-analysis,sci.math,sci.stat.math

Questions on methods for random sampling from the surface
of a sphere keep coming up, and seemingly get lost as
new generations of computer users encounter the problem.

I described the methods that I had used for years in
"Sampling from the surface of a sphere", Ann Math Stat, v43, 1972,
recommending the two that seemed the fastest:

1) Choose standard normal variates x1,x2,x3,
put R=1/sqrt(x1^2+x2^2+x3^2) and return the point
(x1*R,x2*R,x3*R).

2) Generate u and v, uniform in [-1,1] until S=u^2+v^2 < 1,
then return (1-2*S,u*r,v*r), with r=2*sqrt(1-S).

The second method is fast and easiest to program---unless
one already has a fast normal generator in his library.

Method 1) seems by far the fastest (and easily applies to higher
dimensions) if my latest normal generator is used.
It produces normal variates at the rate of seven million
per second in a 300MHz Pc.

George Marsaglia

On 6/5/07, Henrik Sundberg <storangen_at_[hidden]> wrote:
>
> 2007/6/5, Lewis Hyatt <lhyatt_at_[hidden]>:
> > Henrik Sundberg wrote:
> > > The result ought to be more dense at the poles.
> >
> > The posted trig method is correct, it chooses a horizontal cutting plane
> > uniformly. You can show that the surface area of a sphere contained
> > between two z-coordinates only depends on the difference between the two
> > coordinates, not their location, so this generates the correct
> distribution.
>
> Aah, sorry for the noise!
>
> /\$
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