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Index Page qxq_c
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Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       qxq_c ( Quaternion times quaternion )

       void qxq_c ( ConstSpiceDouble    q1   [4],
                    ConstSpiceDouble    q2   [4],
                    SpiceDouble         qout [4]  )
#### Abstract
       Multiply two quaternions.
#### Required_Reading
       ROTATION
#### Keywords
       MATH
       POINTING
       ROTATION
#### Brief_I/O
       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       q1         I   First SPICE quaternion factor.
       q2         I   Second SPICE quaternion factor.
       qout       O   Product of `q1' and `q2'.
#### Detailed_Input
       q1          is a 4-vector representing a SPICE-style quaternion.
                   See the discussion of "Quaternion Styles" in the
                   -Particulars section below.

                   Note that multiple styles of quaternions are in use.
                   This routine will not work properly if the input
                   quaternions do not conform to the SPICE convention.

       q2          is a second SPICE-style quaternion.
#### Detailed_Output
       qout        is 4-vector representing the quaternion product

                      q1 * q2

                   Representing q(i) as the sums of scalar (real)
                   part s(i) and vector (imaginary) part v(i)
                   respectively,

                      q1 = s1 + v1
                      q2 = s2 + v2

                   qout has scalar part s3 defined by

                      s3 = s1 * s2 - <v1, v2>

                   and vector part v3 defined by

                      v3 = s1 * v2  +  s2 * v1  +  v1 x v2

                   where the notation < , > denotes the inner
                   product operator and x indicates the cross
                   product operator.
#### Parameters
       None.
#### Exceptions
       Error free.
#### Files
       None.
#### Particulars
       Quaternion Styles
       -----------------

       There are different "styles" of quaternions used in
       science and engineering applications. Quaternion styles
       are characterized by

       -  The order of quaternion elements

       -  The quaternion multiplication formula

       -  The convention for associating quaternions
          with rotation matrices

       Two of the commonly used styles are

          - "SPICE"

             > Invented by Sir William Rowan Hamilton
             > Frequently used in mathematics and physics textbooks

          - "Engineering"

             > Widely used in aerospace engineering applications

       CSPICE function interfaces ALWAYS use SPICE quaternions.
       Quaternions of any other style must be converted to SPICE
       quaternions before they are passed to CSPICE functions.

       Relationship between SPICE and Engineering Quaternions
       ------------------------------------------------------

       Let `m' be a rotation matrix such that for any vector `v',

          m*v

       is the result of rotating `v' by theta radians in the
       counterclockwise direction about unit rotation axis vector `a'.
       Then the SPICE quaternions representing `m' are

          (+/-) (  cos(theta/2),
                   sin(theta/2) * a(0),
                   sin(theta/2) * a(1),
                   sin(theta/2) * a(2)  )

       while the engineering quaternions representing `m' are

          (+/-) ( -sin(theta/2) * a(0),
                  -sin(theta/2) * a(1),
                  -sin(theta/2) * a(2),
                   cos(theta/2)         )

       For both styles of quaternions, if a quaternion `q' represents
       a rotation matrix `m', then -q represents `m' as well.

       Given an engineering quaternion

          qeng   = ( q0,  q1,  q2,  q3 )

       the equivalent SPICE quaternion is

          qspice = ( q3, -q0, -q1, -q2 )

       Associating SPICE Quaternions with Rotation Matrices
       ----------------------------------------------------

       Let `from' and `to' be two right-handed reference frames, for
       example, an inertial frame and a spacecraft-fixed frame. Let the
       symbols

          v    ,   v
           from     to

       denote, respectively, an arbitrary vector expressed relative to
       the `from' and `to' frames. Let `m' denote the transformation matrix
       that transforms vectors from frame `from' to frame `to'; then

          v   =  m * v
           to         from

       where the expression on the right hand side represents left
       multiplication of the vector by the matrix.

       Then if the unit-length SPICE quaternion `q' represents `m', where

          q = (q0, q1, q2, q3)

       the elements of `m' are derived from the elements of `q' as follows:

            .-                                                           -.
            |            2    2                                           |
            |  1 - 2*( q2 + q3 )   2*(q1*q2 - q0*q3)   2*(q1*q3 + q0*q2)  |
            |                                                             |
            |                                                             |
            |                                2    2                       |
        m = |  2*(q1*q2 + q0*q3)   1 - 2*( q1 + q3 )   2*(q2*q3 - q0*q1)  |
            |                                                             |
            |                                                             |
            |                                                    2    2   |
            |  2*(q1*q3 - q0*q2)   2*(q2*q3 + q0*q1)   1 - 2*( q1 + q2 )  |
            |                                                             |
            `-                                                           -'

       Note that substituting the elements of -q for those of `q' in the
       right hand side leaves each element of `m' unchanged; this shows
       that if a quaternion `q' represents a matrix `m', then so does the
       quaternion -q.

       To map the rotation matrix `m' to a unit quaternion, we start by
       decomposing the rotation matrix as a sum of symmetric
       and skew-symmetric parts:

                                            2
          m = [ I  +  (1-cos(theta)) * omega  ] + [ sin(theta) * omega ]

                           symmetric                 skew-symmetric

       `omega' is a skew-symmetric matrix of the form

                     .-               -.
                     |   0   -n2   n1  |
                     |                 |
           omega  =  |   n2   0   -n0  |
                     |                 |
                     |  -n1   n0   0   |
                     `-               -'

       The vector `n' of matrix entries (n0, n1, n2) is the rotation axis
       of `m' and `theta' is m's rotation angle. Note that `n' and `theta'
       are not unique.

       Let

          cth = cos(theta/2)
          sth = sin(theta/2)

       Then the unit quaternions `q' corresponding to `m' are

          q = +/- ( cth, sth*n0, sth*n1, sth*n2 )

       The mappings between quaternions and the corresponding rotations
       are carried out by the CSPICE routines

          q2m_c {quaternion to matrix}
          m2q_c {matrix to quaternion}

       m2q_c always returns a quaternion with scalar part greater than
       or equal to zero.

       SPICE Quaternion Multiplication Formula
       ---------------------------------------

       Given a SPICE quaternion

          q = ( q0, q1, q2, q3 )

       corresponding to rotation axis `a' and angle `theta' as above, we can
       represent `q' using "scalar + vector" notation as follows:

          s =   q0           = cos(theta/2)

          v = ( q1, q2, q3 ) = sin(theta/2) * a

          q = s + v

       Let `quat1' and `quat2' be SPICE quaternions with respective scalar
       and vector parts `s1', `s2' and `v1', `v2':

          quat1 = s1 + v1
          quat2 = s2 + v2

       We represent the dot product of `v1' and `v2' by

          <v1, v2>

       and the cross product of `v1' and `v2' by

          v1 x v2

       Then the SPICE quaternion product is

          quat1*quat2 = s1*s2 - <v1,v2>  + s1*v2 + s2*v1 + (v1 x v2)

       If `quat1' and `quat2' represent the rotation matrices `m1' and `m2'
       respectively, then the quaternion product

          quat1*quat1

       represents the matrix product

          m1*m2
#### Examples
       The numerical results shown for these examples may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) Given the "basis" quaternions:

             qid:  ( 1.0, 0.0, 0.0, 0.0 )
             qi :  ( 0.0, 1.0, 0.0, 0.0 )
             qj :  ( 0.0, 0.0, 1.0, 0.0 )
             qk :  ( 0.0, 0.0, 0.0, 1.0 )

          the following quaternion products give these results:

              Product       Expected result
             -----------   ----------------------
              qi  * qj     ( 0.0, 0.0, 0.0, 1.0 )
              qj  * qk     ( 0.0, 1.0, 0.0, 0.0 )
              qk  * qi     ( 0.0, 0.0, 1.0, 0.0 )
              qi  * qi     (-1.0, 0.0, 0.0, 0.0 )
              qj  * qj     (-1.0, 0.0, 0.0, 0.0 )
              qk  * qk     (-1.0, 0.0, 0.0, 0.0 )
              qid * qi     ( 0.0, 1.0, 0.0, 0.0 )
              qi  * qid    ( 0.0, 1.0, 0.0, 0.0 )
              qid * qj     ( 0.0, 0.0, 1.0, 0.0 )

          The following code example uses QXQ to produce these results.

          Example code begins here.

          /.
             Program qxq_ex1
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main( )
          {

             /.
             Local variables
             ./
             SpiceDouble          qout   [4];

             /.
             Let `qid', `qi', `qj', `qk' be the "basis"
             quaternions.
             ./
             SpiceDouble          qid    [4] = { 1.0,  0.0,  0.0,  0.0 };
             SpiceDouble          qi     [4] = { 0.0,  1.0,  0.0,  0.0 };
             SpiceDouble          qj     [4] = { 0.0,  0.0,  1.0,  0.0 };
             SpiceDouble          qk     [4] = { 0.0,  0.0,  0.0,  1.0 };

             /.
             Compute:

                qi x qj = qk
                qj x qk = qi
                qk x qi = qj
             ./
             qxq_c ( qi, qj, qout );
             printf( "qi x qj  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "     qk  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qk[0],   qk[1],   qk[2],   qk[3]   );
             printf( " \n" );

             qxq_c ( qj, qk, qout );
             printf( "qj x qk  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "     qi  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qi[0],   qi[1],   qi[2],   qi[3]   );
             printf( " \n" );

             qxq_c ( qk, qi, qout );
             printf( "qk x qi  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "     qj  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qj[0],   qj[1],   qj[2],   qj[3]   );
             printf( " \n" );

             /.
             Compute:

                qi x qi  ==  -qid
                qj x qj  ==  -qid
                qk x qk  ==  -qid
             ./
             qxq_c ( qi, qi, qout );
             printf( "qi x qi  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "     qid = %7.1f %7.1f %7.1f %7.1f\n",
                                 qid[0],   qid[1],  qid[2],  qid[3] );
             printf( " \n" );

             qxq_c ( qj, qj, qout );
             printf( "qj x qj  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "     qid = %7.1f %7.1f %7.1f %7.1f\n",
                                 qid[0],   qid[1],  qid[2],  qid[3] );
             printf( " \n" );

             qxq_c ( qk, qk, qout );
             printf( "qk x qk  = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "     qid = %7.1f %7.1f %7.1f %7.1f\n",
                                 qid[0],   qid[1],  qid[2],  qid[3] );
             printf( " \n" );

             /.
             Compute:

                qid x qi  = qi
                qi  x qid = qi
                qid x qj  = qj
             ./
             qxq_c ( qid, qi, qout );
             printf( "qid x qi = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "      qi = %7.1f %7.1f %7.1f %7.1f\n",
                                 qi[0],   qi[1],   qi[2],   qi[3]   );
             printf( " \n" );

             qxq_c ( qi, qid, qout );
             printf( "qi x qid = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "      qi = %7.1f %7.1f %7.1f %7.1f\n",
                                 qi[0],   qi[1],   qi[2],   qi[3]   );
             printf( " \n" );

             qxq_c ( qid, qj, qout );
             printf( "qid x qj = %7.1f %7.1f %7.1f %7.1f\n",
                                 qout[0], qout[1], qout[2], qout[3] );
             printf( "      qj = %7.1f %7.1f %7.1f %7.1f\n",
                                 qj[0],   qj[1],   qj[2],   qj[3]   );
             printf( " \n" );

             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

          qi x qj  =     0.0     0.0     0.0     1.0
               qk  =     0.0     0.0     0.0     1.0

          qj x qk  =     0.0     1.0     0.0     0.0
               qi  =     0.0     1.0     0.0     0.0

          qk x qi  =     0.0     0.0     1.0     0.0
               qj  =     0.0     0.0     1.0     0.0

          qi x qi  =    -1.0     0.0     0.0     0.0
               qid =     1.0     0.0     0.0     0.0

          qj x qj  =    -1.0     0.0     0.0     0.0
               qid =     1.0     0.0     0.0     0.0

          qk x qk  =    -1.0     0.0     0.0     0.0
               qid =     1.0     0.0     0.0     0.0

          qid x qi =     0.0     1.0     0.0     0.0
                qi =     0.0     1.0     0.0     0.0

          qi x qid =     0.0     1.0     0.0     0.0
                qi =     0.0     1.0     0.0     0.0

          qid x qj =     0.0     0.0     1.0     0.0
                qj =     0.0     0.0     1.0     0.0

       2) Compute the composition of two rotation matrices by
          converting them to quaternions and computing their
          product, and by directly multiplying the matrices.

          Example code begins here.

          /.
             Program qxq_ex2
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main( )
          {

             /.
             Local variables
             ./
             SpiceDouble          cmout  [3][3];
             SpiceDouble          q1     [4];
             SpiceDouble          q2     [4];
             SpiceDouble          qout   [4];

             SpiceDouble          cmat1  [3][3] = { {1.0,  0.0,  0.0},
                                                    {0.0, -1.0,  0.0},
                                                    {0.0,  0.0, -1.0} };

             SpiceDouble          cmat2  [3][3] = { {0.0,  1.0,  0.0},
                                                    {1.0,  0.0,  0.0},
                                                    {0.0,  0.0, -1.0} };

             /.
             Convert the C-matrices to quaternions.
             ./
             m2q_c ( cmat1, q1 );
             m2q_c ( cmat2, q2 );

             /.
             Find the product.
             ./
             qxq_c ( q1, q2, qout );

             /.
             Convert the result to a C-matrix.
             ./
             q2m_c ( qout, cmout );

             printf( "Using quaternion product:\n" );
             printf( "%9.4f %9.4f %9.4f\n",
                     cmout[0][0], cmout[0][1], cmout[0][2] );
             printf( "%9.4f %9.4f %9.4f\n",
                     cmout[1][0], cmout[1][1], cmout[1][2] );
             printf( "%9.4f %9.4f %9.4f\n",
                     cmout[2][0], cmout[2][1], cmout[2][2] );

             /.
             Multiply `cmat1' and `cmat2' directly.
             ./
             mxm_c ( cmat1, cmat2, cmout );

             printf( "Using matrix product:\n" );
             printf( "%9.4f %9.4f %9.4f\n",
                     cmout[0][0], cmout[0][1], cmout[0][2] );
             printf( "%9.4f %9.4f %9.4f\n",
                     cmout[1][0], cmout[1][1], cmout[1][2] );
             printf( "%9.4f %9.4f %9.4f\n",
                     cmout[2][0], cmout[2][1], cmout[2][2] );

             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

          Using quaternion product:
             0.0000    1.0000    0.0000
            -1.0000    0.0000    0.0000
             0.0000    0.0000    1.0000
          Using matrix product:
             0.0000    1.0000    0.0000
            -1.0000    0.0000    0.0000
             0.0000    0.0000    1.0000
#### Restrictions
       None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
#### Version
       -CSPICE Version 1.0.2, 10-AUG-2021 (JDR)

           Edited the header to comply with NAIF standard.

           Created complete code examples from existing example and code
           fragments.

       -CSPICE Version 1.0.1, 27-FEB-2008 (NJB)

           Updated header; added information about SPICE
           quaternion conventions.

       -CSPICE Version 1.0.0, 27-OCT-2005 (NJB)
#### Index_Entries
       quaternion times quaternion
       multiply quaternion by quaternion
#### Link to routine qxq_c source file qxq_c.c

    Fri Dec 31 18:41:11 2021