function r = h3d_get_one_perpendicular(v)
	% r = h3d_get_one_perpendicular(v)
	% r is such that r'v = 0


	% choose any vector perpendicular to axis
	if abs(v' * [1;0;0]) < 0.9 
		r = normalize(cross(v,[1;0;0]));
	else
	    r = normalize(cross(v,[0;1;1]));
	end
	
	if abs(r'*v) > 1e-8 
		error
	end