| cgv2el_c |
|
Table of contents
Procedure
cgv2el_c ( Center and generating vectors to ellipse )
void cgv2el_c ( ConstSpiceDouble center[3],
ConstSpiceDouble vec1 [3],
ConstSpiceDouble vec2 [3],
SpiceEllipse * ellips )
AbstractForm a SPICE ellipse from a center vector and two generating vectors. Required_ReadingELLIPSES KeywordsELLIPSE GEOMETRY Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- center, vec1, vec2 I Center and two generating vectors for an ellipse. ellips O The SPICE ellipse defined by the input vectors. Detailed_Input
center,
vec1,
vec2 are a center and two generating vectors defining
an ellipse in three-dimensional space. The
ellipse is the set of points
center + cos(theta) * vec1 + sin(theta) * vec2
where theta ranges over the interval (-pi, pi].
`vec1' and `vec2' need not be linearly independent.
Detailed_Output
ellips is the SPICE ellipse defined by the input
vectors.
ParametersNone. Exceptions
1) If `vec1' and `vec2' are linearly dependent, `ellips' will be
degenerate. SPICE ellipses are allowed to represent
degenerate geometric ellipses.
FilesNone. Particulars
SPICE ellipses serve to simplify calling sequences and reduce
the chance for error in declaring and describing argument lists
involving ellipses.
The set of ellipse conversion routines is
cgv2el_c ( Center and generating vectors to ellipse )
el2cgv_c ( Ellipse to center and generating vectors )
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) Create a SPICE ellipse given its center and two linearly
independent generating vectors of the ellipse.
Example code begins here.
/.
Program cgv2el_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main( )
{
/.
Local variables.
./
SpiceDouble ecentr [3];
SpiceDouble smajor [3];
SpiceDouble sminor [3];
SpiceEllipse ellips;
/.
Define the center and two linearly independent
generating vectors of an ellipse (the vectors need not
be linearly independent).
./
SpiceDouble center [3] = { -1.0, 1.0, -1.0 };
SpiceDouble vec1 [3] = { 1.0, 1.0, 1.0 };
SpiceDouble vec2 [3] = { 1.0, -1.0, 1.0 };
/.
Create the `ellips'.
./
cgv2el_c ( center, vec1, vec2, &ellips );
/.
In a real application, please use CSPICE API el2cgv_c
to retrieve the center and generating vectors from the
ellipse structure (see next block).
./
printf( "SPICE ellipse:\n" );
printf( " Semi-minor axis: %9.6f %9.6f %9.6f\n",
ellips.semiMinor[0],
ellips.semiMinor[1],
ellips.semiMinor[2] );
printf( " Semi-major axis: %9.6f %9.6f %9.6f\n",
ellips.semiMajor[0],
ellips.semiMajor[1],
ellips.semiMajor[2] );
printf( " Center : %9.6f %9.6f %9.6f\n",
ellips.center[0],
ellips.center[1],
ellips.center[2] );
printf( " \n" );
/.
Obtain the center and generating vectors from the
`ellips'.
./
el2cgv_c ( &ellips, ecentr, smajor, sminor );
printf( "SPICE ellipse (using el2cgv_c):\n" );
printf( " Semi-minor axis: %9.6f %9.6f %9.6f\n",
sminor[0], sminor[1], sminor[2] );
printf( " Semi-major axis: %9.6f %9.6f %9.6f\n",
smajor[0], smajor[1], smajor[2] );
printf( " Center : %9.6f %9.6f %9.6f\n",
ecentr[0], ecentr[1], ecentr[2] );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
SPICE ellipse:
Semi-minor axis: 0.000000 1.414214 0.000000
Semi-major axis: 1.414214 -0.000000 1.414214
Center : -1.000000 1.000000 -1.000000
SPICE ellipse (using el2cgv_c):
Semi-minor axis: 0.000000 1.414214 0.000000
Semi-major axis: 1.414214 -0.000000 1.414214
Center : -1.000000 1.000000 -1.000000
2) Find the intersection of an ellipse with a plane.
Example code begins here.
/.
Program cgv2el_ex2
./
#include <stdio.h>
#include "SpiceUsr.h"
int main( )
{
/.
Local variables.
./
SpiceEllipse ellips;
SpicePlane plane;
SpiceDouble xpts [2][3];
SpiceInt i;
SpiceInt nxpts;
/.
The ellipse is defined by the vectors `center', `vec1', and
`vec2'. The plane is defined by the normal vector `normal'
and the `center'.
./
SpiceDouble center [3] = { 0.0, 0.0, 0.0 };
SpiceDouble vec1 [3] = { 1.0, 7.0, 2.0 };
SpiceDouble vec2 [3] = { -1.0, 1.0, 3.0 };
SpiceDouble normal [3] = { 0.0, 1.0, 0.0 };
/.
Make a SPICE ellipse and a plane.
./
cgv2el_c ( center, vec1, vec2, &ellips );
nvp2pl_c ( normal, center, &plane );
/.
Find the intersection of the ellipse and plane.
`nxpts' is the number of intersection points; `xpts'
are the points themselves.
./
inelpl_c ( &ellips, &plane, &nxpts, xpts[0], xpts[1] );
printf( "Number of intercept points: %2d\n", nxpts );
for ( i = 0; i < nxpts; i++ )
{
printf( " Point %1d: %9.6f %9.6f %9.6f\n", i,
xpts[i][0], xpts[i][1], xpts[i][2] );
}
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Number of intercept points: 2
Point 0: 1.131371 0.000000 -2.687006
Point 1: -1.131371 -0.000000 2.687006
RestrictionsNone. Literature_ReferencesNone. Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) Version
-CSPICE Version 1.1.0, 24-AUG-2021 (JDR)
Edited the header to comply with NAIF standard.
Added complete code examples.
Changed the output argument name "ellipse" to "ellips" for
consistency with other routines.
-CSPICE Version 1.0.0, 05-MAR-1999 (NJB)
Index_Entriescenter and generating vectors to ellipse |
Fri Dec 31 18:41:02 2021