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Diffstat (limited to 'ast/erfa/pap.c')
| -rw-r--r-- | ast/erfa/pap.c | 148 |
1 files changed, 148 insertions, 0 deletions
diff --git a/ast/erfa/pap.c b/ast/erfa/pap.c new file mode 100644 index 0000000..afd8651 --- /dev/null +++ b/ast/erfa/pap.c @@ -0,0 +1,148 @@ +#include "erfa.h" + +double eraPap(double a[3], double b[3]) +/* +** - - - - - - - +** e r a P a p +** - - - - - - - +** +** Position-angle from two p-vectors. +** +** Given: +** a double[3] direction of reference point +** b double[3] direction of point whose PA is required +** +** Returned (function value): +** double position angle of b with respect to a (radians) +** +** Notes: +** +** 1) The result is the position angle, in radians, of direction b with +** respect to direction a. It is in the range -pi to +pi. The +** sense is such that if b is a small distance "north" of a the +** position angle is approximately zero, and if b is a small +** distance "east" of a the position angle is approximately +pi/2. +** +** 2) The vectors a and b need not be of unit length. +** +** 3) Zero is returned if the two directions are the same or if either +** vector is null. +** +** 4) If vector a is at a pole, the result is ill-defined. +** +** Called: +** eraPn decompose p-vector into modulus and direction +** eraPm modulus of p-vector +** eraPxp vector product of two p-vectors +** eraPmp p-vector minus p-vector +** eraPdp scalar product of two p-vectors +** +** Copyright (C) 2013-2016, NumFOCUS Foundation. +** Derived, with permission, from the SOFA library. See notes at end of file. +*/ +{ + double am, au[3], bm, st, ct, xa, ya, za, eta[3], xi[3], a2b[3], pa; + + +/* Modulus and direction of the a vector. */ + eraPn(a, &am, au); + +/* Modulus of the b vector. */ + bm = eraPm(b); + +/* Deal with the case of a null vector. */ + if ((am == 0.0) || (bm == 0.0)) { + st = 0.0; + ct = 1.0; + } else { + + /* The "north" axis tangential from a (arbitrary length). */ + xa = a[0]; + ya = a[1]; + za = a[2]; + eta[0] = -xa * za; + eta[1] = -ya * za; + eta[2] = xa*xa + ya*ya; + + /* The "east" axis tangential from a (same length). */ + eraPxp(eta, au, xi); + + /* The vector from a to b. */ + eraPmp(b, a, a2b); + + /* Resolve into components along the north and east axes. */ + st = eraPdp(a2b, xi); + ct = eraPdp(a2b, eta); + + /* Deal with degenerate cases. */ + if ((st == 0.0) && (ct == 0.0)) ct = 1.0; + } + +/* Position angle. */ + pa = atan2(st, ct); + + return pa; + +} +/*---------------------------------------------------------------------- +** +** +** Copyright (C) 2013-2016, NumFOCUS Foundation. +** All rights reserved. +** +** This library is derived, with permission, from the International +** Astronomical Union's "Standards of Fundamental Astronomy" library, +** available from http://www.iausofa.org. +** +** The ERFA version is intended to retain identical functionality to +** the SOFA library, but made distinct through different function and +** file names, as set out in the SOFA license conditions. The SOFA +** original has a role as a reference standard for the IAU and IERS, +** and consequently redistribution is permitted only in its unaltered +** state. The ERFA version is not subject to this restriction and +** therefore can be included in distributions which do not support the +** concept of "read only" software. +** +** Although the intent is to replicate the SOFA API (other than +** replacement of prefix names) and results (with the exception of +** bugs; any that are discovered will be fixed), SOFA is not +** responsible for any errors found in this version of the library. +** +** If you wish to acknowledge the SOFA heritage, please acknowledge +** that you are using a library derived from SOFA, rather than SOFA +** itself. +** +** +** TERMS AND CONDITIONS +** +** Redistribution and use in source and binary forms, with or without +** modification, are permitted provided that the following conditions +** are met: +** +** 1 Redistributions of source code must retain the above copyright +** notice, this list of conditions and the following disclaimer. +** +** 2 Redistributions in binary form must reproduce the above copyright +** notice, this list of conditions and the following disclaimer in +** the documentation and/or other materials provided with the +** distribution. +** +** 3 Neither the name of the Standards Of Fundamental Astronomy Board, +** the International Astronomical Union nor the names of its +** contributors may be used to endorse or promote products derived +** from this software without specific prior written permission. +** +** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS +** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT +** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS +** FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE +** COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, +** INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, +** BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; +** LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER +** CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +** LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN +** ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE +** POSSIBILITY OF SUCH DAMAGE. +** +*/ |