WCSLIB  6.2
Macros | Enumerations | Functions | Variables
wcshdr.h File Reference
#include "wcs.h"

Go to the source code of this file.

Macros

#define WCSHDR_none   0x00000000
 Bit mask for wcspih() and wcsbth() - reject all extensions. More...
 
#define WCSHDR_all   0x000FFFFF
 Bit mask for wcspih() and wcsbth() - accept all extensions. More...
 
#define WCSHDR_reject   0x10000000
 Bit mask for wcspih() and wcsbth() - reject non-standard keywords. More...
 
#define WCSHDR_strict   0x20000000
 
#define WCSHDR_CROTAia   0x00000001
 Bit mask for wcspih() and wcsbth() - accept CROTAia, iCROTna, TCROTna. More...
 
#define WCSHDR_VELREFa   0x00000002
 Bit mask for wcspih() and wcsbth() - accept VELREFa. More...
 
#define WCSHDR_CD00i00j   0x00000004
 Bit mask for wcspih() and wcsbth() - accept CD00i00j. More...
 
#define WCSHDR_PC00i00j   0x00000008
 Bit mask for wcspih() and wcsbth() - accept PC00i00j. More...
 
#define WCSHDR_PROJPn   0x00000010
 Bit mask for wcspih() and wcsbth() - accept PROJPn. More...
 
#define WCSHDR_CD0i_0ja   0x00000020
 
#define WCSHDR_PC0i_0ja   0x00000040
 
#define WCSHDR_PV0i_0ma   0x00000080
 
#define WCSHDR_PS0i_0ma   0x00000100
 
#define WCSHDR_DOBSn   0x00000200
 Bit mask for wcspih() and wcsbth() - accept DOBSn. More...
 
#define WCSHDR_OBSGLBHn   0x00000400
 
#define WCSHDR_RADECSYS   0x00000800
 Bit mask for wcspih() and wcsbth() - accept RADECSYS. More...
 
#define WCSHDR_EPOCHa   0x00001000
 Bit mask for wcspih() and wcsbth() - accept EPOCHa. More...
 
#define WCSHDR_VSOURCE   0x00002000
 Bit mask for wcspih() and wcsbth() - accept VSOURCEa. More...
 
#define WCSHDR_LONGKEY   0x00004000
 Bit mask for wcspih() and wcsbth() - accept long forms of the alternate binary table and pixel list WCS keywords. More...
 
#define WCSHDR_CNAMn   0x00008000
 Bit mask for wcspih() and wcsbth() - accept iCNAMn, TCNAMn, iCRDEn, TCRDEn, iCSYEn, TCSYEn. More...
 
#define WCSHDR_AUXIMG   0x00010000
 Bit mask for wcspih() and wcsbth() - allow the image-header form of an auxiliary WCS keyword to provide a default value for all images. More...
 
#define WCSHDR_ALLIMG   0x00020000
 Bit mask for wcspih() and wcsbth() - allow the image-header form of all image header WCS keywords to provide a default value for all images. More...
 
#define WCSHDR_IMGHEAD   0x00100000
 Bit mask for wcsbth() - restrict to image header keywords only. More...
 
#define WCSHDR_BIMGARR   0x00200000
 Bit mask for wcsbth() - restrict to binary table image array keywords only. More...
 
#define WCSHDR_PIXLIST   0x00400000
 Bit mask for wcsbth() - restrict to pixel list keywords only. More...
 
#define WCSHDO_none   0x00000
 Bit mask for wcshdo() - don't write any extensions. More...
 
#define WCSHDO_all   0x000FF
 Bit mask for wcshdo() - write all extensions. More...
 
#define WCSHDO_safe   0x0000F
 Bit mask for wcshdo() - write safe extensions only. More...
 
#define WCSHDO_DOBSn   0x00001
 Bit mask for wcshdo() - write DOBSn. More...
 
#define WCSHDO_TPCn_ka   0x00002
 Bit mask for wcshdo() - write TPCn_ka. More...
 
#define WCSHDO_PVn_ma   0x00004
 Bit mask for wcshdo() - write iPVn_ma, TPVn_ma, iPSn_ma, TPSn_ma. More...
 
#define WCSHDO_CRPXna   0x00008
 Bit mask for wcshdo() - write jCRPXna, TCRPXna, iCDLTna, TCDLTna, iCUNIna, TCUNIna, iCTYPna, TCTYPna, iCRVLna, TCRVLna. More...
 
#define WCSHDO_CNAMna   0x00010
 Bit mask for wcshdo() - write iCNAMna, TCNAMna, iCRDEna, TCRDEna, iCSYEna, TCSYEna. More...
 
#define WCSHDO_WCSNna   0x00020
 Bit mask for wcshdo() - write WCSNna instead of TWCSna More...
 
#define WCSHDO_P12   0x01000
 
#define WCSHDO_P13   0x02000
 
#define WCSHDO_P14   0x04000
 
#define WCSHDO_P15   0x08000
 
#define WCSHDO_P16   0x10000
 
#define WCSHDO_P17   0x20000
 
#define WCSHDO_EFMT   0x40000
 

Enumerations

enum  wcshdr_errmsg_enum {
  WCSHDRERR_SUCCESS = 0, WCSHDRERR_NULL_POINTER = 1, WCSHDRERR_MEMORY = 2, WCSHDRERR_BAD_COLUMN = 3,
  WCSHDRERR_PARSER = 4, WCSHDRERR_BAD_TABULAR_PARAMS = 5
}
 

Functions

int wcspih (char *header, int nkeyrec, int relax, int ctrl, int *nreject, int *nwcs, struct wcsprm **wcs)
 FITS WCS parser routine for image headers. More...
 
int wcsbth (char *header, int nkeyrec, int relax, int ctrl, int keysel, int *colsel, int *nreject, int *nwcs, struct wcsprm **wcs)
 FITS WCS parser routine for binary table and image headers. More...
 
int wcstab (struct wcsprm *wcs)
 Tabular construction routine. More...
 
int wcsidx (int nwcs, struct wcsprm **wcs, int alts[27])
 Index alternate coordinate representations. More...
 
int wcsbdx (int nwcs, struct wcsprm **wcs, int type, short alts[1000][28])
 Index alternate coordinate representions. More...
 
int wcsvfree (int *nwcs, struct wcsprm **wcs)
 Free the array of wcsprm structs. More...
 
int wcshdo (int ctrl, struct wcsprm *wcs, int *nkeyrec, char **header)
 Write out a wcsprm struct as a FITS header. More...
 

Variables

const char * wcshdr_errmsg []
 Status return messages. More...
 

Detailed Description

Routines in this suite are aimed at extracting WCS information from a FITS file. The information is encoded via keywords defined in

"Representations of world coordinates in FITS",
Greisen, E.W., & Calabretta, M.R. 2002, A&A, 395, 1061 (WCS Paper I)
"Representations of celestial coordinates in FITS",
Calabretta, M.R., & Greisen, E.W. 2002, A&A, 395, 1077 (WCS Paper II)
"Representations of spectral coordinates in FITS",
Greisen, E.W., Calabretta, M.R., Valdes, F.G., & Allen, S.L.
2006, A&A, 446, 747 (WCS Paper III)
"Representations of distortions in FITS world coordinate systems",
Calabretta, M.R. et al. (WCS Paper IV, draft dated 2004/04/22),
available from http://www.atnf.csiro.au/people/Mark.Calabretta
"Representations of time coordinates in FITS -
Time and relative dimension in space",
Rots, A.H., Bunclark, P.S., Calabretta, M.R., Allen, S.L.,
Manchester, R.N., & Thompson, W.T. 2015, A&A, 574, A36 (WCS Paper VII)

These routines provide the high-level interface between the FITS file and the WCS coordinate transformation routines.

Additionally, function wcshdo() is provided to write out the contents of a wcsprm struct as a FITS header.

Briefly, the anticipated sequence of operations is as follows:

In detail:

Macro Definition Documentation

◆ WCSHDR_none

#define WCSHDR_none   0x00000000

Bit mask for the relax argument of wcspih() and wcsbth() - reject all extensions.

Refer to wcsbth() note 5.

◆ WCSHDR_all

#define WCSHDR_all   0x000FFFFF

Bit mask for the relax argument of wcspih() and wcsbth() - accept all extensions.

Refer to wcsbth() note 5.

◆ WCSHDR_reject

#define WCSHDR_reject   0x10000000

Bit mask for the relax argument of wcspih() and wcsbth() - reject non-standard keywords.

Refer to wcsbth() note 5.

◆ WCSHDR_strict

#define WCSHDR_strict   0x20000000

◆ WCSHDR_CROTAia

#define WCSHDR_CROTAia   0x00000001

Bit mask for the relax argument of wcspih() and wcsbth() - accept CROTAia, iCROTna, TCROTna.

Refer to wcsbth() note 5.

◆ WCSHDR_VELREFa

#define WCSHDR_VELREFa   0x00000002

Bit mask for the relax argument of wcspih() and wcsbth() - accept VELREFa.

Refer to wcsbth() note 5.

◆ WCSHDR_CD00i00j

#define WCSHDR_CD00i00j   0x00000004

Bit mask for the relax argument of wcspih() and wcsbth() - accept CD00i00j.

Refer to wcsbth() note 5.

◆ WCSHDR_PC00i00j

#define WCSHDR_PC00i00j   0x00000008

Bit mask for the relax argument of wcspih() and wcsbth() - accept PC00i00j.

Refer to wcsbth() note 5.

◆ WCSHDR_PROJPn

#define WCSHDR_PROJPn   0x00000010

Bit mask for the relax argument of wcspih() and wcsbth() - accept PROJPn.

Refer to wcsbth() note 5.

◆ WCSHDR_CD0i_0ja

#define WCSHDR_CD0i_0ja   0x00000020

◆ WCSHDR_PC0i_0ja

#define WCSHDR_PC0i_0ja   0x00000040

◆ WCSHDR_PV0i_0ma

#define WCSHDR_PV0i_0ma   0x00000080

◆ WCSHDR_PS0i_0ma

#define WCSHDR_PS0i_0ma   0x00000100

◆ WCSHDR_DOBSn

#define WCSHDR_DOBSn   0x00000200

Bit mask for the relax argument of wcspih() and wcsbth() - accept DOBSn.

Refer to wcsbth() note 5.

◆ WCSHDR_OBSGLBHn

#define WCSHDR_OBSGLBHn   0x00000400

◆ WCSHDR_RADECSYS

#define WCSHDR_RADECSYS   0x00000800

Bit mask for the relax argument of wcspih() and wcsbth() - accept RADECSYS.

Refer to wcsbth() note 5.

◆ WCSHDR_EPOCHa

#define WCSHDR_EPOCHa   0x00001000

Bit mask for the relax argument of wcspih() and wcsbth() - accept EPOCHa.

Refer to wcsbth() note 5.

◆ WCSHDR_VSOURCE

#define WCSHDR_VSOURCE   0x00002000

Bit mask for the relax argument of wcspih() and wcsbth() - accept VSOURCEa.

Refer to wcsbth() note 5.

◆ WCSHDR_LONGKEY

#define WCSHDR_LONGKEY   0x00004000

Bit mask for the relax argument of wcspih() and wcsbth() - accept long forms of the alternate binary table and pixel list WCS keywords.

Refer to wcsbth() note 5.

◆ WCSHDR_CNAMn

#define WCSHDR_CNAMn   0x00008000

Bit mask for the relax argument of wcspih() and wcsbth() - accept iCNAMn, TCNAMn, iCRDEn, TCRDEn, iCSYEn, TCSYEn.

Refer to wcsbth() note 5.

◆ WCSHDR_AUXIMG

#define WCSHDR_AUXIMG   0x00010000

Bit mask for the relax argument of wcspih() and wcsbth() - allow the image-header form of an auxiliary WCS keyword with representation-wide scope to provide a default value for all images.

Refer to wcsbth() note 5.

◆ WCSHDR_ALLIMG

#define WCSHDR_ALLIMG   0x00020000

Bit mask for the relax argument of wcspih() and wcsbth() - allow the image-header form of all image header WCS keywords to provide a default value for all image arrays in a binary table (n.b. not pixel list).

Refer to wcsbth() note 5.

◆ WCSHDR_IMGHEAD

#define WCSHDR_IMGHEAD   0x00100000

Bit mask for the keysel argument of wcsbth() - restrict keyword types considered to image header keywords only.

◆ WCSHDR_BIMGARR

#define WCSHDR_BIMGARR   0x00200000

Bit mask for the keysel argument of wcsbth() - restrict keyword types considered to binary table image array keywords only.

◆ WCSHDR_PIXLIST

#define WCSHDR_PIXLIST   0x00400000

Bit mask for the keysel argument of wcsbth() - restrict keyword types considered to pixel list keywords only.

◆ WCSHDO_none

#define WCSHDO_none   0x00000

Bit mask for the relax argument of wcshdo() - don't write any extensions.

Refer to the notes for wcshdo().

◆ WCSHDO_all

#define WCSHDO_all   0x000FF

Bit mask for the relax argument of wcshdo() - write all extensions.

Refer to the notes for wcshdo().

◆ WCSHDO_safe

#define WCSHDO_safe   0x0000F

Bit mask for the relax argument of wcshdo() - write only extensions that are considered safe.

Refer to the notes for wcshdo().

◆ WCSHDO_DOBSn

#define WCSHDO_DOBSn   0x00001

Bit mask for the relax argument of wcshdo() - write DOBSn, the column-specific analogue of DATE-OBS for use in binary tables and pixel lists.

Refer to the notes for wcshdo().

◆ WCSHDO_TPCn_ka

#define WCSHDO_TPCn_ka   0x00002

Bit mask for the relax argument of wcshdo() - write TPCn_ka if less than eight characters instead of TPn_ka.

Refer to the notes for wcshdo().

◆ WCSHDO_PVn_ma

#define WCSHDO_PVn_ma   0x00004

Bit mask for the relax argument of wcshdo() - write iPVn_ma, TPVn_ma, iPSn_ma, TPSn_ma, if less than eight characters instead of iVn_ma, TVn_ma, iSn_ma, TSn_ma.

Refer to the notes for wcshdo().

◆ WCSHDO_CRPXna

#define WCSHDO_CRPXna   0x00008

Bit mask for the relax argument of wcshdo() - write jCRPXna, TCRPXna, iCDLTna, TCDLTna, iCUNIna, TCUNIna, iCTYPna, TCTYPna, iCRVLna, TCRVLna, if less than eight characters instead of jCRPna, TCRPna, iCDEna, TCDEna, iCUNna, TCUNna, iCTYna, TCTYna, iCRVna, TCRVna.

Refer to the notes for wcshdo().

◆ WCSHDO_CNAMna

#define WCSHDO_CNAMna   0x00010

Bit mask for the relax argument of wcshdo() - write iCNAMna, TCNAMna, iCRDEna, TCRDEna, iCSYEna, TCSYEna, if less than eight characters instead of iCNAna, TCNAna, iCRDna, TCRDna, iCSYna, TCSYna.

Refer to the notes for wcshdo().

◆ WCSHDO_WCSNna

#define WCSHDO_WCSNna   0x00020

Bit mask for the relax argument of wcshdo() - write WCSNna instead of TWCSna.

Refer to the notes for wcshdo().

◆ WCSHDO_P12

#define WCSHDO_P12   0x01000

◆ WCSHDO_P13

#define WCSHDO_P13   0x02000

◆ WCSHDO_P14

#define WCSHDO_P14   0x04000

◆ WCSHDO_P15

#define WCSHDO_P15   0x08000

◆ WCSHDO_P16

#define WCSHDO_P16   0x10000

◆ WCSHDO_P17

#define WCSHDO_P17   0x20000

◆ WCSHDO_EFMT

#define WCSHDO_EFMT   0x40000

Enumeration Type Documentation

◆ wcshdr_errmsg_enum

Enumerator
WCSHDRERR_SUCCESS 
WCSHDRERR_NULL_POINTER 
WCSHDRERR_MEMORY 
WCSHDRERR_BAD_COLUMN 
WCSHDRERR_PARSER 
WCSHDRERR_BAD_TABULAR_PARAMS 

Function Documentation

◆ wcspih()

int wcspih ( char *  header,
int  nkeyrec,
int  relax,
int  ctrl,
int *  nreject,
int *  nwcs,
struct wcsprm **  wcs 
)

wcspih() is a high-level FITS WCS routine that parses an image header, either that of a primary HDU or of an image extension. All WCS keywords defined in Papers I, II, III, IV, and VII are recognized, and also those used by the AIPS convention and certain other keywords that existed in early drafts of the WCS papers as explained in wcsbth() note 5. wcspih() also handles keywords associated with non-standard distortion functions described in the prologue of dis.h.

Given a character array containing a FITS image header, wcspih() identifies and reads all WCS keywords for the primary coordinate representation and up to 26 alternate representations. It returns this information as an array of wcsprm structs.

wcspih() invokes wcstab() on each of the wcsprm structs that it returns.

Use wcsbth() in preference to wcspih() for FITS headers of unknown type; wcsbth() can parse image headers as well as binary table and pixel list headers, although it cannot handle keywords relating to distortion functions, which may only exist in a primary image header.

Parameters
[in,out]headerCharacter array containing the (entire) FITS image header from which to identify and construct the coordinate representations, for example, as might be obtained conveniently via the CFITSIO routine fits_hdr2str().
Each header "keyrecord" (formerly "card image") consists of exactly 80 7-bit ASCII printing characters in the range 0x20 to 0x7e (which excludes NUL, BS, TAB, LF, FF and CR) especially noting that the keyrecords are NOT null-terminated.
For negative values of ctrl (see below), header[] is modified so that WCS keyrecords processed by wcspih() are removed from it.
[in]nkeyrecNumber of keyrecords in header[].
[in]relaxDegree of permissiveness:
  • 0: Recognize only FITS keywords defined by the published WCS standard.
  • WCSHDR_all: Admit all recognized informal extensions of the WCS standard.
Fine-grained control of the degree of permissiveness is also possible as explained in wcsbth() note 5.
[in]ctrlError reporting and other control options for invalid WCS and other header keyrecords:
  • 0: Do not report any rejected header keyrecords.
  • 1: Produce a one-line message stating the number of WCS keyrecords rejected (nreject).
  • 2: Report each rejected keyrecord and the reason why it was rejected.
  • 3: As above, but also report all non-WCS keyrecords that were discarded, and the number of coordinate representations (nwcs) found.
  • 4: As above, but also report the accepted WCS keyrecords, with a summary of the number accepted as well as rejected.
The report is written to stderr by default, or the stream set by wcsprintf_set().
For ctrl < 0, WCS keyrecords processed by wcspih() are removed from header[]:
  • -1: Remove only valid WCS keyrecords whose values were successfully extracted, nothing is reported.
  • -2: As above, but also remove WCS keyrecords that were rejected, reporting each one and the reason that it was rejected.
  • -3: As above, and also report the number of coordinate representations (nwcs) found.
  • -11: Same as -1 but preserving global WCS-related keywords such as '{DATE,MJD}-{OBS,BEG,AVG,END}' and the other basic time-related keywords, and 'OBSGEO-{X,Y,Z,L,B,H}'.
If any keyrecords are removed from header[] it will be null-terminated (NUL not being a legal FITS header character), otherwise it will contain its original complement of nkeyrec keyrecords and possibly not be null-terminated.
[out]nrejectNumber of WCS keywords rejected for syntax errors, illegal values, etc. Keywords not recognized as WCS keywords are simply ignored. Refer also to wcsbth() note 5.
[out]nwcsNumber of coordinate representations found.
[out]wcsPointer to an array of wcsprm structs containing up to 27 coordinate representations.
Memory for the array is allocated by wcspih() which also invokes wcsini() for each struct to allocate memory for internal arrays and initialize their members to default values. Refer also to wcsbth() note 8. Note that wcsset() is not invoked on these structs.
This allocated memory must be freed by the user, first by invoking wcsfree() for each struct, and then by freeing the array itself. A routine, wcsvfree(), is provided to do this (see below).
Returns
Status return value:
  • 0: Success.
  • 1: Null wcsprm pointer passed.
  • 2: Memory allocation failed.
  • 4: Fatal error returned by Flex parser.

Notes:
Refer to wcsbth() notes 1, 2, 3, 5, 7, and 8.

◆ wcsbth()

int wcsbth ( char *  header,
int  nkeyrec,
int  relax,
int  ctrl,
int  keysel,
int *  colsel,
int *  nreject,
int *  nwcs,
struct wcsprm **  wcs 
)

wcsbth() is a high-level FITS WCS routine that parses a binary table header. It handles image array and pixel list WCS keywords which may be present together in one header.

As an extension of the FITS WCS standard, wcsbth() also recognizes image header keywords in a binary table header. These may be used to provide default values via an inheritance mechanism discussed in note 5 (c.f. WCSHDR_AUXIMG and WCSHDR_ALLIMG), or may instead result in wcsprm structs that are not associated with any particular column. Thus wcsbth() can handle primary image and image extension headers in addition to binary table headers (it ignores NAXIS and does not rely on the presence of the TFIELDS keyword).

All WCS keywords defined in Papers I, II, III, and VII are recognized, and also those used by the AIPS convention and certain other keywords that existed in early drafts of the WCS papers as explained in note 5 below.

wcsbth() sets the colnum or colax[] members of the wcsprm structs that it returns with the column number of an image array or the column numbers associated with each pixel coordinate element in a pixel list. wcsprm structs that are not associated with any particular column, as may be derived from image header keywords, have colnum == 0.

Note 6 below discusses the number of wcsprm structs returned by wcsbth(), and the circumstances in which image header keywords cause a struct to be created. See also note 9 concerning the number of separate images that may be stored in a pixel list.

The API to wcsbth() is similar to that of wcspih() except for the addition of extra arguments that may be used to restrict its operation. Like wcspih(), wcsbth() invokes wcstab() on each of the wcsprm structs that it returns.

Parameters
[in,out]headerCharacter array containing the (entire) FITS binary table, primary image, or image extension header from which to identify and construct the coordinate representations, for example, as might be obtained conveniently via the CFITSIO routine fits_hdr2str().
Each header "keyrecord" (formerly "card image") consists of exactly 80 7-bit ASCII printing characters in the range 0x20 to 0x7e (which excludes NUL, BS, TAB, LF, FF and CR) especially noting that the keyrecords are NOT null-terminated.
For negative values of ctrl (see below), header[] is modified so that WCS keyrecords processed by wcsbth() are removed from it.
[in]nkeyrecNumber of keyrecords in header[].
[in]relaxDegree of permissiveness:
  • 0: Recognize only FITS keywords defined by the published WCS standard.
  • WCSHDR_all: Admit all recognized informal extensions of the WCS standard.
Fine-grained control of the degree of permissiveness is also possible, as explained in note 5 below.
[in]ctrlError reporting and other control options for invalid WCS and other header keyrecords:
  • 0: Do not report any rejected header keyrecords.
  • 1: Produce a one-line message stating the number of WCS keyrecords rejected (nreject).
  • 2: Report each rejected keyrecord and the reason why it was rejected.
  • 3: As above, but also report all non-WCS keyrecords that were discarded, and the number of coordinate representations (nwcs) found.
  • 4: As above, but also report the accepted WCS keyrecords, with a summary of the number accepted as well as rejected.
The report is written to stderr by default, or the stream set by wcsprintf_set().
For ctrl < 0, WCS keyrecords processed by wcsbth() are removed from header[]:
  • -1: Remove only valid WCS keyrecords whose values were successfully extracted, nothing is reported.
  • -2: Also remove WCS keyrecords that were rejected, reporting each one and the reason that it was rejected.
  • -3: As above, and also report the number of coordinate representations (nwcs) found.
  • -11: Same as -1 but preserving global WCS-related keywords such as '{DATE,MJD}-{OBS,BEG,AVG,END}' and the other basic time-related keywords, and 'OBSGEO-{X,Y,Z,L,B,H}'.
If any keyrecords are removed from header[] it will be null-terminated (NUL not being a legal FITS header character), otherwise it will contain its original complement of nkeyrec keyrecords and possibly not be null-terminated.
[in]keyselVector of flag bits that may be used to restrict the keyword types considered: If zero, there is no restriction.
Keywords such as EQUIna or RFRQna that are common to binary table image arrays and pixel lists (including WCSNna and TWCSna, as explained in note 4 below) are selected by both WCSHDR_BIMGARR and WCSHDR_PIXLIST. Thus if inheritance via WCSHDR_ALLIMG is enabled as discussed in note 5 and one of these shared keywords is present, then WCSHDR_IMGHEAD and WCSHDR_PIXLIST alone may be sufficient to cause the construction of coordinate descriptions for binary table image arrays.
[in]colselPointer to an array of table column numbers used to restrict the keywords considered by wcsbth().
A null pointer may be specified to indicate that there is no restriction. Otherwise, the magnitude of cols[0] specifies the length of the array:
  • cols[0] > 0: the columns are included,
  • cols[0] < 0: the columns are excluded.
For the pixel list keywords TPn_ka and TCn_ka (and TPCn_ka and TCDn_ka if WCSHDR_LONGKEY is enabled), it is an error for one column to be selected but not the other. This is unlike the situation with invalid keyrecords, which are simply rejected, because the error is not intrinsic to the header itself but arises in the way that it is processed.
[out]nrejectNumber of WCS keywords rejected for syntax errors, illegal values, etc. Keywords not recognized as WCS keywords are simply ignored, refer also to note 5 below.
[out]nwcsNumber of coordinate representations found.
[out]wcsPointer to an array of wcsprm structs containing up to 27027 coordinate representations, refer to note 6 below.
Memory for the array is allocated by wcsbth() which also invokes wcsini() for each struct to allocate memory for internal arrays and initialize their members to default values. Refer also to note 8 below. Note that wcsset() is not invoked on these structs.
This allocated memory must be freed by the user, first by invoking wcsfree() for each struct, and then by freeing the array itself. A routine, wcsvfree(), is provided to do this (see below).
Returns
Status return value:
  • 0: Success.
  • 1: Null wcsprm pointer passed.
  • 2: Memory allocation failed.
  • 3: Invalid column selection.
  • 4: Fatal error returned by Flex parser.

Notes:

  1. wcspih() determines the number of coordinate axes independently for each alternate coordinate representation (denoted by the "a" value in keywords like CTYPEia) from the higher of

    1. NAXIS,
    2. WCSAXESa,
    3. The highest axis number in any parameterized WCS keyword. The keyvalue, as well as the keyword, must be syntactically valid otherwise it will not be considered.

    If none of these keyword types is present, i.e. if the header only contains auxiliary WCS keywords for a particular coordinate representation, then no coordinate description is constructed for it.

    wcsbth() is similar except that it ignores the NAXIS keyword if given an image header to process.

    The number of axes, which is returned as a member of the wcsprm struct, may differ for different coordinate representations of the same image.

  2. wcspih() and wcsbth() enforce correct FITS "keyword = value" syntax with regard to "= " occurring in columns 9 and 10.

    However, they do recognize free-format character (NOST 100-2.0, Sect. 5.2.1), integer (Sect. 5.2.3), and floating-point values (Sect. 5.2.4) for all keywords.

  3. Where CROTAn, CDi_ja, and PCi_ja occur together in one header wcspih() and wcsbth() treat them as described in the prologue to wcs.h.

  4. WCS Paper I mistakenly defined the pixel list form of WCSNAMEa as TWCSna instead of WCSNna; the 'T' is meant to substitute for the axis number in the binary table form of the keyword - note that keywords defined in WCS Papers II, III, and VII that are not parameterized by axis number have identical forms for binary tables and pixel lists. Consequently wcsbth() always treats WCSNna and TWCSna as equivalent.

  5. wcspih() and wcsbth() interpret the relax argument as a vector of flag bits to provide fine-grained control over what non-standard WCS keywords to accept. The flag bits are subject to change in future and should be set by using the preprocessor macros (see below) for the purpose.

    • WCSHDR_none: Don't accept any extensions (not even those in the errata). Treat non-conformant keywords in the same way as non-WCS keywords in the header, i.e. simply ignore them.

    • WCSHDR_all: Accept all extensions recognized by the parser.

    • WCSHDR_reject: Reject non-standard keyrecords (that are not otherwise explicitly accepted by one of the flags below). A message will optionally be printed on stderr by default, or the stream set by wcsprintf_set(), as determined by the ctrl argument, and nreject will be incremented.

      This flag may be used to signal the presence of non-standard keywords, otherwise they are simply passed over as though they did not exist in the header. It is mainly intended for testing conformance of a FITS header to the WCS standard.

      Keyrecords may be non-standard in several ways:

      • The keyword may be syntactically valid but with keyvalue of incorrect type or invalid syntax, or the keycomment may be malformed.

      • The keyword may strongly resemble a WCS keyword but not, in fact, be one because it does not conform to the standard. For example, "CRPIX01" looks like a CRPIXja keyword, but in fact the leading zero on the axis number violates the basic FITS standard. Likewise, "LONPOLE2" is not a valid LONPOLEa keyword in the WCS standard, and indeed there is nothing the parser can sensibly do with it.

      • Use of the keyword may be deprecated by the standard. Such will be rejected if not explicitly accepted via one of the flags below.

    • WCSHDR_strict: As for WCSHDR_reject, but also reject AIPS-convention keywords and all other deprecated usage that is not explicitly accepted.

    • WCSHDR_CROTAia: Accept CROTAia (wcspih()), iCROTna (wcsbth()), TCROTna (wcsbth()).
    • WCSHDR_VELREFa: Accept VELREFa. wcspih() always recognizes the AIPS-convention keywords, CROTAn, EPOCH, and VELREF for the primary representation (a = ' ') but alternates are non-standard.

      wcsbth() accepts EPOCHa and VELREFa only if WCSHDR_AUXIMG is also enabled.

    • WCSHDR_CD00i00j: Accept CD00i00j (wcspih()).
    • WCSHDR_PC00i00j: Accept PC00i00j (wcspih()).
    • WCSHDR_PROJPn: Accept PROJPn (wcspih()). These appeared in early drafts of WCS Paper I+II (before they were split) and are equivalent to CDi_ja, PCi_ja, and PVi_ma for the primary representation (a = ' '). PROJPn is equivalent to PVi_ma with m = n $\le$ 9, and is associated exclusively with the latitude axis.

    • WCSHDR_CD0i_0ja: Accept CD0i_0ja (wcspih()).
    • WCSHDR_PC0i_0ja: Accept PC0i_0ja (wcspih()).
    • WCSHDR_PV0i_0ma: Accept PV0i_0ja (wcspih()).
    • WCSHDR_PS0i_0ma: Accept PS0i_0ja (wcspih()). Allow the numerical index to have a leading zero in doubly- parameterized keywords, for example, PC01_01. WCS Paper I (Sects 2.1.2 & 2.1.4) explicitly disallows leading zeroes. The FITS 3.0 standard document (Sect. 4.1.2.1) states that the index in singly-parameterized keywords (e.g. CTYPEia) "shall not have leading zeroes", and later in Sect. 8.1 that "leading zeroes must not be used" on PVi_ma and PSi_ma. However, by an oversight, it is silent on PCi_ja and CDi_ja.

    • WCSHDR_DOBSn (wcsbth() only): Allow DOBSn, the column-specific analogue of DATE-OBS. By an oversight this was never formally defined in the standard.

    • WCSHDR_OBSGLBHn (wcsbth() only): Allow OBSGLn, OBSGBn, and OBSGHn, the column-specific analogues of OBSGEO-L, OBSGEO-B, and OBSGEO-H. By an oversight these were never formally defined in the standard.

    • WCSHDR_RADECSYS: Accept RADECSYS. This appeared in early drafts of WCS Paper I+II and was subsequently replaced by RADESYSa.

      wcsbth() accepts RADECSYS only if WCSHDR_AUXIMG is also enabled.

    • WCSHDR_EPOCHa: Accept EPOCHa.

    • WCSHDR_VSOURCE: Accept VSOURCEa or VSOUna (wcsbth()). This appeared in early drafts of WCS Paper III and was subsequently dropped in favour of ZSOURCEa and ZSOUna.

      wcsbth() accepts VSOURCEa only if WCSHDR_AUXIMG is also enabled.

    • WCSHDR_LONGKEY (wcsbth() only): Accept long forms of the alternate binary table and pixel list WCS keywords, i.e. with "a" non- blank. Specifically


      jCRPXna TCRPXna : jCRPXn jCRPna TCRPXn TCRPna CRPIXja
      TPCn_ka : ijPCna TPn_ka PCi_ja
      TCDn_ka : ijCDna TCn_ka CDi_ja
      iCDLTna TCDLTna : iCDLTn iCDEna TCDLTn TCDEna CDELTia
      iCUNIna TCUNIna : iCUNIn iCUNna TCUNIn TCUNna CUNITia
      iCTYPna TCTYPna : iCTYPn iCTYna TCTYPn TCTYna CTYPEia
      iCRVLna TCRVLna : iCRVLn iCRVna TCRVLn TCRVna CRVALia
      iPVn_ma TPVn_ma : iVn_ma TVn_ma PVi_ma
      iPSn_ma TPSn_ma : iSn_ma TSn_ma PSi_ma

      where the primary and standard alternate forms together with the image-header equivalent are shown rightwards of the colon.

      The long form of these keywords could be described as quasi- standard. TPCn_ka, iPVn_ma, and TPVn_ma appeared by mistake in the examples in WCS Paper II and subsequently these and also TCDn_ka, iPSn_ma and TPSn_ma were legitimized by the errata to the WCS papers.

      Strictly speaking, the other long forms are non-standard and in fact have never appeared in any draft of the WCS papers nor in the errata. However, as natural extensions of the primary form they are unlikely to be written with any other intention. Thus it should be safe to accept them provided, of course, that the resulting keyword does not exceed the 8-character limit.

      If WCSHDR_CNAMn is enabled then also accept


      iCNAMna TCNAMna : iCNAna TCNAna CNAMEia
      iCRDEna TCRDEna : iCRDna TCRDna CRDERia
      iCSYEna TCSYEna : iCSYna TCSYna CSYERia
      TCZPHna TCZPHna : TCZPna TCZPna CZPHSia
      iCPERna TCPERna : iCPRna TCPRna CPERIia

      Note that CNAMEia, CRDERia, CSYERia, CZPHSia, CPERIia, and their variants are not used by WCSLIB but are stored in the wcsprm struct as auxiliary information.

    • WCSHDR_CNAMn (wcsbth() only): Accept iCNAMn, iCRDEn, iCSYEn, TCZPHn, iCPERn, TCNAMn, TCRDEn, TCSYEn, TCZPHn, and TCPERn, i.e. with "a" blank. While non-standard, these are the obvious analogues of iCTYPn, TCTYPn, etc.

    • WCSHDR_AUXIMG (wcsbth() only): Allow the image-header form of an auxiliary WCS keyword with representation-wide scope to provide a default value for all images. This default may be overridden by the column-specific form of the keyword.

      For example, a keyword like EQUINOXa would apply to all image arrays in a binary table, or all pixel list columns with alternate representation "a" unless overridden by EQUIna.

      Specifically the keywords are:


      LONPOLEa for LONPna
      LATPOLEa for LATPna
      VELREF ... (No column-specific form.)
      VELREFa ... Only if WCSHDR_VELREFa is set.

      whose keyvalues are actually used by WCSLIB, and also keywords providing auxiliary information that is simply stored in the wcsprm struct:


      WCSNAMEa for WCSNna ... Or TWCSna (see below).
      DATE-OBS for DOBSn
      MJD-OBS for MJDOBn
      RADESYSa for RADEna
      RADECSYS for RADEna ... Only if WCSHDR_RADECSYS is set.
      EPOCH ... (No column-specific form.)
      EPOCHa ... Only if WCSHDR_EPOCHa is set.
      EQUINOXa for EQUIna

      where the image-header keywords on the left provide default values for the column specific keywords on the right.

      Note that, according to Sect. 8.1 of WCS Paper III, and Sect. 5.2 of WCS Paper VII, the following are always inherited:


      RESTFREQ for RFRQna
      RESTFRQa for RFRQna
      RESTWAVa for RWAVna

      being those actually used by WCSLIB, together with the following auxiliary keywords, many of which do not have binary table equivalents and therefore can only be inherited:


      TIMESYS
      TREFPOS for MJDAn
      TREFDIR for MJDAn
      PLEPHEM
      TIMEUNIT
      DATEREF
      MJDREF
      MJDREFI
      MJDREFF
      JDREF
      JDREFI
      JDREFF
      TIMEOFFS
      DATE-BEG
      DATE-AVG for DAVGn
      DATE-END
      MJD-BEG
      MJD-AVG for MJDAn
      MJD-END
      JEPOCH
      BEPOCH
      TSTART
      TSTOP
      XPOSURE
      TELAPSE
      TIMSYER
      TIMRDER
      TIMEDEL
      TIMEPIXR
      OBSGEO-X for OBSGXn
      OBSGEO-Y for OBSGYn
      OBSGEO-Z for OBSGZn
      OBSGEO-L for OBSGLn
      OBSGEO-B for OBSGBn
      OBSGEO-H for OBSGHn
      OBSORBIT
      SPECSYSa for SPECna
      SSYSOBSa for SOBSna
      VELOSYSa for VSYSna
      VSOURCEa for VSOUna ... Only if WCSHDR_VSOURCE is set.
      ZSOURCEa for ZSOUna
      SSYSSRCa for SSRCna
      VELANGLa for VANGna

      Global image-header keywords, such as MJD-OBS, apply to all alternate representations, and would therefore provide a default value for all images in the header.

      This auxiliary inheritance mechanism applies to binary table image arrays and pixel lists alike. Most of these keywords have no default value, the exceptions being LONPOLEa and LATPOLEa, and also RADESYSa and EQUINOXa which provide defaults for each other. Thus one potential difficulty in using WCSHDR_AUXIMG is that of erroneously inheriting one of these four keywords.

      Also, beware of potential inconsistencies that may arise where, for example, DATE-OBS is inherited, but MJD-OBS is overridden by MJDOBn and specifies a different time. Pairs in this category are:

      DATE-OBS/DOBSn versus MJD-OBS/MJDOBn
      DATE-AVG/DAVGn versus MJD-AVG/MJDAn
      RESTFRQa/RFRQna versus RESTWAVa/RWAVna
      OBSGEO-[XYZ]/OBSG[XYZ]n versus OBSGEO-[LBH]/OBSG[LBH]n

      The wcsfixi() routines datfix() and obsfix() are provided to check the consistency of these and other such pairs of keywords.

      Unlike WCSHDR_ALLIMG, the existence of one (or all) of these auxiliary WCS image header keywords will not by itself cause a wcsprm struct to be created for alternate representation "a". This is because they do not provide sufficient information to create a non-trivial coordinate representation when used in conjunction with the default values of those keywords that are parameterized by axis number, such as CTYPEia.

    • WCSHDR_ALLIMG (wcsbth() only): Allow the image-header form of *all* image header WCS keywords to provide a default value for all image arrays in a binary table (n.b. not pixel list). This default may be overridden by the column-specific form of the keyword.

      For example, a keyword like CRPIXja would apply to all image arrays in a binary table with alternate representation "a" unless overridden by jCRPna.

      Specifically the keywords are those listed above for WCSHDR_AUXIMG plus


      WCSAXESa for WCAXna

      which defines the coordinate dimensionality, and the following keywords that are parameterized by axis number:


      CRPIXja for jCRPna
      PCi_ja for ijPCna
      CDi_ja for ijCDna
      CDELTia for iCDEna
      CROTAi for iCROTn
      CROTAia ... Only if WCSHDR_CROTAia is set.
      CUNITia for iCUNna
      CTYPEia for iCTYna
      CRVALia for iCRVna
      PVi_ma for iVn_ma
      PSi_ma for iSn_ma
      CNAMEia for iCNAna
      CRDERia for iCRDna
      CSYERia for iCSYna
      CZPHSia for TCZPna
      CPERIia for iCPRna

      where the image-header keywords on the left provide default values for the column specific keywords on the right.

      This full inheritance mechanism only applies to binary table image arrays, not pixel lists, because in the latter case there is no well-defined association between coordinate axis number and column number (see note 9 below).

      Note that CNAMEia, CRDERia, CSYERia, and their variants are not used by WCSLIB but are stored in the wcsprm struct as auxiliary information.

      Note especially that at least one wcsprm struct will be returned for each "a" found in one of the image header keywords listed above:

      • If the image header keywords for "a" are not inherited by a binary table, then the struct will not be associated with any particular table column number and it is up to the user to provide an association.

      • If the image header keywords for "a" are inherited by a binary table image array, then those keywords are considered to be "exhausted" and do not result in a separate wcsprm struct.

    For example, to accept CD00i00j and PC00i00j and reject all other extensions, use

    The parser always treats EPOCH as subordinate to EQUINOXa if both are present, and VSOURCEa is always subordinate to ZSOURCEa.

    Likewise, VELREF is subordinate to the formalism of WCS Paper III, see spcaips().

    Neither wcspih() nor wcsbth() currently recognize the AIPS-convention keywords ALTRPIX or ALTRVAL which effectively define an alternative representation for a spectral axis.

  6. Depending on what flags have been set in its relax argument, wcsbth() could return as many as 27027 wcsprm structs:

    • Up to 27 unattached representations derived from image header keywords.

    • Up to 27 structs for each of up to 999 columns containing an image arrays.

    • Up to 27 structs for a pixel list.

    Note that it is considered legitimate for a column to contain an image array and also form part of a pixel list, and in particular that wcsbth() does not check the TFORM keyword for a pixel list column to check that it is scalar.

    In practice, of course, a realistic binary table header is unlikely to contain more than a handful of images.

    In order for wcsbth() to create a wcsprm struct for a particular coordinate representation, at least one WCS keyword that defines an axis number must be present, either directly or by inheritance if WCSHDR_ALLIMG is set.

    When the image header keywords for an alternate representation are inherited by a binary table image array via WCSHDR_ALLIMG, those keywords are considered to be "exhausted" and do not result in a separate wcsprm struct. Otherwise they do.

  7. Neither wcspih() nor wcsbth() check for duplicated keywords, in most cases they accept the last encountered.

  8. wcspih() and wcsbth() use wcsnpv() and wcsnps() (refer to the prologue of wcs.h) to match the size of the pv[] and ps[] arrays in the wcsprm structs to the number in the header. Consequently there are no unused elements in the pv[] and ps[] arrays, indeed they will often be of zero length.

  9. The FITS WCS standard for pixel lists assumes that a pixel list defines one and only one image, i.e. that each row of the binary table refers to just one event, e.g. the detection of a single photon or neutrino, for which the device "pixel" coordinates are stored in separate scalar columns of the table.

    In the absence of a standard for pixel lists - or even an informal description! - let alone a formal mechanism for identifying the columns containing pixel coordinates (as opposed to pixel values or metadata recorded at the time the photon or neutrino was detected), WCS Paper I discusses how the WCS keywords themselves may be used to identify them.

    In practice, however, pixel lists have been used to store multiple images. Besides not specifying how to identify columns, the pixel list convention is also silent on the method to be used to associate table columns with image axes.

    An additional shortcoming is the absence of a formal method for associating global binary-table WCS keywords, such as WCSNna or MJDOBn, with a pixel list image, whether one or several.

    In light of these uncertainties, wcsbth() simply collects all WCS keywords for a particular pixel list coordinate representation (i.e. the "a" value in TCTYna) into one wcsprm struct. However, these alternates need not be associated with the same table columns and this allows a pixel list to contain up to 27 separate images. As usual, if one of these representations happened to contain more than two celestial axes, for example, then an error would result when wcsset() is invoked on it. In this case the "colsel" argument could be used to restrict the columns used to construct the representation so that it only contained one pair of celestial axes.

    Global, binary-table WCS keywords are considered to apply to the pixel list image with matching alternate (e.g. the "a" value in LONPna or EQUIna), regardless of the table columns the image occupies. In other words, the column number is ignored (the "n" value in LONPna or EQUIna). This also applies for global, binary-table WCS keywords that have no alternates, such as MJDOBn and OBSGXn, which match all images in a pixel list. Take heed that this may lead to counterintuitive behaviour, especially where such a keyword references a column that does not store pixel coordinates, and moreso where the pixel list stores only a single image. In fact, as the column number, n, is ignored for such keywords, it would make no difference even if they referenced non-existent columns. Moreover, there is no requirement for consistency in the column numbers used for such keywords, even for OBSGXn, OBSGYn, and OBSGZn which are meant to define the elements of a coordinate vector. Although it would surely be perverse to construct a pixel list like this, such a situation may still arise in practice where columns are deleted from a binary table.

    The situation with global, binary-table WCS keywords becomes potentially even more confusing when image arrays and pixel list images coexist in one binary table. In that case, a keyword such as MJDOBn may legitimately appear multiple times with n referencing different image arrays. Which then is the one that applies to the pixel list images? In this implementation, it is the last instance that appears in the header, whether or not it is also associated with an image array.

◆ wcstab()

int wcstab ( struct wcsprm wcs)

wcstab() assists in filling in the information in the wcsprm struct relating to coordinate lookup tables.

Tabular coordinates ('TAB') present certain difficulties in that the main components of the lookup table - the multidimensional coordinate array plus an index vector for each dimension - are stored in a FITS binary table extension (BINTABLE). Information required to locate these arrays is stored in PVi_ma and PSi_ma keywords in the image header.

wcstab() parses the PVi_ma and PSi_ma keywords associated with each 'TAB' axis and allocates memory in the wcsprm struct for the required number of tabprm structs. It sets as much of the tabprm struct as can be gleaned from the image header, and also sets up an array of wtbarr structs (described in the prologue of wtbarr.h) to assist in extracting the required arrays from the BINTABLE extension(s).

It is then up to the user to allocate memory for, and copy arrays from the BINTABLE extension(s) into the tabprm structs. A CFITSIO routine, fits_read_wcstab(), has been provided for this purpose, see getwcstab.h. wcsset() will automatically take control of this allocated memory, in particular causing it to be free'd by wcsfree(); the user must not attempt to free it after wcsset() has been called.

Note that wcspih() and wcsbth() automatically invoke wcstab() on each of the wcsprm structs that they return.

Parameters
[in,out]wcsCoordinate transformation parameters (see below).
wcstab() sets ntab, tab, nwtb and wtb, allocating memory for the tab and wtb arrays. This allocated memory will be free'd automatically by wcsfree().
Returns
Status return value:
  • 0: Success.
  • 1: Null wcsprm pointer passed.
  • 2: Memory allocation failed.
  • 3: Invalid tabular parameters.
For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().

◆ wcsidx()

int wcsidx ( int  nwcs,
struct wcsprm **  wcs,
int  alts[27] 
)

wcsidx() returns an array of 27 indices for the alternate coordinate representations in the array of wcsprm structs returned by wcspih(). For the array returned by wcsbth() it returns indices for the unattached (colnum == 0) representations derived from image header keywords - use wcsbdx() for those derived from binary table image arrays or pixel lists keywords.

Parameters
[in]nwcsNumber of coordinate representations in the array.
[in]wcsPointer to an array of wcsprm structs returned by wcspih() or wcsbth().
[out]altsIndex of each alternate coordinate representation in the array: alts[0] for the primary, alts[1] for 'A', etc., set to -1 if not present.
For example, if there was no 'P' representation then
alts['P'-'A'+1] == -1;

Otherwise, the address of its wcsprm struct would be
wcs + alts['P'-'A'+1];
Returns
Status return value:
  • 0: Success.
  • 1: Null wcsprm pointer passed.

◆ wcsbdx()

int wcsbdx ( int  nwcs,
struct wcsprm **  wcs,
int  type,
short  alts[1000][28] 
)

wcsbdx() returns an array of 999 x 27 indices for the alternate coordinate representions for binary table image arrays xor pixel lists in the array of wcsprm structs returned by wcsbth(). Use wcsidx() for the unattached representations derived from image header keywords.

Parameters
[in]nwcsNumber of coordinate representations in the array.
[in]wcsPointer to an array of wcsprm structs returned by wcsbth().
[in]typeSelect the type of coordinate representation:
  • 0: binary table image arrays,
  • 1: pixel lists.
[out]altsIndex of each alternate coordinate represention in the array: alts[col][0] for the primary, alts[col][1] for 'A', to alts[col][26] for 'Z', where col is the 1-relative column number, and col == 0 is used for unattached image headers. Set to -1 if not present.
alts[col][27] counts the number of coordinate representations of the chosen type for each column.
For example, if there was no 'P' represention for column 13 then
alts[13]['P'-'A'+1] == -1;

Otherwise, the address of its wcsprm struct would be
wcs + alts[13]['P'-'A'+1];
Returns
Status return value:
  • 0: Success.
  • 1: Null wcsprm pointer passed.

◆ wcsvfree()

int wcsvfree ( int *  nwcs,
struct wcsprm **  wcs 
)

wcsvfree() frees the memory allocated by wcspih() or wcsbth() for the array of wcsprm structs, first invoking wcsfree() on each of the array members.

Parameters
[in,out]nwcsNumber of coordinate representations found; set to 0 on return.
[in,out]wcsPointer to the array of wcsprm structs; set to 0x0 on return.
Returns
Status return value:
  • 0: Success.
  • 1: Null wcsprm pointer passed.

◆ wcshdo()

int wcshdo ( int  ctrl,
struct wcsprm wcs,
int *  nkeyrec,
char **  header 
)

wcshdo() translates a wcsprm struct into a FITS header. If the colnum member of the struct is non-zero then a binary table image array header will be produced. Otherwise, if the colax[] member of the struct is set non-zero then a pixel list header will be produced. Otherwise, a primary image or image extension header will be produced.

If the struct was originally constructed from a header, e.g. by wcspih(), the output header will almost certainly differ in a number of respects:

  • The output header only contains WCS-related keywords. In particular, it does not contain syntactically-required keywords such as SIMPLE, NAXIS, BITPIX, or END.
  • Elements of the PCi_ja matrix will be written if and only if they differ from the unit matrix. Thus, if the matrix is unity then no elements will be written.
  • The redundant keywords MJDREF, JDREF, JDREFI, JDREFF, all of which duplicate MJDREFI + MJDREFF, are never written. OBSGEO-[LBH] are not written if OBSGEO-[XYZ] are defined.
  • Deprecated (e.g. CROTAn, RESTFREQ, VELREF, RADECSYS, EPOCH, VSOURCEa) or non-standard usage will be translated to standard (this is partially dependent on whether wcsfix() was applied).
  • Additional keywords such as WCSAXESa, CUNITia, LONPOLEa and LATPOLEa may appear.
  • Quantities will be converted to the units used internally, basically SI with the addition of degrees.
  • Floating-point quantities may be given to a different decimal precision.
  • The original keycomments will be lost, although wcshdo() tries hard to write meaningful comments.
  • Keyword order will almost certainly be changed.

Keywords can be translated between the image array, binary table, and pixel lists forms by manipulating the colnum or colax[] members of the wcsprm struct.

Parameters
[in]ctrlVector of flag bits that controls the degree of permissiveness in departing from the published WCS standard, and also controls the formatting of floating-point keyvalues. Set it to zero to get the default behaviour.
Flag bits for the degree of permissiveness:
  • WCSHDO_none: Recognize only FITS keywords defined by the published WCS standard.
  • WCSHDO_all: Admit all recognized informal extensions of the WCS standard.
Fine-grained control of the degree of permissiveness is also possible as explained in the notes below.
As for controlling floating-point formatting, by default wcshdo() uses "%20.12G" for non-parameterized keywords such as LONPOLEa, and attempts to make the header more human-readable by using the same "f" format for all values of each of the following parameterized keywords: CRPIXja, PCi_ja, and CDELTia (n.b. excluding CRVALia). Each has the same field width and precision so that the decimal points line up. The precision, allowing for up to 15 significant digits, is chosen so that there are no excess trailing zeroes. A similar formatting scheme applies by default for distortion function parameters.
However, where the values of, for example, CDELTia differ by many orders of magnitude, the default formatting scheme may cause unacceptable loss of precision for the lower-valued keyvalues. Thus the default behaviour may be overridden:
  • WCSHDO_P12: Use "%20.12G" format for all floating- point keyvalues (12 significant digits).
  • WCSHDO_P13: Use "%21.13G" format for all floating- point keyvalues (13 significant digits).
  • WCSHDO_P14: Use "%22.14G" format for all floating- point keyvalues (14 significant digits).
  • WCSHDO_P15: Use "%23.15G" format for all floating- point keyvalues (15 significant digits).
  • WCSHDO_P16: Use "%24.16G" format for all floating- point keyvalues (16 significant digits).
  • WCSHDO_P17: Use "%25.17G" format for all floating- point keyvalues (17 significant digits).
If more than one of the above flags are set, the highest number of significant digits prevails. In addition, there is an anciliary flag:
  • WCSHDO_EFMT: Use "E" format instead of the default "G" format above.
Note that excess trailing zeroes are stripped off the fractional part with "G" (which never occurs with "E"). Note also that the higher-precision options eat into the keycomment area. In this regard, WCSHDO_P14 causes minimal disruption with "G" format, while WCSHDO_P13 is appropriate with "E".
[in,out]wcsPointer to a wcsprm struct containing coordinate transformation parameters. Will be initialized if necessary.
[out]nkeyrecNumber of FITS header keyrecords returned in the "header" array.
[out]headerPointer to an array of char holding the header. Storage for the array is allocated by wcshdo() in blocks of 2880 bytes (32 x 80-character keyrecords) and must be free'd by the user to avoid memory leaks.
Each keyrecord is 80 characters long and is *NOT* null-terminated, so the first keyrecord starts at (*header)[0], the second at (*header)[80], etc.
Returns
Status return value (associated with wcs_errmsg[]):
  • 0: Success.
  • 1: Null wcsprm pointer passed.
  • 2: Memory allocation failed.
  • 3: Linear transformation matrix is singular.
  • 4: Inconsistent or unrecognized coordinate axis types.
  • 5: Invalid parameter value.
  • 6: Invalid coordinate transformation parameters.
  • 7: Ill-conditioned coordinate transformation parameters.
For returns > 1, a detailed error message is set in wcsprm::err if enabled, see wcserr_enable().

Notes:
wcshdo() interprets the relax argument as a vector of flag bits to provide fine-grained control over what non-standard WCS keywords to write. The flag bits are subject to change in future and should be set by using the preprocessor macros (see below) for the purpose.

  • WCSHDO_none: Don't use any extensions.

  • WCSHDO_all: Write all recognized extensions, equivalent to setting each flag bit.

  • WCSHDO_safe: Write all extensions that are considered to be safe and recommended.

  • WCSHDO_DOBSn: Write DOBSn, the column-specific analogue of DATE-OBS for use in binary tables and pixel lists. WCS Paper III introduced DATE-AVG and DAVGn but by an oversight DOBSn (the obvious analogy) was never formally defined by the standard. The alternative to using DOBSn is to write DATE-OBS which applies to the whole table. This usage is considered to be safe and is recommended.

  • WCSHDO_TPCn_ka: WCS Paper I defined

    • TPn_ka and TCn_ka for pixel lists

    but WCS Paper II uses TPCn_ka in one example and subsequently the errata for the WCS papers legitimized the use of

    • TPCn_ka and TCDn_ka for pixel lists

    provided that the keyword does not exceed eight characters. This usage is considered to be safe and is recommended because of the non-mnemonic terseness of the shorter forms.

  • WCSHDO_PVn_ma: WCS Paper I defined

    • iVn_ma and iSn_ma for bintables and
    • TVn_ma and TSn_ma for pixel lists

    but WCS Paper II uses iPVn_ma and TPVn_ma in the examples and subsequently the errata for the WCS papers legitimized the use of

    • iPVn_ma and iPSn_ma for bintables and
    • TPVn_ma and TPSn_ma for pixel lists

    provided that the keyword does not exceed eight characters. This usage is considered to be safe and is recommended because of the non-mnemonic terseness of the shorter forms.

  • WCSHDO_CRPXna: For historical reasons WCS Paper I defined

    • jCRPXn, iCDLTn, iCUNIn, iCTYPn, and iCRVLn for bintables and
    • TCRPXn, TCDLTn, TCUNIn, TCTYPn, and TCRVLn for pixel lists

    for use without an alternate version specifier. However, because of the eight-character keyword constraint, in order to accommodate column numbers greater than 99 WCS Paper I also defined

    • jCRPna, iCDEna, iCUNna, iCTYna and iCRVna for bintables and
    • TCRPna, TCDEna, TCUNna, TCTYna and TCRVna for pixel lists

    for use with an alternate version specifier (the "a"). Like the PC, CD, PV, and PS keywords there is an obvious tendency to confuse these two forms for column numbers up to 99. It is very unlikely that any parser would reject keywords in the first set with a non-blank alternate version specifier so this usage is considered to be safe and is recommended.

  • WCSHDO_CNAMna: WCS Papers I and III defined

    • iCNAna, iCRDna, and iCSYna for bintables and
    • TCNAna, TCRDna, and TCSYna for pixel lists

    By analogy with the above, the long forms would be

    • iCNAMna, iCRDEna, and iCSYEna for bintables and
    • TCNAMna, TCRDEna, and TCSYEna for pixel lists

    Note that these keywords provide auxiliary information only, none of them are needed to compute world coordinates. This usage is potentially unsafe and is not recommended at this time.

  • WCSHDO_WCSNna: In light of wcsbth() note 4, write WCSNna instead of TWCSna for pixel lists. While wcsbth() treats WCSNna and TWCSna as equivalent, other parsers may not. Consequently, this usage is potentially unsafe and is not recommended at this time.

Variable Documentation

◆ wcshdr_errmsg

const char * wcshdr_errmsg[]

Error messages to match the status value returned from each function. Use wcs_errmsg[] for status returns from wcshdo().