C+
C NAME:
C	quaternion_multiply
C PURPOSE:
C	Multiply two quaternions
C CATEGORY:
C	ucsd/gen/for/lib
C CALLING SEQUENCE:
	subroutine quaternion_multiply(q1,q2,q)
C INPUTS:
C	q1(4)	double precision	quaternion on the left of the multiplication
C	q2(4)	double precision	quaternion on the right of the multiplication
C OUTPUTS:
C	q(4)	double precision	final quaternion
C MODIFICATION HISTORY:
C	MAR-2003, Aaron Smith (UCSD/CASS)
C	NOV-2004, Paul Hick (UCSD/CASS; pphick@ucsd.edu)
C		Changed order of quaternion components from
C		q0,qx,qy,qz to qx,qy,qz,q0
C-
	    double precision	q1(4)
	    double precision	q2(4)
	    double precision	q (4)

	double precision	qq(4)

	qq(1) = q1(1)*q2(4) + q1(4)*q2(1) - q1(3)*q2(2) + q1(2)*q2(3)
	qq(2) = q1(2)*q2(4) + q1(3)*q2(1) + q1(4)*q2(2) - q1(1)*q2(3)
	qq(3) = q1(3)*q2(4) - q1(2)*q2(1) + q1(1)*q2(2) + q1(4)*q2(3)
	qq(4) = q1(4)*q2(4) - q1(1)*q2(1) - q1(2)*q2(2) - q1(3)*q2(3)

	q(1) = qq(1)
	q(2) = qq(2)
	q(3) = qq(3)
	q(4) = qq(4)

	return
	end