Index Page m2eul_c

A B C D E F G H I J K L M N O P Q R S T U V W X

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

       void  m2eul_c ( ConstSpiceDouble    r[3][3],
                       SpiceInt            axis3,
                       SpiceInt            axis2,
                       SpiceInt            axis1,
                       SpiceDouble       * angle3,
                       SpiceDouble       * angle2,
                       SpiceDouble       * angle1  )

#### Abstract

       Factor a rotation matrix as a product of three rotations about
       specified coordinate axes.

#### Required_Reading

       ROTATION

#### Keywords

       ANGLE
       MATRIX
       ROTATION
       TRANSFORMATION

#### Brief_I/O

       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       r          I   A rotation matrix to be factored.
       axis3,
       axis2,
       axis1      I   Numbers of third, second, and first rotation axes.
       angle3,
       angle2,
       angle1     O   Third, second, and first Euler angles, in radians.

#### Detailed_Input

       r              is a 3x3 rotation matrix that is to be factored as
                      a product of three rotations about a specified
                      coordinate axes.  The angles of these rotations are
                      called `Euler angles'.

       axis3,
       axis2,
       axis1          are the indices of the rotation axes of the
                      `factor' rotations, whose product is r.  r is
                      factored as

                         r = [ angle3 ]     [ angle2 ]     [ angle1 ]   .
                                      axis3          axis2          axis1

                      The axis numbers must belong to the set {1, 2, 3}.
                      The second axis number MUST differ from the first
                      and third axis numbers.

                      See the Particulars section below for details
                      concerning this notation.

#### Detailed_Output

       angle3,
       angle2,
       angle1         are the Euler angles corresponding to the matrix
                      r and the axes specified by axis3, axis2, and
                      axis1.  These angles satisfy the equality

                         r = [ angle3 ]     [ angle2 ]     [ angle1 ]
                                     axis3           axis2          axis1

                      See the Particulars section below for details
                      concerning this notation.

                      The range of angle3 and angle1 is (-pi, pi].

                      The range of angle2 depends on the exact set of
                      axes used for the factorization.  For
                      factorizations in which the first and third axes
                      are the same,

                         r = [R]  [S]  [T] ,
                                a    b    a

                      the range of angle2 is [0, pi].

                      For factorizations in which the first and third
                      axes are different,

                         r = [R]  [S]  [T] ,
                                a    b    c

                      the range of angle2 is [-pi/2, pi/2].

                      For rotations such that angle3 and angle1 are not
                      uniquely determined, angle3 will always be set to
                      zero; angle1 is then uniquely determined.

#### Parameters

       None.

#### Exceptions

       1)   If any of axis3, axis2, or axis1 do not have values in

               { 1, 2, 3 },

            then the error SPICE(INPUTOUTOFRANGE) is signalled.

       2)   An arbitrary rotation matrix cannot be expressed using
            a sequence of Euler angles unless the second rotation axis
            differs from the other two.  If axis2 is equal to axis3 or
            axis1, then then error SPICE(BADAXISNUMBERS) is signalled.

       3)   If the input matrix r is not a rotation matrix, the error
            SPICE(NOTAROTATION) is signalled.

       4)   If angle3 and angle1 are not uniquely determined, angle3
            is set to zero, and angle1 is determined.

#### Files

       None.

#### Particulars

       A word about notation:  the symbol

          [ x ]
               i

       indicates a coordinate system rotation of x radians about the
       ith coordinate axis.  To be specific, the symbol

          [ x ]
               1

       indicates a coordinate system rotation of x radians about the
       first, or x-, axis; the corresponding matrix is

          +-                    -+
          |  1      0       0    |
          |                      |
          |  0    cos(x)  sin(x) |.
          |                      |
          |  0   -sin(x)  cos(x) |
          +-                    -+

       Remember, this is a COORDINATE SYSTEM rotation by x radians; this
       matrix, when applied to a vector, rotates the vector by -x
       radians, not x radians.  Applying the matrix to a vector yields
       the vector's representation relative to the rotated coordinate
       system.

       The analogous rotation about the second, or y-, axis is
       represented by

          [ x ]
               2

       which symbolizes the matrix

          +-                    -+
          | cos(x)   0   -sin(x) |
          |                      |
          |  0       1      0    |,
          |                      |
          | sin(x)   0    cos(x) |
          +-                    -+

       and the analogous rotation about the third, or z-, axis is
       represented by

          [ x ]
               3

       which symbolizes the matrix

          +-                    -+
          |  cos(x)  sin(x)   0  |
          |                      |
          | -sin(x)  cos(x)   0  |.
          |                      |
          |  0        0       1  |
          +-                    -+

       The input matrix is assumed to be the product of three
       rotation matrices, each one of the form

          +-                    -+
          |  1      0       0    |
          |                      |
          |  0    cos(r)  sin(r) |     (rotation of r radians about the
          |                      |      x-axis),
          |  0   -sin(r)  cos(r) |
          +-                    -+

          +-                    -+
          | cos(s)   0   -sin(s) |
          |                      |
          |  0       1      0    |     (rotation of s radians about the
          |                      |      y-axis),
          | sin(s)   0    cos(s) |
          +-                    -+

       or

          +-                    -+
          |  cos(t)  sin(t)   0  |
          |                      |
          | -sin(t)  cos(t)   0  |     (rotation of t radians about the
          |                      |      z-axis),
          |  0        0       1  |
          +-                    -+

       where the second rotation axis is not equal to the first or
       third.  Any rotation matrix can be factored as a sequence of
       three such rotations, provided that this last criterion is met.

       This routine is related to the CSPICE routine EUL2M, which
       produces a rotation matrix, given a sequence of Euler angles.
       This routine is a `right inverse' of EUL2M:  the sequence of
       calls

          m2eul_c ( r,  axis3,   axis2,   axis1,
                        angle3,  angle2,  angle1     );

          eul2m_c (     angle3,  angle2,  angle1,
                        axis3,   axis2,   axis1,   r );

       preserves r, except for round-off error.

       On the other hand, the sequence of calls

          eul2m_c ( angle3,  angle2,  angle1,
                    axis3,   axis2,   axis1,   r );

          m2eul_c ( r,  axis3,   axis2,   axis1,
                        angle3,  angle2,  angle1 );

       preserve angle3, angle2, and angle1 only if these angles start
       out in the ranges that m2eul_c's outputs are restricted to.

#### Examples

       1)  Conversion of instrument pointing from a matrix representation
           to Euler angles:

           Suppose we want to find camera pointing in alpha, delta, and
           kappa, given the inertial-to-camera coordinate transformation

           ticam =

       +-                                                               -+
       |  0.49127379678135830  0.50872620321864170  0.70699908539882417  |
       |                                                                 |
       | -0.50872620321864193 -0.49127379678135802  0.70699908539882428  |
       |                                                                 |
       |  0.70699908539882406 -0.70699908539882439  0.01745240643728360  |
       +-                                                               -+

           We want to find angles alpha, delta, kappa such that

              ticam  =  [ kappa ]  [ pi/2 - delta ]  [ pi/2 + alpha ] .
                                 3                 1                 3

           The code fragment

              m2eul_c ( ticam, 3, 1, 3, &kappa, &ang2, &ang1 );

              alpha  =  ang1       - halfpi_c();
              delta  =  halfpi_c() - ang2;

           calculates the desired angles.  If we wish to make sure that
           alpha, delta, and kappa are in the ranges [0, 2pi),
           [-pi/2, pi/2], and [0, 2pi) respectively, we may add the code

              if ( alpha < 0. )
                 {
                 alpha = alpha + twopi_c();
                 }

              if ( kappa < 0. )
                 {
                 kappa = kappa + twopi_c();
                 }

           Note that delta is already in the correct range, since ang2
           is in the range [0, pi] when the first and third input axes
           are equal.

           If we wish to print out the results in degrees, we might
           use the code

              printf ( "Alpha = %25.17f\n"
                       "Delta = %25.17f\n"
                       "Kappa = %25.17f\n",
                       dpr_c() * alpha,
                       dpr_c() * delta,
                       dpr_c() * kappa     );

           We should see something like

              Alpha =     315.00000000000000000
              Delta =       1.00000000000000000
              Kappa =      45.00000000000000000

           possibly formatted a little differently, or degraded slightly
           by round-off.

       2)  Conversion of instrument pointing angles from a non-J2000,
           not necessarily inertial frame to J2000-relative RA, Dec,
           and Twist.

           Suppose that we have pointing for some instrument expressed as

              [ gamma ]  [ beta ]  [ alpha ]
                       3         2          3

           with respect to some coordinate system S.  For example, S
           could be a spacecraft-fixed system.

           We will suppose that the transformation from J2000
           coordinates to system S coordinates is given by the rotation
           matrix j2s.

           The rows of j2s are the unit basis vectors of system S, given
           in J2000 coordinates.

           We want to express the pointing with respect to the J2000
           system as the sequence of rotations

              [ kappa ]  [ pi/2 - delta ]  [ pi/2 + alpha ] .
                       3                 1                 3

           First, we use subroutine eul2m_c to form the transformation
           from system S to instrument coordinates s2inst.

              eul2m_c ( gamma, beta, alpha, 3, 2, 3, s2inst );

           Next, we form the transformation from J2000 to instrument
           coordinates j2inst.

              mxm_c ( s2inst, j2s, j2inst );

           Finally, we express j2inst using the desired Euler angles, as
           in the first example:

              m2eul_c ( j2inst, 3, 1, 3, &twist, &ang2, &ang3 );

              ra   =  ang3       - halfpi_c();
              dec  =  halfpi_c() - ang2;

           If we wish to make sure that ra, dec, and twist are in
           the ranges [0, 2pi), [-pi/2, pi/2], and [0, 2pi)
           respectively, we may add the code

              if ( ra < 0. )
                 {
                 ra = ra + twopi_c();
                 }

              if ( twist < 0. )
                 {
                 twist = twist + twopi_c();
                 }

           Note that dec is already in the correct range, since ang2
           is in the range [0, pi] when the first and third input axes
           are equal.

           Now ra, dec, and twist express the instrument pointing
           as RA, Dec, and Twist, relative to the J2000 system.

           A warning note:  more than one definition of RA, Dec, and
           Twist is extant.  Before using this example in an application,
           check that the definition given here is consistent with that
           used in your application.

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman   (JPL)

#### Version

       -CSPICE Version 1.3.1, 13-OCT-2004 (NJB)

          Fixed header typo.

       -CSPICE Version 1.3.0, 21-OCT-1998 (NJB)

          Made input matrix const.

       -CSPICE Version 1.2.0, 13-FEB-1998 (EDW)

          Minor corrections to header.

       -CSPICE Version 1.2.0, 08-FEB-1998 (NJB)

          Removed local variables used for temporary capture of outputs.

       -CSPICE Version 1.0.0 25-OCT-1997 (NJB)

          Based on SPICELIB Version 1.1.1, 10-MAR-1992 (WLT)

#### Index_Entries

       matrix to euler angles

#### Link to routine m2eul_c source file m2eul_c.c

    Wed Apr  5 17:54:38 2017