Index Page dskx02_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 dskx02_c ( SpiceInt               handle,
                       ConstSpiceDLADescr   * dladsc,
                       ConstSpiceDouble       vertex  [3],
                       ConstSpiceDouble       raydir  [3],
                       SpiceInt             * plid,
                       SpiceDouble            xpt     [3],
                       SpiceBoolean         * found        )

#### Abstract

       Determine the plate ID and body-fixed coordinates of the
       intersection of a specified ray with the surface defined by a
       type 2 DSK plate model.

#### Required_Reading

        None.

#### Keywords

        GEOMETRY
        SHAPES

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       handle     I   Handle of DSK kernel containing plate model.
       dladsc     I   DLA descriptor of plate model segment.
       vertex     I   Ray's vertex in the  body fixed frame.
       raydir     I   Ray direction in the body fixed frame.
       plid       O   ID code of the plate intersected by the ray.
       xpt        O   Intercept.
       found      O   Flag indicating whether intercept exists.

#### Detailed_Input

       handle     is the DAS file handle of a DSK file containing a shape
                  model for a target body.  The shape model is stored in a
                  type 2 DSK segment.

       dladsc     is the DLA descriptor of a type 2 DSK segment containing
                  plate model data representing the surface of the target
                  body.  Normally this descriptor will be obtained by a
                  search through a DSK file using the DLA search routines;
                  see the Examples header section below for a working code
                  example illustrating a simple search.

       vertex     is the vertex of a ray.  `vertex' is expressed relative
                  to the body fixed reference frame associated with the
                  target body.  This reference frame is the same frame
                  relative to which the vertices of the plate model are
                  expressed.  Units are km.

                  The vertex is required to be outside the target body.

       raydir     is the ray's direction vector.  `raydir' is expressed
                  relative to the body fixed reference frame associated
                  with the target body.

#### Detailed_Output

       plid       is the ID of the plate closest to the input ray's
                  vertex at which a ray-surface intercept exists.
                  If no intercept exists, `plid' is undefined.

       xpt        is the ray-target intercept closest to the ray's vertex,
                  if an intercept exists. `xpt' is expressed relative to
                  the body-fixed reference frame associated with the target
                  body.  Units are km.

                  If no intercept exists, `xpt' is undefined.

       found      is a logical flag that indicates whether or not the ray
                  does indeed intersect the target.  If the ray intersects a
                  plate, `found' is SPICETRUE.  Otherwise `found' is
                  SPICEFALSE.

#### Parameters

       See header files

          SpiceDLA.h
          SpiceDSK.h

#### Exceptions

       1)  If the input handle is invalid, the error will be diagnosed by
           routines in the call tree of this routine.

       2)  If a file read error occurs, the error will be diagnosed by
           routines in the call tree of this routine.

       3)  If the input DLA descriptor is invalid, the effect of this
           routine is undefined. The error *may* be diagnosed by routines
           in the call  tree of this routine, but there are no
           guarantees.

       4)  If an error occurs while trying to look up any component
           of the shape model, the error will be diagnosed by routines
           in the call tree of this routine.

       5)  If the input ray direction is the zero vector, the error
           SPICE(ZEROVECTOR) is signaled.

       6)  If the coarse voxel grid scale of the shape model is less than
           1, the error SPICE(VALUEOUTOFRANGE) is signaled.

       7)  If the coarse voxel grid of the shape model contains more
           than SPICE_DSK_MAXCGR (see SpiceDSK.h) voxels, the error
           SPICE(GRIDTOOLARGE) is signaled.

       8)  If the plate list for any intersected voxel is too large
           for this routine to buffer, the error SPICE(ARRAYTOOSMALL)
           is signaled.

       9)  Due to round-off errors, results from this routine may
           differ across platforms.  Results also may differ from
           those expected---and not necessarily by a small amount.
           For example, a ray may miss a plate it was expected to
           hit and instead hit another plate considerably farther
           from the ray's vertex, or miss the target entirely.

       10) In the event that an intercept point lies on multiple
           plates (that is, the point is on an edge or vertex),
           a plate will be selected.  Due to round-off error, the
           selection may vary across platforms.

#### Files

       See the description of the input argument `handle'.

#### Particulars

       This routine solves the ray-surface intercept problem for a
       specified ray and a surface represented by triangular plate model.
       The surface representation is provided by data in a type 2 segment
       of a DSK file.

       This routine does not assume that the segment from which the surface
       model data are read represents the entire surface of the target
       body.  A program could call this routine repeatedly to find the
       surface intercept of a ray and a shape model partitioned into
       multiple segments.

       In general, this routine should be expected to run faster when used
       with smaller shape models.

#### Examples

       1) Find the surface intercept points corresponding to a latitude/
          longitude grid of a specified resolution, for a specified
          DSK file.  This simple program assumes the shape model for the
          target body of interest is stored in a single type 2 DSK segment,
          and that this segment is the first one in the DSK file to which
          it belongs.

             #include <stdio.h>
             #include <math.h>
             #include "SpiceUsr.h"
             #include "SpiceDLA.h"
             #include "SpiceDSK.h"

             int main()
             {
                /.
                Local parameters
                ./
                #define  FILSIZ             256
                #define  NLAT               9
                #define  NLON               9
                #define  TOL                ( 1.e-12 )
                #define  SPICE_MXCOR3_IDX   21

                /.
                Local variables
                ./
                SpiceBoolean            found;

                SpiceChar               dsk [ FILSIZ ];

                SpiceDLADescr           dladsc;
                SpiceDSKDescr           dskdsc;

                SpiceDouble             lat;
                SpiceDouble             lon;
                SpiceDouble             maxr;
                SpiceDouble             r;
                SpiceDouble             raydir [3];
                SpiceDouble             vertex [3];
                SpiceDouble             xlat;
                SpiceDouble             xlon;
                SpiceDouble             xpt    [3];
                SpiceDouble             xr;

                SpiceInt                handle;
                SpiceInt                i;
                SpiceInt                j;
                SpiceInt                plid;

                /.
                Prompt for the name of the DSK to read.
                ./
                prompt_c ( "Enter DSK name > ", FILSIZ, dsk );

                /.
                Open the DSK file for read access.
                We use the DAS-level interface for
                this function.
                ./
                dasopr_c ( dsk, &handle );

                /.
                Begin a forward search through the
                kernel, treating the file as a DLA.
                In this example, it's a very short
                search.
                ./
                dlabfs_c ( handle, &dladsc, &found );

                if ( !found  )
                {
                   /.
                   We arrive here only if the kernel
                   contains no segments.  This is
                   unexpected, but we're prepared for it.
                   ./
                   setmsg_c ( "No segments found in DSK file #.");
                   errch_c  ( "#",  dsk                         );
                   sigerr_c ( "SPICE(NODATA)"                   );
                }

                /.
                If we made it this far, DLADSC is the
                DLA descriptor of the first segment.

                We're going to generate the intercept points
                using a set of rays which point toward the
                origin and whose vertices are on a specified
                specified lat/lon grid.  To start out we
                must pick a reasonable range from the origin
                for the vertices:  the range must be large
                enough so that the vertices are guaranteed
                to be outside the target body but small
                enough that we don't lose too much precision
                in the surface intercept computation.

                We'll look up the upper bound for the target
                radius, then use 2 times this value as the
                vertex magnitude.
                ./

                dskgd_c ( handle, &dladsc, &dskdsc );

                maxr =  dskdsc.co3max;
                r    = 2.0 * maxr;

                /.
                Now generate the intercept points. We generate
                intercepts along latitude bounds, working from
                north to south. Latitude ranges from +80 to -80
                degrees. Longitude ranges from 0 to 320 degrees.
                The increment is 20 degrees for latitude and 40
                degrees for longitude.
                ./
                for ( i = 0;  i < NLAT;  i++  )
                {
                   lat = rpd_c() * ( 80.0 - 20.0*i );

                   for ( j = 0;  j < NLON;  j++  )
                   {
                      lon = rpd_c() * 40.0*j;

                      /.
                      Produce a ray vertex for the current
                      lat/lon value.  Negate the vertex to
                      produce the ray's direction vector.
                      ./
                      latrec_c ( r, lon, lat, vertex );
                      vminus_c ( vertex,      raydir );

                      /.
                      Find the surface intercept for this
                      ray.
                      ./
                      dskx02_c ( handle, &dladsc, vertex, raydir,
                                 &plid,  xpt,     &found          );

                      /.
                      Since the ray passes through the origin on
                      the body-fixed frame associated with the
                      target body, we'd rarely expect to find that
                      the ray failed to intersect the surface.
                      For safety, we check the FOUND flag.  (A
                      "not found" condition could be a sign of
                      a bug.)
                      ./
                      if ( ! found  )
                      {
                         printf ( "\n"
                                  "Intercept not found!\n"
                                  "   Ray vertex:\n"
                                  "   Longitude (deg): %f\n"
                                  "   Latitude  (deg): %f\n"
                                  "   Radius     (km): %e\n"
                                  "\n",
                                  lon * dpr_c(),
                                  lat * dpr_c(),
                                  r                           );
                      }
                      else
                      {
                         /.
                         This is the normal case.  Display the
                         intercept plate ID and the intercept
                         point in both cartesian and latitudinal
                         coordinates.  Show the corresponding ray
                         vertex to facilitate validation of results.

                         Use recrad_c rather than reclat_c to produce
                         non-negative longitudes.
                         ./
                         recrad_c ( xpt, &xr, &xlon, &xlat );

                         printf ( "\n"
                                  "Intercept found:\n"
                                  "   Plate ID:                 %d\n"
                                  "   Cartesian Coordinates:    (%e %e %e)\n"
                                  "   Latitudinal Coordinates:\n"
                                  "   Longitude (deg): %f\n"
                                  "   Latitude  (deg): %f\n"
                                  "   Radius     (km): %e\n"
                                  "\n"
                                  "   Ray vertex:\n"
                                  "   Longitude (deg): %f\n"
                                  "   Latitude  (deg): %f\n"
                                  "   Radius     (km): %e\n"
                                  "\n",
                                  (int)plid,
                                  xpt[0], xpt[1], xpt[2],
                                  xlon * dpr_c(),
                                  xlat * dpr_c(),
                                  xr,
                                  lon  * dpr_c(),
                                  lat  * dpr_c(),
                                  r                                           );

                         /.
                         Perform sanity checks on the intercept
                         coordinates.  Stop the program if any error
                         is larger than our tolerance value.
                         ./
                         if ( fabs(xlat-lat) > TOL )
                         {
                            sigerr_c ( "Latitude error!" );
                         }

                         if (  (xlon - lon)  > pi_c()  )
                         {
                            xlon -= twopi_c();
                         }

                         if (  (xlon - lon)  > TOL  )
                         {
                            sigerr_c ( "Longitude error!" );
                         }

                         if ( xr  > (1.0+TOL)*maxr  )
                         {
                            sigerr_c ( "Radius error!" );
                         }
                      }
                      /.
                      End of longitude loop.
                      ./
                   }
                   /.
                   End of latitude loop.
                   ./
                }

                /.
                Close the kernel.  This isn't necessary in a stand-
                alone program, but it's good practice in subroutines
                because it frees program and system resources.
                ./
                dascls_c ( handle );

                return ( 0 );
             }

       The program outputs:

          Intercept found:
             Plate ID:                 306238
             Cartesian Coordinates:    (1.520878e+00 0.000000e+00 8.625327e+00)
             Latitudinal Coordinates:
             Longitude (deg): 0.000000
             Latitude  (deg): 80.000000
             Radius     (km): 8.758387e+00

             Ray vertex:
             Longitude (deg): 0.000000
             Latitude  (deg): 80.000000
             Radius     (km): 2.802354e+01

          Intercept found:
             Plate ID:                 317112
             Cartesian Coordinates:    (1.189704e+00 9.982799e-01 8.807772e+00)
             Latitudinal Coordinates:
             Longitude (deg): 40.000000
             Latitude  (deg): 80.000000
             Radius     (km): 8.943646e+00

             Ray vertex:
             Longitude (deg): 40.000000
             Latitude  (deg): 80.000000
             Radius     (km): 2.802354e+01

          Intercept found:
             Plate ID:                 324141
             Cartesian Coordinates:    (2.777752e-01 1.575341e+00 9.072029e+00)
             Latitudinal Coordinates:
             Longitude (deg): 80.000000
             Latitude  (deg): 80.000000
             Radius     (km): 9.211980e+00

             Ray vertex:
             Longitude (deg): 80.000000
             Latitude  (deg): 80.000000
             Radius     (km): 2.802354e+01

             ...

          Intercept found:
             Plate ID:                 1893
             Cartesian Coordinates:    (-7.783952e-01 -1.348220e+00 -8.828997e+00)
             Latitudinal Coordinates:
             Longitude (deg): 240.000000
             Latitude  (deg): -80.000000
             Radius     (km): 8.965198e+00

             Ray vertex:
             Longitude (deg): 240.000000
             Latitude  (deg): -80.000000
             Radius     (km): 2.802354e+01

          Intercept found:
             Plate ID:                 2467
             Cartesian Coordinates:    (2.666822e-01 -1.512430e+00 -8.709738e+00)
             Latitudinal Coordinates:
             Longitude (deg): 280.000000
             Latitude  (deg): -80.000000
             Radius     (km): 8.844099e+00

             Ray vertex:
             Longitude (deg): 280.000000
             Latitude  (deg): -80.000000
             Radius     (km): 2.802354e+01

          Intercept found:
             Plate ID:                 3187
             Cartesian Coordinates:    (1.132409e+00 -9.502036e-01 -8.383597e+00)
             Latitudinal Coordinates:
             Longitude (deg): 320.000000
             Latitude  (deg): -80.000000
             Radius     (km): 8.512928e+00

             Ray vertex:
             Longitude (deg): 320.000000
             Latitude  (deg): -80.000000
             Radius     (km): 2.802354e+01

#### Restrictions

       None.

#### Literature_References

       Woo, A. "Fast Ray-Box Intersection", Graphic Gems I, 395-396.

#### Author_and_Institution

       N.J. Bachman    (JPL)
       J.A. Bytof      (JPL)
       W.L. Taber      (JPL)
       E.D. Wright     (JPL)

#### Version

        -CSPICE Version 1.0.3, 04-APR-2017 (EDW)

           Edit to example program to use "%d" with explicit casts
           to int for printing SpiceInts with printf.

       -CSPICE Version 1.0.1 20-JUL-2011 (NJB)

           Header correction: the detailed input section
           now says that the ray's vertex *is* required to
           be outside the target body.

       -CSPICE Version 1.0.0, 05-JUN-2010 (NJB) (EDW) (JAB) (WLT)

#### Index_Entries

       plate and plate model point intersected by ray
       intersection of ray and surface

#### Link to routine dskx02_c source file dskx02_c.c

    Wed Apr  5 17:54:32 2017