dskxv_c |

## Procedurevoid 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[] ) ## AbstractCompute ray-surface intercepts for a set of rays, using data provided by multiple loaded DSK segments. ## Required_ReadingCK DSK FRAMES PCK SPK TIME ## KeywordsGEOMETRY INTERCEPT SURFACE TOPOGRAPHY ## Brief_I/OVARIABLE 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_Inputpri 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_Outputxptarr 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. ## ParametersSee 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. ## Exceptions1) 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. ## FilesAppropriate 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 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. ## ExamplesThe 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" ); ## Restrictions1) 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) ## Version-CSPICE Version 1.0.0, 26-FEB-2016 (NJB) ## Index_Entriesvectorized ray-surface intercept vectorized ray-dsk intercept |

Wed Apr 5 17:54:32 2017