Index Page dskxv_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 dskxv_c ( SpiceBoolean         pri,
                      ConstSpiceChar     * target,
                      SpiceInt             nsurf,
                      ConstSpiceInt        srflst[],
                      SpiceDouble          et,
                      ConstSpiceChar     * fixref,
                      SpiceInt             nrays,
                      ConstSpiceDouble     vtxarr[][3],
                      ConstSpiceDouble     dirarr[][3],
                      SpiceDouble          xptarr[][3],
                      SpiceBoolean         fndarr[]     )

#### Abstract

       Compute ray-surface intercepts for a set of rays, using data
       provided by multiple loaded DSK segments.

#### Required_Reading

       CK
       DSK
       FRAMES
       PCK
       SPK
       TIME

#### Keywords

       GEOMETRY
       INTERCEPT
       SURFACE
       TOPOGRAPHY

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       pri        I   Data prioritization flag.
       target     I   Target body name.
       nsurf      I   Number of surface IDs in list.
       srflst     I   Surface ID list.
       et         I   Epoch, expressed as seconds past J2000 TDB.
       fixref     I   Name of target body-fixed reference frame.
       nrays      I   Number of rays.
       vtxarr     I   Array of vertices of rays.
       dirarr     I   Array of direction vectors of rays.
       xptarr     O   Intercept point array.
       fndarr     O   Found flag array.

#### Detailed_Input

       pri        is a logical flag indicating whether to perform
                  a prioritized or unprioritized DSK segment search.
                  In an unprioritized search, no segment masks another:
                  data from all specified segments are used to
                  define the surface of interest.

                  The search is unprioritized if and only if `pri'
                  is set to SPICEFALSE. In the N0066 SPICE Toolkit, this
                  is the only allowed value.

       target     is the name of the target body on which a surface
                  intercept is sought.

       nsurf,
       srflst     are, respectively, a count of surface ID codes in a
                  list and an containing the list. Only DSK segments for
                  for the body designated by `target' and having surface
                  IDs in this list will considered in the intercept
                  computation. If the list is empty, all DSK segments
                  for `target' will be considered.

       et         is the epoch of the intersection computation,
                  expressed as seconds past J2000 TDB. This epoch is
                  used only for DSK segment selection. Segments used
                  the intercept computation must include `et' in their
                  time coverage intervals.

       fixref     is the name of a body-fixed, body-centered reference
                  frame associated with the target. The input ray vectors
                  are specified in this frame, as is the output intercept
                  point.

                  The frame designated by `fixref' must have a fixed
                  orientation relative to the frame of any DSK segment
                  used in the computation.

       nrays,
       vtxarr,
       dirarr     are, respectively, a count of rays, an array containing
                  the vertices of rays, and an array containing the
                  direction vectors of the rays.

                  The ray's vertices are considered to represent offsets
                  from the center of the target body.

                  The rays' vertices and direction vectors are
                  represented in the reference frame designated by
                  `fixref'.

#### Detailed_Output

       xptarr     is an array containing the intercepts of the input
                  rays on the surface specified by the inputs

                     pri
                     target
                     nsurf
                     srflst
                     et

                  The ith element of `xptarr' is the intercept
                  corresponding to the ith ray, if such an intercept
                  exists. If a ray intersects the surface at multiple
                  points, the intercept closest to the ray's vertex is
                  selected.

                  The ith element of `xptarr' is defined if and only if the
                  ith element of `fndarr' is SPICETRUE.

                  Units are km.

       fndarr     is an array of logical flags indicating whether the
                  input rays intersect the surface. The ith element of
                  `fndarr' is set to SPICETRUE if and only if an intercept
                  was found for the ith ray.

#### Parameters

       See the include file

          SpiceDtl.h

       for the values of tolerance parameters used by default by the
       ray-surface intercept algorithm. These are discussed in in the
       Particulars section below.

#### Exceptions

       1)  If the input prioritization flag `pri' is set to SPICETRUE,
           the error SPICE(NOPRIORITIZATION) is signaled.

       2)  If the input body name `target' cannot be mapped to an
           ID code, the error SPICE(IDCODENOTFOUND) is signaled.

       3)  If the input frame name `fixref' cannot be mapped to an
           ID code, the error SPICE(IDCODENOTFOUND) is signaled.

       4)  If the frame center associated with `fixref' cannot be
           retrieved, the error SPICE(NOFRAMEINFO) is signaled.

       5)  If the frame center associated with `fixref' is not
           the target body, the error SPICE(INVALIDFRAME) is signaled.

       6)  If `nrays' is less than 1, the error SPICE(INVALIDCOUNT)
           is signaled.

       7)  Any errors that occur during the intercept computation
           will be signaled by routines in the call tree of this
           routine.

       8)  If any input string argument pointer is null, the error
           SPICE(NULLPOINTER) will be signaled.

       9)  If any input string argument is empty, the error
           SPICE(EMPTYSTRING) will be signaled.

#### Files

       Appropriate kernels must be loaded by the calling program before
       this routine is called.

       The following data are required:

          - SPK data: ephemeris data for the positions of the centers
            of DSK reference frames relative to the target body are
            required if those frames are not centered at the target
            body center.

            Typically ephemeris data are made available by loading one
            or more SPK files via furnsh_c.

          - DSK data: DSK files containing topographic data for the
            target body must be loaded. If a surface list is specified,
            data for at least one of the listed surfaces must be loaded.

          - Frame data: if a frame definition is required to convert
            DSK segment data to the body-fixed frame designated by
            `fixref', the target, that definition must be available in the
            kernel pool. Typically the definitions of frames not already
            built-in to SPICE are supplied by loading a frame kernel.

          - CK data: if the frame to which `fixref' refers is a CK frame,
            and if any DSK segments used in the computation have a
            different frame, at least one CK file will be needed to
            permit transformation of vectors between that frame and both
            the J2000 and the target body-fixed frames.

          - SCLK data: if a CK file is needed, an associated SCLK
            kernel is required to enable conversion between encoded SCLK
            (used to time-tag CK data) and barycentric dynamical time
            (TDB).

       In all cases, kernel data are normally loaded once per program
       run, NOT every time this routine is called.

#### Particulars

       This routine is suitable for efficient ray-surface intercept
       computations in which the relative observer-target geometry is
       constant but the rays vary.

       For cases in which it is necessary to know the source of the
       data defining the surface on which an intercept was found,
       use the CSPICE routine dskxsi_c.

       For cases in which a ray's vertex is not explicitly known but is
       defined by relative observer-target geometry, the CSPICE
       ray-surface intercept routine sincpt_c should be used.

       This routine works with multiple DSK files. It places no
       restrictions on the data types or coordinate systems of the DSK
       segments used in the computation. DSK segments using different
       reference frames may be used in a single computation. The only
       restriction is that any pair of reference frames used directly or
       indirectly are related by a constant rotation.

       Using DSK data
       ==============

          DSK loading and unloading
          -------------------------

          DSK files providing data used by this routine are loaded by
          calling furnsh_c and can be unloaded by calling unload_c or
          kclear_c. See the documentation of furnsh_c for limits on numbers
          of loaded DSK files.

          For run-time efficiency, it's desirable to avoid frequent
          loading and unloading of DSK files. When there is a reason to
          use multiple versions of data for a given target body---for
          example, if topographic data at varying resolutions are to be
          used---the surface list can be used to select DSK data to be
          used for a given computation. It is not necessary to unload
          the data that are not to be used. This recommendation presumes
          that DSKs containing different versions of surface data for a
          given body have different surface ID codes.

          DSK data priority
          -----------------

          A DSK coverage overlap occurs when two segments in loaded DSK
          files cover part or all of the same domain---for example, a
          given longitude-latitude rectangle---and when the time
          intervals of the segments overlap as well.

          When DSK data selection is prioritized, in case of a coverage
          overlap, if the two competing segments are in different DSK
          files, the segment in the DSK file loaded last takes
          precedence. If the two segments are in the same file, the
          segment located closer to the end of the file takes
          precedence.

          When DSK data selection is unprioritized, data from competing
          segments are combined. For example, if two competing segments
          both represent a surface as sets of triangular plates, the
          union of those sets of plates is considered to represent the
          surface.

          Currently only unprioritized data selection is supported.
          Because prioritized data selection may be the default behavior
          in a later version of the routine, the presence of the `pri'
          argument is required.

          Round-off errors and mitigating algorithms
          ------------------------------------------

          When topographic data are used to represent the surface of a
          target body, round-off errors can produce some results that
          may seem surprising.

          Note that, since the surface in question might have mountains,
          valleys, and cliffs, the points of intersection found for
          nearly identical sets of inputs may be quite far apart from
          each other: for example, a ray that hits a mountain side in a
          nearly tangent fashion may, on a different host computer, be
          found to miss the mountain and hit a valley floor much farther
          from the observer, or even miss the target altogether.

          Round-off errors can affect segment selection: for example, a
          ray that is expected to intersect the target body's surface
          near the boundary between two segments might hit either
          segment, or neither of them; the result may be
          platform-dependent.

          A similar situation exists when a surface is modeled by a set
          of triangular plates, and the ray is expected to intersect the
          surface near a plate boundary.

          To avoid having the routine fail to find an intersection when
          one clearly should exist, this routine uses two "greedy"
          algorithms:

             1) If the ray passes sufficiently close to any of the
                boundary surfaces of a segment (for example, surfaces of
                maximum and minimum longitude or latitude), that segment
                is tested for an intersection of the ray with the
                surface represented by the segment's data.

                This choice prevents all of the segments from being
                missed when at least one should be hit, but it could, on
                rare occasions, cause an intersection to be found in a
                segment other than the one that would be found if higher
                precision arithmetic were used.

             2) For type 2 segments, which represent surfaces as
                sets of triangular plates, each plate is expanded very
                slightly before a ray-plate intersection test is
                performed. The default plate expansion factor is

                   1 + XFRACT

                where XFRACT is declared in

                   SpiceDtl.h

                For example, given a value for XFRACT of 1.e-10, the
                sides of the plate are lengthened by 1/10 of a micron
                per km. The expansion keeps the centroid of the plate
                fixed.

                Plate expansion prevents all plates from being missed
                in cases where clearly at least one should be hit.

                As with the greedy segment selection algorithm, plate
                expansion can occasionally cause an intercept to be
                found on a different plate than would be found if higher
                precision arithmetic were used. It also can occasionally
                cause an intersection to be found when the ray misses
                the target by a very small distance.

#### 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) Compute surface intercepts of rays emanating from a set of
          vertices distributed on a longitude-latitude grid. All
          vertices are outside the target body, and all rays point
          toward the target's center.

          Check intercepts against expected values. Indicate the
          number of errors, the number of computations, and the
          number of intercepts found.

          Use the meta-kernel shown below to load example SPICE
          kernels.

              KPL/MK

              File: dskxv_ex1.tm

              This meta-kernel is intended to support operation of SPICE
              example programs. The kernels shown here should not be
              assumed to contain adequate or correct versions of data
              required by SPICE-based user applications.

              In order for an application to use this meta-kernel, the
              kernels referenced here must be present in the user's
              current working directory.

              The names and contents of the kernels referenced
              by this meta-kernel are as follows:

                 File name                        Contents
                 ---------                        --------
                 phobos512.bds                    DSK based on
                                                  Gaskell ICQ Q=512
                                                  plate model
              \begindata

                 PATH_SYMBOLS    = 'GEN'
                 PATH_VALUES     = '/ftp/pub/naif/generic_kernels'

                 KERNELS_TO_LOAD = ( '$GEN/dsk/phobos/phobos512.bds' )

              \begintext

       Example code begins here.

          /.
          Multi-segment, vectorized spear program.

          This program expects all loaded DSKs
          to represent the same body and surface.

          Syntax: vspear <meta-kernel>
          ./

          #include <stdio.h>
          #include <stdlib.h>
          #include "SpiceUsr.h"

          int main( int argc,  char **argv )
          {
             /.
             Local constants
             ./
             #define  DTOL           1.0e-14
             #define  FILSIZ         256
             #define  FRNMLN         33
             #define  BDNMLN         37
             #define  TYPLEN         5
             #define  INTLEN         12
             #define  MAXN           100000
             #define  MAXSRF         100

             /.
             Local variables
             ./
             static SpiceBoolean     fndarr [MAXN];
             SpiceBoolean            found;

             SpiceChar               dsk1   [FILSIZ];
             SpiceChar               fixref [FRNMLN];
             SpiceChar               source [FILSIZ];
             SpiceChar               filtyp [TYPLEN];
             SpiceChar               target [BDNMLN];

             SpiceDLADescr           dladsc;

             SpiceDSKDescr           dskdsc;

             SpiceDouble             d;
             static SpiceDouble      dirarr[MAXN][3];
             SpiceDouble             et;
             SpiceDouble             lat;
             SpiceDouble             latcrd[3];
             SpiceDouble             latstp;
             SpiceDouble             lon;
             SpiceDouble             lonstp;
             SpiceDouble             polmrg;
             SpiceDouble             r;
             SpiceDouble             radius;
             SpiceDouble             vlat;
             SpiceDouble             vlon;
             SpiceDouble             vrad;
             static SpiceDouble      vtxarr[MAXN][3];
             static SpiceDouble      xptarr[MAXN][3];
             SpiceDouble             xyzhit[3];

             SpiceInt                bodyid;
             SpiceInt                framid;
             SpiceInt                handle;
             SpiceInt                i;
             SpiceInt                nderr;
             SpiceInt                nhits;
             SpiceInt                nlstep;
             SpiceInt                nrays;
             SpiceInt                nsurf;
             static SpiceInt         srflst [MAXSRF];
             SpiceInt                surfid;

             chkin_c ( "vspear" );

             /.
             Get meta-kernel name from the command line.
             ./
             if ( argc != 2 )
             {
                printf ( "Command syntax:  spearv <meta-kernel>\n" );
                exit(1);
             }

             /.
             Load the meta-kernel.
             ./
             furnsh_c ( argv[1] );

             /.
             Get a handle for one of the loaded DSKs,
             then find the first segment and extract
             the body and surface IDs.
             ./
             kdata_c ( 0,    "DSK",  FILSIZ, TYPLEN,  FILSIZ,
                       dsk1, filtyp, source, &handle, &found );
             if ( !found )
             {
                sigerr_c ( "SPICE(NOINFO)" );
             }

             dlabfs_c ( handle, &dladsc, &found );

             if ( !found )
             {
                sigerr_c ( "SPICE(NOSEGMENT)" );
             }

             dskgd_c ( handle, &dladsc, &dskdsc );

             bodyid = dskdsc.center;
             surfid = dskdsc.surfce;
             framid = dskdsc.frmcde;

             bodc2n_c ( bodyid, BDNMLN, target, &found );

             if ( !found )
             {
                setmsg_c ( "Cannot map body ID # to a name." );
                errint_c ( "#", bodyid                       );
                sigerr_c ( "SPICE(BODYNAMENOTFOUND)"         );
             }

             frmnam_c ( framid, FRNMLN, fixref );

             if ( eqstr_c( fixref, " " ) )
             {
                setmsg_c ( "Cannot map frame ID # to a name." );
                errint_c ( "#", framid                        );
                sigerr_c ( "SPICE(BODYNAMENOTFOUND)"          );
             }

             /.
             Set the magnitude of the ray vertices. Use a large
             number to ensure the vertices are outside of
             any realistic target.
             ./
             r = 1.0e10;

             /.
             Spear the target with rays pointing toward
             the origin.  Use a grid of ray vertices
             located on a sphere enclosing the target.

             The variable `polmrg' ("pole margin") can
             be set to a small positive value to reduce
             the number of intercepts done at the poles.
             This may speed up the computation for
             the multi-segment case, since rays parallel
             to the Z axis will cause all segments converging
             at the pole of interest to be tested for an
             intersection.
             ./

             polmrg =    0.5;
             latstp =    1.0;
             lonstp =    2.0;

             nhits  =    0;
             nderr  =    0;

             lon    = -180.0;
             lat    =   90.0;
             nlstep =    0;
             nrays  =    0;

             /.
             Generate rays.
             ./
             while ( lon < 180.0 )
             {
                while ( nlstep <= 180 )
                {
                   if ( lon == 180.0 )
                   {
                      lat = 90.0 - nlstep*latstp;
                   }
                   else
                   {
                      if ( nlstep == 0 )
                      {
                         lat =  90.0 - polmrg;
                      }
                      else if ( nlstep == 180 )
                      {
                         lat = -90.0 + polmrg;
                      }
                      else
                      {
                         lat =  90.0 - nlstep*latstp;
                      }
                   }

                   latrec_c ( r,             lon*rpd_c(),
                              lat*rpd_c(),   vtxarr[nrays] );

                   vminus_c ( vtxarr[nrays], dirarr[nrays] );

                   ++ nrays;
                   ++ nlstep;
                }

                lon   += lonstp;
                lat    = 90.0;
                nlstep = 0;
             }

             /.
             Assign surface ID list.

             Note that, if we knew that all files had the desired
             surface ID, we could set `nsurf' to 0 and omit the
             initialization of the surface ID list.
             ./
             nsurf     = 1;
             srflst[0] = surfid;

             printf ( "Computing intercepts...\n" );

             dskxv_c ( SPICEFALSE,  target,  nsurf,  srflst,
                       et,          fixref,  nrays,  vtxarr,
                       dirarr,      xptarr,  fndarr         );

             printf ( "Done.\n" );

             /.
             Check results.
             ./
             for ( i = 0;  i < nrays;  i++ )
             {
                if ( fndarr[i] )
                {
                   /.
                   Record that a new intercept was found.
                   ./
                   ++ nhits;

                   /.
                   Compute the latitude and longitude of
                   the intercept. Make sure these agree
                   well with those of the vertex.
                   ./
                   reclat_c ( xptarr[i], latcrd, latcrd+1, latcrd+2 );
                   radius = latcrd[0];

                   /.
                   Recover the vertex longitude and latitude.
                   ./
                   reclat_c ( vtxarr[i], &vrad, &vlon, &vlat  );
                   latrec_c ( radius,     vlon,  vlat, xyzhit );

                   d = vdist_c( xptarr[i], xyzhit );

                   if ( d/r > DTOL )
                   {
                      printf ( "===========================\n" );
                      printf ( "Lon = %f;  Lat = %f\n",
                               lon, lat                        );
                      printf ( "Bad intercept\n"               );
                      printf ( "Distance error = %e\n", d      );
                      printf ( "xpt    = (%e %e %e)\n",
                               xptarr[i][0], xptarr[i][1], xptarr[i][2] );
                      printf ( "xyzhit = (%e %e %e)\n",
                               xyzhit[0], xyzhit[1], xyzhit[2] );

                      ++ nderr;
                   }
                }
                else
                {
                   /.
                   Missing the target entirely is a fatal error.

                   This is true only for this program, not in
                   general. For example, if the target shape is
                   a torus, many rays would miss the target.
                   ./
                   printf ( "===========================\n" );
                   printf ( "Lon = %f;  Lat = %f\n",
                            lon, lat                        );
                   printf ( "No intercept\n"                );
                   exit( 1 );
                }
             }
             printf( "nrays = %d\n", (int)nrays );
             printf( "nhits = %d\n", (int)nhits );
             printf( "nderr = %d\n", (int)nderr );

             return ( 0 );
          }

       When this program was executed on a PC/Linux/gcc 64-bit
       platform, the output was:

          Computing intercepts...
          Done.
          nrays = 32580
          nhits = 32580
          nderr = 0

#### Restrictions

       1)  The frame designated by `fixref' must have a fixed
           orientation relative to the frame of any DSK segment
           used in the computation. This routine has no
           practical way of ensuring that this condition is met;
           so this responsibility is delegated to the calling
           application.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman    (JPL)

#### Version

       -CSPICE Version 1.0.0, 26-FEB-2016 (NJB)

#### Index_Entries

       vectorized ray-surface intercept
       vectorized ray-dsk intercept

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

    Wed Apr  5 17:54:32 2017