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Index Page inelpl_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
       inelpl_c ( Intersection of ellipse and plane )

       void inelpl_c ( ConstSpiceEllipse  * ellips,
                       ConstSpicePlane    * plane,
                       SpiceInt           * nxpts,
                       SpiceDouble          xpt1[3],
                       SpiceDouble          xpt2[3] )
#### Abstract
       Find the intersection of an ellipse and a plane.
#### Required_Reading
       ELLIPSES
       PLANES
#### Keywords
       ELLIPSE
       GEOMETRY
       MATH
#### Brief_I/O
       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       ellips     I   A SPICE ellipse.
       plane      I   A SPICE plane.
       nxpts      O   Number of intersection points of plane and ellipse.
       xpt1,
       xpt2       O   Intersection points.
#### Detailed_Input
       ellips      is a SPICE ellipse. The ellipse is allowed to
                   be degenerate: one or both semi-axes may have
                   zero length.

       plane       is a SPICE plane. The intersection of plane
                   and ellipse is sought.
#### Detailed_Output
       nxpts       is the number of points of intersection of the
                   geometric plane and ellipse represented by `plane' and
                   `ellips'. `nxpts' may take the values 0, 1, 2 or -1.
                   The value -1 indicates that the ellipse consists of
                   more than one point lies in the plane, so the number
                   of intersection points is infinite.

                   When the ellipse consists of a single point and
                   lies in the plane, `nxpts' is set to 1.

       xpt1,
       xpt2        are the points of intersection of the input plane
                   and ellipse. If there is only one intersection
                   point, both xpt1 and xpt2 contain that point. If
                   the number of intersection points is zero or
                   infinite, the contents of xpt1 and xpt2 are
                   undefined.
#### Parameters
       None.
#### Exceptions
       1)  If the input plane is invalid, the error SPICE(INVALIDPLANE)
           is signaled. The input plane must be a SPICE plane: the normal
           vector must be non-zero and the constant must be non-negative.

       2)  If the input ellipse has non-orthogonal axes, the error
           SPICE(INVALIDELLIPSE) is signaled.

       3)  The input ellipse is allowed to be a line segment or a point;
           these cases are not considered to be errors. If the ellipse
           consists of a single point and lies in the plane, the number
           of intersection points is set to 1 (rather than -1) and
           the output arguments `xpt1' and `xpt2' are assigned the value
           of the ellipse's center.
#### Files
       None.
#### Particulars
       This routine computes the intersection set of a non-degenerate
       plane with a possibly degenerate ellipse. The ellipse is allowed
       to consist of a line segment or a point.

       A plane may intersect an ellipse in 0, 1, 2, or infinitely many
       points. For there to be an infinite set of intersection points,
       the ellipse must lie in the plane and consist of more than one
#### Examples
       1)  If we want to find the angle of some ray above the limb of an
           ellipsoid, where the angle is measured in a plane containing
           the ray and a "down" vector, we can follow the procedure
           given below. We assume the ray does not intersect the
           ellipsoid. The result we seek is called angle, imaginatively
           enough.

           We assume that all vectors are given in body-fixed
           coordinates.

              #include "SpiceUsr.h"
                  .
                  .
                  .
         /.
         Find the limb of the ellipsoid as seen from the
         point observ. Here a, b, and c are the lengths of
         the semi-axes of the ellipsoid. The limb is
         returned as a SpiceEllipse.
         ./

         edlimb_c ( a, b, c, observ, &limb );

         /.
         The ray direction vector is raydir, so the ray is the
         set of points

            observ  +  t * raydir

         where t is any non-negative real number.

         The `down' vector is just -observ. The vectors
         observ and raydir are spanning vectors for the plane
         we're interested in. We can use psv2pl_c to represent
         this plane by a SPICE plane.
         ./
         psv2pl_c ( observ, observ, raydir, &plane );

         /.
         Find the intersection of the plane defined by observ
         and raydir with the limb.
         ./
         inelpl_c ( limb, plane, nxpts, xpt1, xpt2 );

         /.
         We always expect two intersection points, if the vector
         down is valid.
         ./
         if ( nxpts < 2 )
         {
            [ do something about the error ]
         }

         /.
         Form the vectors from observ to the intersection
         points. Find the angular separation between the
         boresight ray and each vector from observ to the
         intersection points.
         ./
         vsub_c   ( xpt1, observ, vec1 );
         vsub_c   ( xpt2, observ, vec2 );

         sep1 = vsep_c ( vec1, raydir );
         sep2 = vsep_c ( vec2, raydir );

         /.
         The angular separation we're after is the minimum of
         the two separations we've computed.
         ./
         angle = mind_c ( 2, sep1, sep2 );
#### Restrictions
       None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
#### Version
       -CSPICE Version 2.1.1, 24-AUG-2021 (JDR)

           Edited the header to comply with NAIF standard.

           Removed non-standard header section "Revisions", moving its
           contents to the relevant entry in -Version section.

       -CSPICE Version 2.1.0, 07-OCT-2011 (NJB)

           Relaxed ellipse semi-axes orthogonality test limit
           SEPLIM from 1.D-12 TO 1.D-9 radians. The angular
           separation of the axes of the input ellipse must not
           differ from pi/2 radians by more than this limit.

       -CSPICE Version 2.0.0, 14-JAN-2008 (NJB)

           Bug fix: the routine's specification and behavior have been
           updated so the routine now returns a meaningful result for the
           case of an ellipse consisting of a single point. In this case,
           if an intersection is found, the number of intersection points
           is set to 1 and both intersection arguments are set equal to
           the ellipse's center.

           Bug fix: in the degenerate case where the input ellipse is a
           line segment of positive length, and this segment intersects
           the plane, the number of intersection points is set to 1
           rather than 2.

           Invalid input planes and ellipses are now diagnosed.
           Error handling code has been added to trap errors that had
           been erroneously passed off to lower level routines for
           diagnosis.

       -CSPICE Version 1.0.0, 28-AUG-2001 (NJB)
#### Index_Entries
       intersection of ellipse and plane
#### Link to routine inelpl_c source file inelpl_c.c

    Fri Dec 31 18:41:08 2021