spc.h

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/*============================================================================

  WCSLIB 4.4 - an implementation of the FITS WCS standard.
  Copyright (C) 1995-2009, Mark Calabretta

  This file is part of WCSLIB.

  WCSLIB is free software: you can redistribute it and/or modify it under the
  terms of the GNU Lesser General Public License as published by the Free
  Software Foundation, either version 3 of the License, or (at your option)
  any later version.

  WCSLIB is distributed in the hope that it will be useful, but WITHOUT ANY
  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for
  more details.

  You should have received a copy of the GNU Lesser General Public License
  along with WCSLIB.  If not, see <http://www.gnu.org/licenses/>.

  Correspondence concerning WCSLIB may be directed to:
    Internet email: mcalabre@atnf.csiro.au
    Postal address: Dr. Mark Calabretta
                    Australia Telescope National Facility, CSIRO
                    PO Box 76
                    Epping NSW 1710
                    AUSTRALIA

  Author: Mark Calabretta, Australia Telescope National Facility
  http://www.atnf.csiro.au/~mcalabre/index.html
  $Id: spc_8h-source.html,v 1.1 2009/09/14 20:25:24 irby Exp $
*=============================================================================
*
* WCSLIB 4.4 - C routines that implement the spectral coordinate systems
* recognized by the FITS World Coordinate System (WCS) standard.  Refer to
*
*   "Representations of world coordinates in FITS",
*   Greisen, E.W., & Calabretta, M.R. 2002, A&A, 395, 1061 (Paper I)
*
*   "Representations of spectral coordinates in FITS",
*   Greisen, E.W., Calabretta, M.R., Valdes, F.G., & Allen, S.L.
*   2006, A&A, 446, 747 (Paper III)
*
* Refer to the README file provided with WCSLIB for an overview of the
* library.
*
*
* Summary of the spc routines
* ---------------------------
* These routines implement the part of the FITS WCS standard that deals with
* spectral coordinates.  They define methods to be used for computing spectral
* world coordinates from intermediate world coordinates (a linear
* transformation of image pixel coordinates), and vice versa.  They are based
* on the spcprm struct which contains all information needed for the
* computations.  The struct contains some members that must be set by the
* user, and others that are maintained by these routines, somewhat like a
* C++ class but with no encapsulation.
*
* Routine spcini() is provided to initialize the spcprm struct with default
* values, and another, spcprt(), to print its contents.
*
* A setup routine, spcset(), computes intermediate values in the spcprm struct
* from parameters in it that were supplied by the user.  The struct always
* needs to be set up by spcset() but it need not be called explicitly - refer
* to the explanation of spcprm::flag.
*
* spcx2s() and spcs2x() implement the WCS spectral coordinate transformations.
* In fact, they are high level driver routines for the lower level spectral
* coordinate transformation routines described in spx.h.
*
* A number of routines are provided to aid in analysing or synthesising sets
* of FITS spectral axis keywords:
*
*   - spctyp() checks a spectral CTYPEia keyword for validity and returns
*     information derived from it.
*
*   - Spectral keyword analysis routine spcspx() computes the values of the
*     X-type spectral variables for the S-type variables supplied.
*
*   - Spectral keyword synthesis routine, spcxps(), computes the S-type
*     variables for the X-types supplied.
*
*   - Given a set of spectral keywords, a translation routine, spctrn(),
*     produces the corresponding set for the specified spectral CTYPEia.
*
* Spectral variable types - S, P, and X:
* --------------------------------------
* A few words of explanation are necessary regarding spectral variable types
* in FITS.
*
* Every FITS spectral axis has three associated spectral variables:
*
*   S-type: the spectral variable in which coordinates are to be
*     expressed.  Each S-type is encoded as four characters and is
*     linearly related to one of four basic types as follows:
*
*     F: frequency
*       'FREQ':  frequency
*       'AFRQ':  angular frequency
*       'ENER':  photon energy
*       'WAVN':  wave number
*       'VRAD':  radio velocity
*
*     W: wavelength in vacuo
*       'WAVE':  wavelength
*       'VOPT':  optical velocity
*       'ZOPT':  redshift
*
*     A: wavelength in air
*       'AWAV':  wavelength in air
*
*     V: velocity
*       'VELO':  relativistic velocity
*       'BETA':  relativistic beta factor
*
*     The S-type forms the first four characters of the CTYPEia keyvalue,
*     and CRVALia and CDELTia are expressed as S-type quantities so that
*     they provide a first-order approximation to the S-type variable at
*     the reference point.
*
*     Note that 'AFRQ', angular frequency, is additional to the variables
*     defined in WCS Paper III.
*
*   P-type: the basic spectral variable (F, W, A, or V) with which the
*     S-type variable is associated (see list above).
*
*     For non-grism axes, the P-type is encoded as the eighth character of
*     CTYPEia.
*
*   X-type: the basic spectral variable (F, W, A, or V) for which the
*     spectral axis is linear, grisms excluded (see below).
*
*     For non-grism axes, the X-type is encoded as the sixth character of
*     CTYPEia.
*
*   Grisms: Grism axes have normal S-, and P-types but the axis is linear,
*     not in any spectral variable, but in a special "grism parameter".
*     The X-type spectral variable is either W or A for grisms in vacuo or
*     air respectively, but is encoded as 'w' or 'a' to indicate that an
*     additional transformation is required to convert to or from the
*     grism parameter.  The spectral algorithm code for grisms also has a
*     special encoding in CTYPEia, either 'GRI' (in vacuo) or 'GRA' (in air).
*
* In the algorithm chain, the non-linear transformation occurs between the
* X-type and the P-type variables; the transformation between P-type and
* S-type variables is always linear.
*
* When the P-type and X-type variables are the same, the spectral axis is
* linear in the S-type variable and the second four characters of CTYPEia
* are blank.  This can never happen for grism axes.
*
* As an example, correlating radio spectrometers always produce spectra that
* are regularly gridded in frequency; a redshift scale on such a spectrum is
* non-linear.  The required value of CTYPEia would be 'ZOPT-F2W', where the
* desired S-type is 'ZOPT' (redshift), the P-type is necessarily 'W'
* (wavelength), and the X-type is 'F' (frequency) by the nature of the
* instrument.
*
* Argument checking:
* ------------------
* The input spectral values are only checked for values that would result in
* floating point exceptions.  In particular, negative frequencies and
* wavelengths are allowed, as are velocities greater than the speed of
* light.  The same is true for the spectral parameters - rest frequency and
* wavelength.
*
* Accuracy:
* ---------
* No warranty is given for the accuracy of these routines (refer to the
* copyright notice); intending users must satisfy for themselves their
* adequacy for the intended purpose.  However, closure effectively to within
* double precision rounding error was demonstrated by test routine tspc.c
* which accompanies this software.
*
*
* spcini() - Default constructor for the spcprm struct
* ----------------------------------------------------
* spcini() sets all members of a spcprm struct to default values.  It should
* be used to initialize every spcprm struct.
*
* Given and returned:
*   spc       struct spcprm*
*                       Spectral transformation parameters.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         1: Null spcprm pointer passed.
*
*
* spcprt() - Print routine for the spcprm struct
* ----------------------------------------------
* spcprt() prints the contents of a spcprm struct.
*
* Given:
*   spc       const struct spcprm*
*                       Spectral transformation parameters.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         1: Null spcprm pointer passed.
*
*
* spcset() - Setup routine for the spcprm struct
* ----------------------------------------------
* spcset() sets up a spcprm struct according to information supplied within
* it.
*
* Note that this routine need not be called directly; it will be invoked by
* spcx2s() and spcs2x() if spcprm::flag is anything other than a predefined
* magic value.
*
* Given and returned:
*   spc       struct spcprm*
*                       Spectral transformation parameters.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         1: Null spcprm pointer passed.
*                         2: Invalid spectral parameters.
*
*
* spcx2s() - Transform to spectral coordinates
* --------------------------------------------
* spcx2s() transforms intermediate world coordinates to spectral coordinates.
*
* Given and returned:
*   spc       struct spcprm*
*                       Spectral transformation parameters.
*
* Given:
*   nx        int       Vector length.
*   sx        int       Vector stride.
*   sspec     int       Vector stride.
*   x         const double[]
*                       Intermediate world coordinates, in SI units.
*
* Returned:
*   spec      double[]  Spectral coordinates, in SI units.
*   stat      int[]     Status return value status for each vector element:
*                         0: Success.
*                         1: Invalid value of x.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         1: Null spcprm pointer passed.
*                         2: Invalid spectral parameters.
*                         3: One or more of the x coordinates were invalid,
*                            as indicated by the stat vector.
*
*
* spcs2x() - Transform spectral coordinates
* -----------------------------------------
* spcs2x() transforms spectral world coordinates to intermediate world
* coordinates.
*
* Given and returned:
*   spc       struct spcprm*
*                       Spectral transformation parameters.
*
* Given:
*   nspec     int       Vector length.
*   sspec     int       Vector stride.
*   sx        int       Vector stride.
*   spec      const double[]
*                       Spectral coordinates, in SI units.
*
* Returned:
*   x         double[]  Intermediate world coordinates, in SI units.
*   stat      int[]     Status return value status for each vector element:
*                         0: Success.
*                         1: Invalid value of spec.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         1: Null spcprm pointer passed.
*                         2: Invalid spectral parameters.
*                         4: One or more of the spec coordinates were
*                            invalid, as indicated by the stat vector.
*
*
* spctyp() - Spectral CTYPEia keyword analysis
* --------------------------------------------
* spctyp() checks whether a CTYPEia keyvalue is a valid spectral axis type and
* if so returns information derived from it relating to the associated S-, P-,
* and X-type spectral variables (see explanation above).  It recognizes and
* translates AIPS-convention spectral CTYPEia keyvalues.
*
* The return arguments are guaranteed not be modified if CTYPEia is not a
* valid spectral type; zero-pointers may be specified for any that are not of
* interest.
*
* Given:
*   ctype     const char[9]
*                       The CTYPEia keyvalue, (eight characters with null
*                       termination).
*
* Returned:
*   stype     char[]    The four-letter name of the S-type spectral variable
*                       copied or translated from ctype.  If a non-zero
*                       pointer is given, the array must accomodate a null-
*                       terminated string of length 5.
*   scode     char[]    The three-letter spectral algorithm code copied or
*                       translated from ctype.  Logarithmic ('LOG') and
*                       tabular ('TAB') codes are also recognized.  If a
*                       non-zero pointer is given, the array must accomodate a
*                       null-terminated string of length 4.
*   sname     char[]    Descriptive name of the S-type spectral variable.
*                       If a non-zero pointer is given, the array must
*                       accomodate a null-terminated string of length 22.
*   units     char[]    SI units of the S-type spectral variable.  If a
*                       non-zero pointer is given, the array must accomodate a
*                       null-terminated string of length 8.
*   ptype     char*     Character code for the P-type spectral variable
*                       derived from ctype, one of 'F', 'W', 'A', or 'V'.
*   xtype     char*     Character code for the X-type spectral variable
*                       derived from ctype, one of 'F', 'W', 'A', or 'V'.
*                       Also, 'w' and 'a' are synonymous to 'W' and 'A' for
*                       grisms in vacuo and air respectively.  Set to 'L' or
*                       'T' for logarithmic ('LOG') and tabular ('TAB') axes.
*   restreq   int*      Multivalued flag that indicates whether rest
*                       frequency or wavelength is required to compute
*                       spectral variables for this CTYPEia:
*                         0: Not required.
*                         1: Required for the conversion between S- and
*                            P-types (e.g. 'ZOPT-F2W').
*                         2: Required for the conversion between P- and
*                            X-types (e.g. 'BETA-W2V').
*                         3: Required for the conversion between S- and
*                            P-types, and between P- and X-types, but not
*                            between S- and X-types (this applies only for
*                            'VRAD-V2F', 'VOPT-V2W', and 'ZOPT-V2W').
*                        Thus the rest frequency or wavelength is required for
*                        spectral coordinate computations (i.e. between S- and
*                        X-types) only if restreq%3 != 0.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         2: Invalid spectral parameters (not a spectral
*                            CTYPEia).
*
*
* spcspx() - Spectral keyword analysis
* ------------------------------------
* spcspx() analyses the CTYPEia and CRVALia FITS spectral axis keyword values
* and returns information about the associated X-type spectral variable.
*
* Given:
*   ctypeS    const char[9]
*                       Spectral axis type, i.e. the CTYPEia keyvalue, (eight
*                       characters with null termination).  For non-grism
*                       axes, the character code for the P-type spectral
*                       variable in the algorithm code (i.e. the eighth
*                       character of CTYPEia) may be set to '?' (it will not
*                       be reset).
*   crvalS    double    Value of the S-type spectral variable at the reference
*                       point, i.e. the CRVALia keyvalue, SI units.
*   restfrq,
*   restwav   double    Rest frequency [Hz] and rest wavelength in vacuo [m],
*                       only one of which need be given, the other should be
*                       set to zero.  Neither are required if the translation
*                       is between wave-characteristic types, or between
*                       velocity-characteristic types.  E.g., required for
*                       'FREQ'     -> 'ZOPT-F2W', but not required for
*                       'VELO-F2V' -> 'ZOPT-F2W'.
*
* Returned:
*   ptype     char*     Character code for the P-type spectral variable
*                       derived from ctypeS, one of 'F', 'W', 'A', or 'V'.
*   xtype     char*     Character code for the X-type spectral variable
*                       derived from ctypeS, one of 'F', 'W', 'A', or 'V'.
*                       Also, 'w' and 'a' are synonymous to 'W' and 'A' for
*                       grisms in vacuo and air respectively; crvalX and dXdS
*                       (see below) will conform to these.
*   restreq   int*      Multivalued flag that indicates whether rest frequency
*                       or wavelength is required to compute spectral
*                       variables for this CTYPEia, as for spctyp().
*   crvalX    double*   Value of the X-type spectral variable at the reference
*                       point, SI units.
*   dXdS      double*   The derivative, dX/dS, evaluated at the reference
*                       point, SI units.  Multiply the CDELTia keyvalue by
*                       this to get the pixel spacing in the X-type spectral
*                       coordinate.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         2: Invalid spectral parameters.
*
*
* spcxps() - Spectral keyword synthesis
* -------------------------------------
* spcxps(), for the spectral axis type specified and the value provided for
* the X-type spectral variable at the reference point, deduces the value of
* the FITS spectral axis keyword CRVALia and also the derivative dS/dX which
* may be used to compute CDELTia.  See above for an explanation of the S-,
* P-, and X-type spectral variables.
*
* Given:
*   ctypeS    const char[9]
*                       The required spectral axis type, i.e. the CTYPEia
*                       keyvalue, (eight characters with null termination).
*                       For non-grism axes, the character code for the P-type
*                       spectral variable in the algorithm code (i.e. the
*                       eighth character of CTYPEia) may be set to '?' (it
*                       will not be reset).
*   crvalX    double    Value of the X-type spectral variable at the reference
*                       point (N.B. NOT the CRVALia keyvalue), SI units.
*   restfrq,
*   restwav   double    Rest frequency [Hz] and rest wavelength in vacuo [m],
*                       only one of which need be given, the other should be
*                       set to zero.  Neither are required if the translation
*                       is between wave-characteristic types, or between
*                       velocity-characteristic types.  E.g., required for
*                       'FREQ'     -> 'ZOPT-F2W', but not required for
*                       'VELO-F2V' -> 'ZOPT-F2W'.
*
* Returned:
*   ptype     char*     Character code for the P-type spectral variable
*                       derived from ctypeS, one of 'F', 'W', 'A', or 'V'.
*   xtype     char*     Character code for the X-type spectral variable
*                       derived from ctypeS, one of 'F', 'W', 'A', or 'V'.
*                       Also, 'w' and 'a' are synonymous to 'W' and 'A' for
*                       grisms; crvalX and cdeltX must conform to these.
*   restreq   int*      Multivalued flag that indicates whether rest frequency
*                       or wavelength is required to compute spectral
*                       variables for this CTYPEia, as for spctyp().
*   crvalS    double*   Value of the S-type spectral variable at the reference
*                       point (i.e. the appropriate CRVALia keyvalue), SI
*                       units.
*   dSdX      double*   The derivative, dS/dX, evaluated at the reference
*                       point, SI units.  Multiply this by the pixel spacing
*                       in the X-type spectral coordinate to get the CDELTia
*                       keyvalue.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         2: Invalid spectral parameters.
*
*
* spctrn() - Spectral keyword translation
* ---------------------------------------
* spctrn() translates a set of FITS spectral axis keywords into the
* corresponding set for the specified spectral axis type.  For example, a
* 'FREQ' axis may be translated into 'ZOPT-F2W' and vice versa.
*
* Given:
*   ctypeS1   const char[9]
*                       Spectral axis type, i.e. the CTYPEia keyvalue, (eight
*                       characters with null termination).  For non-grism
*                       axes, the character code for the P-type spectral
*                       variable in the algorithm code (i.e. the eighth
*                       character of CTYPEia) may be set to '?' (it will not
*                       be reset).  AIPS-convention spectral types are
*                       accepted for ctypeS1 but the Doppler frame encoded
*                       within them will not be used.
*   crvalS1   double    Value of the S-type spectral variable at the reference
*                       point, i.e. the CRVALia keyvalue, SI units.
*   cdeltS1   double    Increment of the S-type spectral variable at the
*                       reference point, SI units.
*   restfrq,
*   restwav   double    Rest frequency [Hz] and rest wavelength in vacuo [m],
*                       only one of which need be given, the other should be
*                       set to zero.  Neither are required if the translation
*                       is between wave-characteristic types, or between
*                       velocity-characteristic types.  E.g., required for
*                       'FREQ'     -> 'ZOPT-F2W', but not required for
*                       'VELO-F2V' -> 'ZOPT-F2W'.
*
* Given and returned:
*   ctypeS2   char[9]   Required spectral axis type (eight characters with
*                       null termination).  The first four characters are
*                       required to be given and are never modified.  The
*                       remaining four, the algorithm code, are completely
*                       determined by, and must be consistent with, ctypeS1
*                       and the first four characters of ctypeS2.  A non-zero
*                       status value will be returned if they are inconsistent
*                       (see below).  However, if the final three characters
*                       are specified as "???", or if just the eighth
*                       character is specified as '?', the correct algorithm
*                       code will be substituted (applies for grism axes as
*                       well as non-grism).
*
*                       AIPS-convention spectral types are not accepted for
*                       ctypeS2.
*
* Returned:
*   crvalS2   double*   Value of the new S-type spectral variable at the
*                       reference point, i.e. the new CRVALia keyvalue, SI
*                       units.
*   cdeltS2   double*   Increment of the new S-type spectral variable at the
*                       reference point, i.e. the new CDELTia keyvalue, SI
*                       units.
*
* Function return value:
*             int       Status return value:
*                         0: Success.
*                         2: Invalid spectral parameters.
*
*                       A status value of 2 will be returned if restfrq or
*                       restwav are not specified when required, or if ctypeS1
*                       or ctypeS2 are self-inconsistent, or have different
*                       spectral X-type variables.
*
*
* spcprm struct - Spectral transformation parameters
* --------------------------------------------------
* The spcprm struct contains information required to transform spectral
* coordinates.  It consists of certain members that must be set by the user
* ("given") and others that are set by the WCSLIB routines ("returned").  Some
* of the latter are supplied for informational purposes while others are for
* internal use only.
*
*   int flag
*     (Given and returned) This flag must be set to zero whenever any of the
*     following spcprm structure members are set or changed:
*
*       - spcprm::type,
*       - spcprm::code,
*       - spcprm::crval,
*       - spcprm::restfrq,
*       - spcprm::restwav,
*       - spcprm::pv[].
*
*     This signals the initialization routine, spcset(), to recompute the
*     returned members of the spcprm struct.  spcset() will reset flag to
*     indicate that this has been done.
*
*   char type[8]
*     (Given) Four-letter spectral variable type, e.g "ZOPT" for
*     CTYPEia = 'ZOPT-F2W'.  (Declared as char[8] for alignment reasons.)
*
*   char code[4]
*     (Given) Three-letter spectral algorithm code, e.g "F2W" for
*     CTYPEia = 'ZOPT-F2W'.
*
*   double crval
*     (Given) Reference value (CRVALia), SI units.
*
*   double restfrq
*     (Given) The rest frequency [Hz], and ...
*
*   double restwav
*     (Given) ... the rest wavelength in vacuo [m], only one of which need be
*     given, the other should be set to zero.  Neither are required if the
*     X and S spectral variables are both wave-characteristic, or both
*     velocity-characteristic, types.
*
*   double pv[7]
*     (Given) Grism parameters for 'GRI' and 'GRA' algorithm codes:
*       - 0: G, grating ruling density.
*       - 1: m, interference order.
*       - 2: alpha, angle of incidence [deg].
*       - 3: n_r, refractive index at the reference wavelength, lambda_r.
*       - 4: n'_r, dn/dlambda at the reference wavelength, lambda_r (/m).
*       - 5: epsilon, grating tilt angle [deg].
*       - 6: theta, detector tilt angle [deg].
*
* The remaining members of the spcprm struct are maintained by spcset() and
* must not be modified elsewhere:
*
*   double w[6]
*     (Returned) Intermediate values:
*       - 0: Rest frequency or wavelength (SI).
*       - 1: The value of the X-type spectral variable at the reference point
*           (SI units).
*       - 2: dX/dS at the reference point (SI units).
*      The remainder are grism intermediates.
*
*   int isGrism
*     (Returned) Grism coordinates?
*       - 0: no,
*       - 1: in vacuum,
*       - 2: in air.
*
*   int padding
*     (An unused variable inserted for alignment purposes only.)
*
*   int (*spxX2P)(SPX_ARGS)
*     (Returned) The first and ...
*   int (*spxP2S)(SPX_ARGS)
*     (Returned) ... the second of the pointers to the transformation
*     functions in the two-step algorithm chain X -> P -> S in the
*     pixel-to-spectral direction where the non-linear transformation is from
*     X to P.  The argument list, SPX_ARGS, is defined in spx.h.
*
*   int (*spxS2P)(SPX_ARGS)
*     (Returned) The first and ...
*   int (*spxP2X)(SPX_ARGS)
*     (Returned) ... the second of the pointers to the transformation
*     functions in the two-step algorithm chain S -> P -> X in the
*     spectral-to-pixel direction where the non-linear transformation is from
*     P to X.  The argument list, SPX_ARGS, is defined in spx.h.
*
*
* Global variable: const char *spc_errmsg[] - Status return messages
* ------------------------------------------------------------------
* Error messages to match the status value returned from each function.
*
*===========================================================================*/

#ifndef WCSLIB_SPC
#define WCSLIB_SPC

#include "spx.h"

#ifdef __cplusplus
extern "C" {
#endif


extern const char *spc_errmsg[];


struct spcprm {
  /* Initialization flag (see the prologue above).                          */
  /*------------------------------------------------------------------------*/
  int   flag;                   /* Set to zero to force initialization.     */

  /* Parameters to be provided (see the prologue above).                    */
  /*------------------------------------------------------------------------*/
  char   type[8];               /* Four-letter spectral variable type.      */
  char   code[4];               /* Three-letter spectral algorithm code.    */

  double crval;                 /* Reference value (CRVALia), SI units.     */
  double restfrq;               /* Rest frequency, Hz.                      */
  double restwav;               /* Rest wavelength, m.                      */

  double pv[7];                 /* Grism parameters:                        */
                                /*   0: G, grating ruling density.          */
                                /*   1: m, interference order.              */
                                /*   2: alpha, angle of incidence.          */
                                /*   3: n_r, refractive index at lambda_r.  */
                                /*   4: n'_r, dn/dlambda at lambda_r.       */
                                /*   5: epsilon, grating tilt angle.        */
                                /*   6: theta, detector tilt angle.         */

  /* Information derived from the parameters supplied.                      */
  /*------------------------------------------------------------------------*/
  double w[6];                  /* Intermediate values.                     */
                                /*   0: Rest frequency or wavelength (SI).  */
                                /*   1: CRVALX (SI units).                  */
                                /*   2: CDELTX/CDELTia = dX/dS (SI units).  */
                                /* The remainder are grism intermediates.   */

  int isGrism;                  /* Grism coordinates?  1: vacuum, 2: air.   */
  int padding;                  /* (Dummy inserted for alignment purposes.) */

  int (*spxX2P)(SPX_ARGS);      /* Pointers to the transformation functions */
  int (*spxP2S)(SPX_ARGS);      /* in the two-step algorithm chain in the   */
                                /* pixel-to-spectral direction.             */

  int (*spxS2P)(SPX_ARGS);      /* Pointers to the transformation functions */
  int (*spxP2X)(SPX_ARGS);      /* in the two-step algorithm chain in the   */
                                /* spectral-to-pixel direction.             */
};

/* Size of the spcprm struct in int units, used by the Fortran wrappers. */
#define SPCLEN (sizeof(struct spcprm)/sizeof(int))


int spcini(struct spcprm *spc);

int spcprt(const struct spcprm *spc);

int spcset(struct spcprm *spc);

int spcx2s(struct spcprm *spc, int nx, int sx, int sspec,
           const double x[], double spec[], int stat[]);

int spcs2x(struct spcprm *spc, int nspec, int sspec, int sx,
           const double spec[], double x[], int stat[]);

int spctyp(const char ctype[], char stype[], char scode[], char sname[],
           char units[], char *ptype, char *xtype, int *restreq);

int spcspx(const char ctypeS[], double crvalS, double restfrq, double restwav,
           char *ptype, char *xtype, int *restreq, double *crvalX,
           double *dXdS);

int spcxps(const char ctypeS[], double crvalX, double restfrq, double restwav,
           char *ptype, char *xtype, int *restreq, double *crvalS,
           double *dSdX);

int spctrn(const char ctypeS1[], double crvalS1, double cdeltS1,
           double restfrq, double restwav, char ctypeS2[], double *crvalS2,
           double *cdeltS2);


/* Deprecated. */
#define spcini_errmsg spc_errmsg
#define spcprt_errmsg spc_errmsg
#define spcset_errmsg spc_errmsg
#define spcx2s_errmsg spc_errmsg
#define spcs2x_errmsg spc_errmsg

#ifdef __cplusplus
}
#endif

#endif /* WCSLIB_SPC */

---