| dskxsi_c |
|
Table of contents
Procedure
dskxsi_c (DSK, ray-surface intercept with source information)
void dskxsi_c ( SpiceBoolean pri,
ConstSpiceChar * target,
SpiceInt nsurf,
ConstSpiceInt srflst [],
SpiceDouble et,
ConstSpiceChar * fixref,
ConstSpiceDouble vertex [3],
ConstSpiceDouble raydir [3],
SpiceInt maxd,
SpiceInt maxi,
SpiceDouble xpt [3],
SpiceInt * handle,
SpiceDLADescr * dladsc,
SpiceDSKDescr * dskdsc,
SpiceDouble dc [],
SpiceInt ic [],
SpiceBoolean * found )
AbstractCompute a ray-surface intercept using data provided by multiple loaded DSK segments. Return information about the source of the data defining the surface on which the intercept was found: DSK handle, DLA and DSK descriptors, and DSK data type-dependent parameters. Required_ReadingCK DSK FRAMES PCK SPK TIME KeywordsGEOMETRY 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.
vertex I Vertex of ray.
raydir I Direction vector of ray.
maxd I Size of DC array.
maxi I Size of IC array.
xpt O Intercept point.
handle O Handle of segment contributing surface data.
dladsc O DLA descriptor of segment.
dskdsc O DSK descriptor of segment.
dc O Double precision component of source info.
ic O Integer component of source info.
found O Found flag.
SPICE_DSKXSI_DCSIZE
P Required size of DC array.
SPICE_DSKXSI_ICSIZE
P Required size of IC 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 array containing the list. Only DSK segments for
the body designated by `target' and having surface IDs in
this list will be 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 in 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.
vertex,
raydir are, respectively, the vertex and direction vector of the
ray to be used in the intercept computation.
Both the vertex and ray's direction vector must be
represented in the reference frame designated by `fixref'.
The vertex is considered to be an offset from the target
body.
maxd,
maxi are, respectively, the declared sizes of the arrays `dc' and
`ic'. `maxd' must be at least SPICE_DSKXSI_DCSIZE, while `maxi'
must be at least SPICE_DSKXSI_ICSIZE. See the -Parameters
section for details.
Detailed_Output
xpt is the intercept of the input ray on the surface
specified by the inputs
pri
target
nsurf
srflst
et
if such an intercept exists. If the ray intersects the
surface at multiple points, the one closest to the ray's
vertex is selected.
`xpt' is defined if and only if `found' is SPICETRUE.
Units are km.
handle,
dladsc,
dskdsc are, respectively, the DSK file handle, DLA descriptor,
and DSK descriptor of the DSK file and segment that
contributed the surface data on which the intercept was
found.
These outputs are defined if and only if `found' is SPICETRUE.
dc,
ic are, respectively, double precision and integer arrays
that may contain additional information associated with
the segment contributing the surface data on which the
intercept was found. The information is DSK data
type-dependent.
For DSK type 2 segments
ic[0] is the intercept plate ID. `dc' is unused.
These outputs are defined if and only if `found' is
SPICETRUE.
The declared length of `dc' must be at least
SPICE_DSKXSI_DCSIZE; the declared length of `ic' must be at
least SPICE_DSKXSI_ICSIZE. See the -Parameters section for
details.
found is a logical flag that is set to SPICETRUE if and only if
and intercept was found.
Parameters
See the header file
SpiceDSK.h
for declarations of size parameters for the output arguments
dc
ic
See the header files
SpiceDLA.h
SpiceDSK.h
for declarations of DLA and DSK descriptor sizes and
documentation of the contents of these descriptors.
See the header file
SpiceDtl.h
for the values of tolerance parameters used by default by the
ray-surface intercept algorithm. These are discussed in the
-Particulars section below.
Exceptions
1) If the input prioritization flag `pri' is set to SPICETRUE, the
error SPICE(BADPRIORITYSPEC) is signaled by a routine in the
call tree of this routine.
2) If the input body name `target' cannot be mapped to an ID code,
the error SPICE(IDCODENOTFOUND) is signaled by a routine in
the call tree of this routine.
3) If the input frame name `fixref' cannot be mapped to an ID code,
the error SPICE(IDCODENOTFOUND) is signaled by a routine in
the call tree of this routine.
4) If the frame center associated with `fixref' cannot be
retrieved, the error SPICE(NOFRAMEINFO) is signaled by a
routine in the call tree of this routine.
5) If the frame center associated with `fixref' is not the target
body, the error SPICE(INVALIDFRAME) is signaled by a routine
in the call tree of this routine.
6) If `maxd' is less than SPICE_DSKXSI_DCSIZE or `maxi' is less
than SPICE_DSKXSI_ICSIZE, the error SPICE(ARRAYTOOSMALL) is
signaled by a routine in the call tree of this routine.
7) If `nsurf' is less than 0, the error SPICE(INVALIDCOUNT)
is signaled by a routine in the call tree of this routine.
8) If an error occurs during the intercept computation, the error
is signaled by a routine in the call tree of this routine.
9) If any of the `target' or `fixref' input string pointers is
null, the error SPICE(NULLPOINTER) is signaled.
10) If any of the `target' or `fixref' input strings has zero
length, the error SPICE(EMPTYSTRING) is 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.
ParticularsThis is the lowest-level public interface for computing ray-surface intercepts, where the surface is modeled using topographic data provided by DSK files. The highest-level interface for this purpose is sincpt_c. In cases where the data source information returned by this routine are not needed, the routine dskxv_c may be more suitable. 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. This routine enables calling applications to identify the source of the data defining the surface on which an intercept was found. The file, segment, and segment-specific information such as a DSK type 2 plate ID are returned. This routine can be used for improved efficiency in situations in which multiple ray-surface intercepts are to be performed using a constant ray vertex. 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 this example 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: dskxsi_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
KERNELS_TO_LOAD = ( 'phobos512.bds' )
\begintext
End of meta-kernel
Example code begins here.
/.
Program dskxsi_ex1
Multi-segment spear program.
This program expects all loaded DSKs
to represent the same body and surface.
./
#include <stdio.h>
#include <stdlib.h>
#include "SpiceUsr.h"
int main( )
{
/.
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
#define MAXD SPICE_DSKXSI_DCSIZE
#define MAXI SPICE_DSKXSI_ICSIZE
/.
Local variables
./
SpiceBoolean found;
SpiceChar dsk1 [FILSIZ];
SpiceChar filtyp [TYPLEN];
SpiceChar fixref [FRNMLN];
SpiceChar meta [FILSIZ];
SpiceChar source [FILSIZ];
SpiceChar target [BDNMLN];
SpiceDLADescr dladsc;
SpiceDLADescr xptDLAdsc;
SpiceDSKDescr dskdsc;
SpiceDSKDescr xptDSKdsc;
SpiceDouble d;
SpiceDouble dc [1];
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 xpt [3];
SpiceDouble xyzhit[3];
SpiceInt bodyid;
SpiceInt framid;
SpiceInt handle;
SpiceInt i;
SpiceInt ic [1];
SpiceInt nderr;
SpiceInt nhits;
SpiceInt nlstep;
SpiceInt nrays;
SpiceInt nsurf;
static SpiceInt srflst [MAXSRF];
SpiceInt surfid;
SpiceInt xpthan;
chkin_c ( "spear" );
/.
Prompt for the name of the meta-kernel.
./
prompt_c ( "Enter meta-kernel name > ", FILSIZ, meta );
/.
Load the meta-kernel.
./
furnsh_c ( meta );
/.
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;
/.
Set the epoch for interval selection.
./
et = 0.0;
/.
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 ( "\nComputing intercepts...\n" );
for ( i = 0; i < nrays; i++ )
{
/.
Find the surface intercept of the ith ray.
./
dskxsi_c ( SPICEFALSE, target, nsurf, srflst,
et, fixref, vtxarr[i], dirarr[i],
MAXD, MAXI, xpt, &xpthan,
&xptDLAdsc, &xptDSKdsc, dc, ic,
&found );
if ( found )
{
/.
Record that a new intercept was found.
./
++ nhits;
/.
Check results.
Compute the latitude and longitude of
the intercept. Make sure these agree
well with those of the vertex.
./
reclat_c ( xpt, 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( xpt, 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",
xpt[0], xpt[1], xpt[2] );
printf ( "xyzhit = (%e %e %e)\n",
xyzhit[0], xyzhit[1], xyzhit[2] );
/.
Display the intercept segment's plate ID if
applicable.
./
if ( xptDSKdsc.dtype == 2 )
{
printf ( "Plate ID = %d\n", (int)ic[0] );
}
++ 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 ( "Done.\n\n" );
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 Mac/Intel/cc/64-bit
platform, using as input the meta-kernel dskxsi_ex1.tm, the
output was:
Enter meta-kernel name > dskxsi_ex1.tm
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_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) Version
-CSPICE Version 1.0.1, 06-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
Updated code example to prompt for input meta-kernel name and
set input time to zero.
Added exception entry #7.
-CSPICE Version 1.0.0, 04-APR-2017 (NJB)
Index_EntriesDSK ray-surface intercept with source information DSK ray-surface intercept with handle and descriptors |
Fri Dec 31 18:41:05 2021