edlimb_c

Index Page edlimb_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 edlimb_c ( SpiceDouble           a,
                       SpiceDouble           b,
                       SpiceDouble           c,
                       ConstSpiceDouble      viewpt[3],
                       SpiceEllipse        * limb      )
#### Abstract
       Find the limb of a triaxial ellipsoid, viewed from a specified
       point.
#### Required_Reading
       ELLIPSES
#### Keywords
       ELLIPSE
       ELLIPSOID
       GEOMETRY
       MATH
#### Brief_I/O
       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       a          I   Length of ellipsoid semi-axis lying on the x-axis.
       b          I   Length of ellipsoid semi-axis lying on the y-axis.
       c          I   Length of ellipsoid semi-axis lying on the z-axis.
       viewpt     I   Location of viewing point.
       limb       O   Limb of ellipsoid as seen from viewing point.
#### Detailed_Input
       a,
       b,
       c              are the lengths of the semi-axes of a triaxial
                      ellipsoid.  The ellipsoid is centered at the
                      origin and oriented so that its axes lie on the
                      x, y and z axes.  a, b, and c are the lengths of
                      the semi-axes that point in the x, y, and z
                      directions respectively.

       viewpt         is a point from which the ellipsoid is viewed.
                      viewpt must be outside of the ellipsoid.
#### Detailed_Output
       limb           is a CSPICE ellipse that represents the limb of
                      the ellipsoid.
#### Parameters
       None.
#### Exceptions
       1)  If the length of any semi-axis of the ellipsoid is
           non-positive, the error DEGENERATECASE is signaled.
           limb is not modified.

       2)  If the length of any semi-axis of the ellipsoid is zero after
           the semi-axis lengths are scaled by the reciprocal of the
           magnitude of the longest semi-axis and then squared, the error
           SPICE(DEGENERATECASE) is signaled.  limb is not modified.

       3)  If the viewing point viewpt is inside the ellipse, the error
           SPICE(INVALIDPOINT) is signaled.  limb is not modified.

       4)  If the geometry defined by the input ellipsoid and viewing
           point is so extreme that the limb cannot be found, the error
           SPICE(DEGENERATECASE) is signaled.

       5)  If the shape of the ellipsoid and the viewing geometry are
           such that the limb is an excessively flat ellipsoid, the
           limb may be a degenerate ellipse.  You must determine whether
           this possibility poses a problem for your application.
#### Files
       None.
#### Particulars
       The limb of a body, as seen from a viewing point, is the boundary
       of the portion of the body's surface that is visible from that
       viewing point.  In this definition, we consider a surface point
       to be `visible' if it can be connected to the viewing point by a
       line segment that doen't pass through the body.  This is a purely
       geometrical definition that ignores the matter of which portions
       of the surface are illuminated, or whether the view is obscured by
       any additional objects.

       If a body is modelled as a triaxial ellipsoid, the limb is always
       an ellipse.  The limb is determined by its center, a semi-major
       axis vector, and a semi-minor axis vector.

       We note that the problem of finding the limb of a triaxial
       ellipsoid is mathematically identical to that of finding its
       terminator, if one makes the simplifying assumption that the
       terminator is the limb of the body as seen from the vertex of the
       umbra.  So, this routine can be used to solve this simplified
       version of the problem of finding the terminator.
#### Examples
       1)  We'd like to find the apparent limb of Jupiter, corrected for
           light time and stellar aberration, as seen from a spacecraft's
           position at time ET.

              /.
              Find the viewing point in Jupiter-fixed coordinates.  To do
              this, find the apparent position of Jupiter as seen from the
              spacecraft in Jupiter-fixed coordinates and negate this
              vector.  In this case we'll use light time and stellar
              aberration corrections to arrive at the apparent limb. jstat
              is the Jupiter's state (position and velocity) as seen
              from the spacecraft.  scpos is the spacecraft's
              position relative to Jupiter.
              ./
              spkez_c( jupid, et, "IAU_JUPITER", "LT+S", scid, scstat, &lt);

              vminus_c ( scstat, scpos );

              /.
              Get Jupiter's semi-axis lengths.
              ./
              bodvcd_c ( jupid, "RADII", 3, &n, rad );

              /.
              Find the apparent limb.  limb is a CSPICE ellipse
              representing the limb.
              ./
              edlimb_c ( rad[0], rad[1], rad[2], scpos, &limb );

              /.
              lcentr, smajor, and sminor are the limb's center,
              semi-major axis, and semi-minor axis.
              ./
              el2cgv_c ( &limb, center, smajor, sminor );
#### Restrictions
       None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman   (JPL)
#### Version
       -CSPICE Version 1.0.1, 24-OCT-2005 (NJB)

           Header update: reference to bodvar_c was replaced with
           reference to bodvcd_c.

       -CSPICE Version 1.0.0, 13-JUN-1999 (NJB)
#### Index_Entries
       ellipsoid limb
#### Link to routine edlimb_c source file edlimb_c.c

    Thu Apr 16 08:57:49 2009