mtimes.m

Robot tool box - provides many functions that are useful in robotics including such things as kinem

%MTIMES Multiply two quaternion objects
%
% Invoked by the * operator, handle two cases:
%
% q1*q2	standard quaternion multiplication
% q1*v	rotate vector v by quaternion
% q1*s	multiply vector v by scalar

% Copyright (C) 1999-2008, by Peter I. Corke
%
% This file is part of The Robotics Toolbox for Matlab (RTB).
% 
% RTB is free software: you can redistribute it and/or modify
% it under the terms of the GNU Lesser General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
% 
% RTB is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU Lesser General Public License for more details.
% 
% You should have received a copy of the GNU Leser General Public License
% along with RTB.  If not, see <http://www.gnu.org/licenses/>.

function qp = mtimes(q1, q2)

	if isa(q1, 'quaternion') & isa(q2, 'quaternion')
	%QQMUL	Multiply unit-quaternion by unit-quaternion
	%
	%	QQ = qqmul(Q1, Q2)
	%
	%	Return a product of unit-quaternions.
	%
	%	See also: TR2Q

	%	Copyright (C) 1993 Peter Corke

		% decompose into scalar and vector components
		s1 = q1.s;	v1 = q1.v;
		s2 = q2.s;	v2 = q2.v;

		% form the product
		qp = quaternion([s1*s2-v1*v2' s1*v2+s2*v1+cross(v1,v2)]);

	elseif isa(q1, 'quaternion') & isa(q2, 'double'),

	%QVMUL	Multiply vector by unit-quaternion
	%
	%	VT = qvmul(Q, V)
	%
	%	Rotate the vector V by the unit-quaternion Q.
	%
	%	See also: QQMUL, QINV

	%	Copyright (C) 1993 Peter Corke

	% MOD HISTORY
	%	fixed error in q-v product, added inv(q1) on RHS

		if length(q2) == 3,
			qp = q1 * quaternion([0 q2(:)']) * inv(q1);
			qp = qp.v;
		elseif length(q2) == 1,
			qp = quaternion( double(q1)*q2);
		else
			error('quaternion-vector product: must be a 3-vector or scalar');
		end

	elseif isa(q2, 'quaternion') & isa(q1, 'double'),
		if length(q1) == 3,
			qp = q2 * quaternion([0 q1(:)']) * inv(q2);
			qp = qp.v;
		elseif length(q1) == 1,
			qp = quaternion( double(q2)*q1);
		else
			error('quaternion-vector product: must be a 3-vector or scalar');
		end
	end