/// <summary>
/// Uniform vector inside the open unit sphere (i.e. no vector
/// ends on the surface of the sphere).
/// </summary>
public static V3d UniformV3dOpenSphere(this IRandomUniform rnd)
{
double r2;
V3d p;
do
{
p = (rnd.UniformV3dOpen() + c_shift) * 2.0;
r2 = p.LengthSquared;
}while (r2 >= 1.0);
return(p);
}