Wcs.cc

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// -*- lsst-c++ -*-

/* 
 * LSST Data Management System
 * Copyright 2008, 2009, 2010 LSST Corporation.
 * 
 * This product includes software developed by the
 * LSST Project (http://www.lsst.org/).
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program 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 General Public License for more details.
 * 
 * You should have received a copy of the LSST License Statement and 
 * the GNU General Public License along with this program.  If not, 
 * see <http://www.lsstcorp.org/LegalNotices/>.
 */

#include <iostream>
#include <sstream>
#include <cmath>
#include <cstring>

#include "boost/format.hpp"

#include "wcslib/wcs.h"
#include "wcslib/wcsfix.h"
#include "wcslib/wcshdr.h"

#include "lsst/daf/base.h"
#include "lsst/daf/base/Citizen.h"
#include "lsst/afw/formatters/Utils.h"
#include "lsst/afw/formatters/WcsFormatter.h"
#include "lsst/pex/exceptions.h"
#include "lsst/afw/image/ImageUtils.h"
#include "lsst/afw/image/Wcs.h"
#include "lsst/afw/coord/Coord.h"
#include "lsst/afw/geom/Angle.h"
#include "lsst/afw/table/io/OutputArchive.h"
#include "lsst/afw/table/io/InputArchive.h"
#include "lsst/afw/table/io/CatalogVector.h"

namespace except = lsst::pex::exceptions; 
namespace afwImg = lsst::afw::image;
namespace afwCoord = lsst::afw::coord;
namespace afwGeom = lsst::afw::geom;


using namespace std;

typedef lsst::daf::base::PropertySet PropertySet;
typedef lsst::daf::base::PropertyList PropertyList;
typedef lsst::afw::image::Wcs Wcs;
typedef lsst::afw::geom::Point2D GeomPoint;
typedef lsst::afw::coord::Coord::Ptr CoordPtr;
typedef lsst::afw::image::XYTransformFromWcsPair XYTransformFromWcsPair;

//The amount of space allocated to strings in wcslib
const int STRLEN = 72;

//Set internal params for wcslib
void lsst::afw::image::Wcs::_setWcslibParams()
{
    _wcsfixCtrl =                       // ctrl for wcsfix
        2;                              // Translate "H" to "h"
    _wcshdrCtrl =                       // ctrl for wcspih
        2;                              // Report each rejected keyrecord and the reason why it was rejected
    _relax =                            // relax parameter for wcspih;
        WCSHDR_all;                     // Accept all extensions recognized by the parser
}

const int lsstToFitsPixels = +1;
const int fitsToLsstPixels = -1;

//
// Constructors
//


lsst::afw::image::Wcs::Wcs() :
    daf::base::Citizen(typeid(this)),
    _wcsInfo(NULL), _nWcsInfo(0), _relax(0), _wcsfixCtrl(0), _wcshdrCtrl(0), _nReject(0),
    _coordSystem(static_cast<afwCoord::CoordSystem>(-1)) {
    _setWcslibParams();
    _initWcs();    
}


Wcs::Wcs(CONST_PTR(lsst::daf::base::PropertySet) const& fitsMetadata):
    daf::base::Citizen(typeid(this)),
    _wcsInfo(NULL), 
    _nWcsInfo(0), 
    _relax(0), 
    _wcsfixCtrl(0), 
    _wcshdrCtrl(0),
    _nReject(0),
    _coordSystem(static_cast<afwCoord::CoordSystem>(-1))
{
    _setWcslibParams();

    initWcsLibFromFits(fitsMetadata);
    _initWcs();
}

/*
 * Set some internal variables that we need to refer to
 */
void Wcs::_initWcs()
{
    if (_wcsInfo) {
        _coordSystem = afwCoord::makeCoordEnum(_wcsInfo->radesys);

        // tell WCSlib that values have been updated
        _wcsInfo->flag = 0;
        // and then tell it to do its internal magic.
        int status = wcsset(_wcsInfo);
        if (status != 0) {
            throw LSST_EXCEPT(except::RuntimeError,
                              (boost::format("Failed to setup wcs structure with wcsset. Status %d: %s") %
                               status % wcs_errmsg[status] ).str());
        }
    }
}

Wcs::Wcs(GeomPoint const & crval, GeomPoint const & crpix, Eigen::Matrix2d const & CD, 
         std::string const & ctype1, std::string const & ctype2,
         double equinox, std::string const & raDecSys,
         std::string const & cunits1, std::string const & cunits2
):
    daf::base::Citizen(typeid(this)),
    _wcsInfo(NULL), 
    _nWcsInfo(0), 
    _relax(0), 
    _wcsfixCtrl(0), 
    _wcshdrCtrl(0),
    _nReject(0),
    _coordSystem(static_cast<afwCoord::CoordSystem>(-1))
{
    _setWcslibParams();
    initWcsLib(crval, crpix, CD, 
               ctype1, ctype2,
               equinox, raDecSys,
               cunits1, cunits2);
    _initWcs();
}
               
    
void Wcs::initWcsLibFromFits(CONST_PTR(lsst::daf::base::PropertySet) const& header){
    class HeaderAccess {
    public:
        CONST_PTR(lsst::daf::base::PropertySet) const& toRead() { return _constHeader; }
        PTR(lsst::daf::base::PropertySet) const& toWrite() {
            if (!_hackHeader) {
                _hackHeader = _constHeader->deepCopy();
                _constHeader = _hackHeader;
            }
            return _hackHeader;
        }
        
        HeaderAccess(CONST_PTR(lsst::daf::base::PropertySet) const& header) :
            _constHeader(header), _hackHeader() {}

    private:
        CONST_PTR(lsst::daf::base::PropertySet) _constHeader;
        PTR(lsst::daf::base::PropertySet) _hackHeader;
    };

    HeaderAccess access(header);

    // Some headers (e.g. SDSS ones from FNAL) have EQUINOX as a string.  Fix this,
    // as wcslib 4.4.4 refuses to handle it
    {
        std::string const& key = "EQUINOX";
        if (access.toRead()->exists(key) && access.toRead()->typeOf(key) == typeid(std::string)) {
            double equinox = ::atof(access.toRead()->getAsString(key).c_str());
            access.toWrite()->set(key, equinox);
        }
    }

    //Check header isn't empty
    int nCards = lsst::afw::formatters::countFitsHeaderCards(access.toRead());
    if (nCards <= 0) {
        string msg = "Could not parse FITS WCS: no header cards found";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }

    // Scamp produces PVi_xx header cards that are inconsistent with WCS Paper 2
    // and cause WCSLib to choke.  Aggressively, rename all PV keywords to X_PV
    for (int j=1; j<3; j++) {
        for (int i=0; i<=99; i++) {
            std::string key = (boost::format("PV%i_%i") % j % i).str();
            if (!access.toRead()->exists(key)) {
                break;
            }
            double val = access.toRead()->getAsDouble(key);
            access.toWrite()->remove(key);
            access.toWrite()->add("X_"+key, val);
        }
    }

    //While the standard does not insist on CRVAL and CRPIX being present, it 
    //is almost certain their absence indicates a problem.   
    //Check for CRPIX
    if( !access.toRead()->exists("CRPIX1") && !access.toRead()->exists("CRPIX1a")) {
        string msg = "Neither CRPIX1 not CRPIX1a found";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }

    if( !access.toRead()->exists("CRPIX2") && !access.toRead()->exists("CRPIX2a")) {
        string msg = "Neither CRPIX2 not CRPIX2a found";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }

    //And the same for CRVAL
    if( !access.toRead()->exists("CRVAL1") && !access.toRead()->exists("CRVAL1a")) {
        string msg = "Neither CRVAL1 not CRVAL1a found";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }

    if( !access.toRead()->exists("CRVAL2") && !access.toRead()->exists("CRVAL2a")) {
        string msg = "Neither CRVAL2 not CRVAL2a found";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }
    /*
     * According to Greisen and Calabretta (A&A 395, 1061–1075 (2002)) it's illegal to mix PCi_j and CDi_j
     * headers; unfortunately Subaru puts both in its headers.  It actually uses PC001002 instead of PC1_2
     * (dating to a proposed FITS standard from 1996) and at least sometimes fails to include CDELT[12],
     * so the CD and PC matrices are inconsistent
     *
     * If we detect any part of a CD matrix, delete all PC matrices
     */
    if(access.toRead()->exists("CD1_1") || access.toRead()->exists("CD1_2") ||
       access.toRead()->exists("CD2_1") || access.toRead()->exists("CD2_2")) {
        for (int i = 1; i <= 2; ++i) {
            for (int j = 1; j <= 2; ++j) {
                std::string key = (boost::format("PC%i_%i") % j % i).str();
                if (access.toRead()->exists(key)) {
                    double const val = access.toRead()->getAsDouble(key);
                    access.toWrite()->remove(key);
                    access.toWrite()->add("X_" + key, val);
                }

                key = (boost::format("PC%03d%03d") % j % i).str();
                if (access.toRead()->exists(key)) {
                    double const val = access.toRead()->getAsDouble(key);
                    access.toWrite()->remove(key);
                    access.toWrite()->add("X_" + key, val);
                }
            }
        }
    }

    //Pass the header into wcslib's formatter to extract & setup the Wcs. First need
    //to convert to a C style string, so the compile doesn't complain about constness
    std::string metadataStr = lsst::afw::formatters::formatFitsProperties(access.toRead());
    // We own the data, and wcslib is slack about constness, so no qualms with casting away const
    char *hdrString = const_cast<char*>(metadataStr.c_str());
    //printf("wcspih string:\n%s\n", hdrString);
    
    nCards = lsst::afw::formatters::countFitsHeaderCards(access.toRead()); // we may have dropped some
    int pihStatus = wcspih(hdrString, nCards, _relax, _wcshdrCtrl, &_nReject, &_nWcsInfo, &_wcsInfo);

    if (pihStatus != 0) {
        throw LSST_EXCEPT(except::RuntimeError,
                          (boost::format("Could not parse FITS WCS: wcspih status = %d (%s)") %
                           pihStatus % wcs_errmsg[pihStatus]).str());
    }    

    //Run wcsfix on _wcsInfo to try and fix any problems it knows about.
    const int *naxes = NULL;            // should be {NAXIS1, NAXIS2, ...} to check cylindrical projections
    int stats[NWCSFIX];                 // status returns from wcsfix
    int fixStatus = wcsfix(_wcsfixCtrl, naxes, _wcsInfo, stats);
    if (fixStatus != 0) {
        std::stringstream errStream;
        errStream << "Could not parse FITS WCS: wcsfix failed " << std::endl;
        for (int ii = 0; ii < NWCSFIX; ++ii) {
            if (stats[ii] >= 0) {
                errStream << "\t" << ii << ": " << stats[ii] << " " << wcsfix_errmsg[stats[ii]] << std::endl;
            } else {
                errStream << "\t" << ii << ": " << stats[ii] << std::endl;
            }
        }
    }
    
    //The Wcs standard requires a default value for RADESYS if the keyword
    //doesn't exist in header, but wcslib doesn't set it. So we do so here. This code 
    //conforms to Calabretta & Greisen 2002 \S 3.1
    if (!(access.toRead()->exists("RADESYS") || access.toRead()->exists("RADESYSa"))) {
        // If RADECSYS exists, use that (counter to Calabretta & Greisen 2002 \S 3.1, but commonly used).
        // If equinox exist and < 1984, use FK4. If >= 1984, use FK5
        if (access.toRead()->exists("RADECSYS")) {
            strncpy(_wcsInfo->radesys, access.toRead()->getAsString("RADECSYS").c_str(), STRLEN);
        } else if (access.toRead()->exists("EQUINOX") || access.toRead()->exists("EQUINOXa")) {
            std::string const EQUINOX = access.toRead()->exists("EQUINOX") ? "EQUINOX" : "EQUINOXa";
            double const equinox = access.toRead()->getAsDouble(EQUINOX);
            if(equinox < 1984) {
                strncpy(_wcsInfo->radesys, "FK4", STRLEN);
            } else {
                strncpy(_wcsInfo->radesys, "FK5", STRLEN);
            }
        } else {
            //If Equinox doesn't exist, default to ICRS
            strncpy(_wcsInfo->radesys, "ICRS", STRLEN);
        }
    }
    // strip trailing whitespace
    {
        for(int i = strlen(_wcsInfo->radesys) - 1; i >= 0; i--) {
            if (isspace(_wcsInfo->radesys[i])) {
                _wcsInfo->radesys[i] = '\0';
            }
        }
    }
    //
    // If there are no CDi_j cards in the header, set CDi_j from PCi_j
    // CDi_j == CDELTi*PCi_j
    //
    if ((_wcsInfo->altlin & 2) == 0) {  // no CDi_j cards were found in the header
        double const *cdelt = _wcsInfo->cdelt;
        double const *pc = _wcsInfo->pc;
        double *cd = _wcsInfo->cd;

        cd[0] = cdelt[0]*pc[0];         // 1_1
        cd[1] = cdelt[0]*pc[1];         // 1_2
        cd[2] = cdelt[1]*pc[2];         // 2_1
        cd[3] = cdelt[1]*pc[3];         // 2_2
    }
}

void Wcs::initWcsLib(GeomPoint const & crval, GeomPoint const & crpix, Eigen::Matrix2d const & CD, 
                     std::string const & ctype1, std::string const & ctype2,
                     double equinox, std::string const & raDecSys,
                     std::string const & cunits1, std::string const & cunits2) {

    //Check CD is a valid size
    if( (CD.rows() != 2) || (CD.cols() != 2) ) {
        string msg = "CD is not a 2x2 matrix";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }

    //Check that cunits are legitimate values
    bool isValid = (cunits1 == "deg");
    isValid |= (cunits1 == "arcmin");
    isValid |= (cunits1 == "arcsec");
    isValid |= (cunits1 == "mas");
    
    if (!isValid) {
        string msg =  "CUNITS1 must be one of {deg|arcmin|arcsec|mas}";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }        

    isValid = (cunits2 == "deg");
    isValid |= (cunits2 == "arcmin");
    isValid |= (cunits2 == "arcsec");
    isValid |= (cunits2 == "mas");
    
    if (!isValid) {
        string msg =  "CUNITS2 must be one of {deg|arcmin|arcsec|mas}";
        throw LSST_EXCEPT(except::InvalidParameterError, msg);
    }        

    //Initialise the wcs struct
    _wcsInfo = static_cast<struct wcsprm *>(malloc(sizeof(struct wcsprm)));
    if (_wcsInfo == NULL) {
        throw LSST_EXCEPT(except::MemoryError, "Cannot allocate WCS info");
    }

    _wcsInfo->flag = -1;
    int status = wcsini(true, 2, _wcsInfo);   //2 indicates a naxis==2, a two dimensional image
    if(status != 0) {
        throw LSST_EXCEPT(except::MemoryError,
                          (boost::format("Failed to allocate memory with wcsini. Status %d: %s") %
                           status % wcs_errmsg[status] ).str());
    }
    
    //Set crval, crpix and CD. Internally to the class, we use fits units for consistency with
    //wcslib.
    _wcsInfo->crval[0] = crval.getX();
    _wcsInfo->crval[1] = crval.getY();
    _wcsInfo->crpix[0] = crpix.getX() + lsstToFitsPixels;
    _wcsInfo->crpix[1] = crpix.getY() + lsstToFitsPixels;

    //Set the CD matrix
    for (int i=0; i<2; ++i) {
        for (int j=0; j<2; ++j) {
            _wcsInfo->cd[(2*i) + j] = CD(i,j);
        }
    }

    //Specify that we have a CD matrix, but no PC or CROTA
    _wcsInfo->altlin = 2;
    _wcsInfo->flag   = 0;   //values have been updated

    //This is a work around for what I think is a bug in wcslib. ->types is neither
    //initialised or set to NULL by default, so if I try to delete a Wcs object,
    //wcslib then attempts to free non-existent space, and the code can crash.
    _wcsInfo->types = NULL;

    //Set the coordinate system
    strncpy(_wcsInfo->ctype[0], ctype1.c_str(), STRLEN);
    strncpy(_wcsInfo->ctype[1], ctype2.c_str(), STRLEN);
    strncpy(_wcsInfo->radesys, raDecSys.c_str(), STRLEN);
    _wcsInfo->equinox = equinox;
    
    //Set the units
    strncpy(_wcsInfo->cunit[0], cunits1.c_str(), STRLEN);
    strncpy(_wcsInfo->cunit[1], cunits2.c_str(), STRLEN);
    
    _nWcsInfo = 1;   //Specify that we have only one coordinate representation

    //Tell wcslib that we are need to set up internal values
    status=wcsset(_wcsInfo);
    if(status != 0) {
        throw LSST_EXCEPT(except::RuntimeError,
                          (boost::format("Failed to setup wcs structure with wcsset. Status %d: %s") %
                           status % wcs_errmsg[status] ).str());

    }
}


Wcs::Wcs(afwImg::Wcs const & rhs) : 
    daf::base::Citizen(typeid(this)),
    _wcsInfo(NULL), 
    _nWcsInfo(rhs._nWcsInfo), 
    _relax(rhs._relax), 
    _wcsfixCtrl(rhs._wcsfixCtrl), 
    _wcshdrCtrl(rhs._wcshdrCtrl),
    _nReject(rhs._nReject),
    _coordSystem(static_cast<afwCoord::CoordSystem>(-1))
{
    
    if (rhs._nWcsInfo > 0) {
        _wcsInfo = static_cast<struct wcsprm *>(calloc(rhs._nWcsInfo, sizeof(struct wcsprm)));
        if (_wcsInfo == NULL) {
            throw LSST_EXCEPT(lsst::pex::exceptions::MemoryError, "Cannot allocate WCS info");
        }

        _wcsInfo->flag = -1;
        int alloc = 1;                  //Unconditionally allocate memory when calling
        for (int i = 0; i != rhs._nWcsInfo; ++i) {
            int status = wcscopy(alloc, &rhs._wcsInfo[i], &_wcsInfo[i]);
            if (status != 0) {
                wcsvfree(&i, &_wcsInfo);
                throw LSST_EXCEPT(lsst::pex::exceptions::MemoryError,
                                  (boost::format("Could not copy WCS: wcscopy status = %d : %s") %
                                   status % wcs_errmsg[status]).str());
            }
        }
    }
    _initWcs();
}
       
bool Wcs::operator==(Wcs const & other) const {
    if (&other == this) return true;
    // We do a bidirectional test with a virtual member function in case one of us is a derived
    // class with members we don't know about here.
    // This is not the most efficient possible implementation, but I think it's the easiest one
    // with which to ensure correctness, and I think that's more important in this case.
    return this->_isSubset(other) && other._isSubset(*this);
}

// convenience functions and a macro for implementing isSubset
namespace {

inline bool compareArrays(double const * a, double const * b, int n) {
    for (int i = 0; i < n; ++i) if (a[i] != b[i]) return false;
    return true;
}

template <typename T>
inline bool compareStringArrays(T a, T b, int n) {
    for (int i = 0; i < n; ++i) if (strcmp(a[i], b[i]) != 0) return false;
    return true;
}

#define CHECK_NULLS(a, b)                       \
    do {                                        \
        if ((a) == NULL) {                      \
            if ((b) == NULL) return true;       \
            return false;                       \
        }                                       \
        if ((b) == NULL) return false;          \
    } while (false)

} // anonymous

bool Wcs::_isSubset(Wcs const & rhs) const {
    CHECK_NULLS(_wcsInfo, rhs._wcsInfo);
    CHECK_NULLS(_wcsInfo->crval, rhs._wcsInfo->crval);
    CHECK_NULLS(_wcsInfo->cd, rhs._wcsInfo->cd);
    CHECK_NULLS(_wcsInfo->crpix, rhs._wcsInfo->crpix);
    CHECK_NULLS(_wcsInfo->cunit, rhs._wcsInfo->cunit);
    CHECK_NULLS(_wcsInfo->ctype, rhs._wcsInfo->ctype);
    return _nWcsInfo == rhs._nWcsInfo &&
        _coordSystem == rhs._coordSystem &&
        _wcsInfo->naxis == rhs._wcsInfo->naxis &&
        _wcsInfo->equinox == rhs._wcsInfo->equinox &&
        _wcsInfo->altlin == rhs._wcsInfo->altlin &&
        compareArrays(_wcsInfo->crval, rhs._wcsInfo->crval, 2) &&
        compareArrays(_wcsInfo->crpix, rhs._wcsInfo->crpix, 2) &&
        compareArrays(_wcsInfo->cd, rhs._wcsInfo->cd, 4) &&
        compareStringArrays(_wcsInfo->cunit, rhs._wcsInfo->cunit, 2) &&
        compareStringArrays(_wcsInfo->ctype, rhs._wcsInfo->ctype, 2) &&
        skyToPixel(_wcsInfo->crval[0] * afwGeom::degrees,
                   _wcsInfo->crval[1] * afwGeom::degrees) ==
        rhs.skyToPixel(_wcsInfo->crval[0] * afwGeom::degrees,
                       _wcsInfo->crval[1] * afwGeom::degrees) &&
        *pixelToSky(_wcsInfo->crpix[0], _wcsInfo->crpix[1]) ==
        *rhs.pixelToSky(_wcsInfo->crpix[0], _wcsInfo->crpix[1]);
}

Wcs::~Wcs() {
    if (_wcsInfo != NULL) {
        wcsvfree(&_nWcsInfo, &_wcsInfo);
    }
}
    

Wcs::Ptr Wcs::clone(void) const {
    return Wcs::Ptr(new Wcs(*this));
}

//
// Accessors
//

CoordPtr Wcs::getSkyOrigin() const {
    assert(_wcsInfo);
    return makeCorrectCoord(_wcsInfo->crval[0] * afwGeom::degrees, _wcsInfo->crval[1] * afwGeom::degrees);
}

GeomPoint Wcs::getPixelOrigin() const {
    assert(_wcsInfo);
    //Convert from fits units back to lsst units
    double p1 = _wcsInfo->crpix[0] + fitsToLsstPixels;
    double p2 = _wcsInfo->crpix[1] + fitsToLsstPixels;
    return afwGeom::Point2D(p1, p2);
}

Eigen::Matrix2d Wcs::getCDMatrix() const {
    assert(_wcsInfo);
    int const naxis = _wcsInfo->naxis;

    //If naxis != 2, I'm not sure if any of what follows is correct
    assert(naxis == 2);
    
    Eigen::Matrix2d C;

    for (int i=0; i< naxis; ++i){
        for (int j=0; j<naxis; ++j) {
            C(i,j) = _wcsInfo->cd[ (i*naxis) + j ];
        }
    }

    return C;
}

void Wcs::flipImage(int flipLR, int flipTB, afwGeom::Extent2I dimensions) const {
    assert(_wcsInfo);
    
    int const naxis = _wcsInfo->naxis;

    //If naxis != 2, I'm not sure if any of what follows is correct
    assert(naxis == 2);
    if (flipLR) {
        _wcsInfo->cd[0] = -_wcsInfo->cd[0];
        _wcsInfo->cd[2] = -_wcsInfo->cd[2];
        _wcsInfo->crpix[0] = -_wcsInfo->crpix[0] + dimensions.getX();
    }
    if (flipTB) {
        _wcsInfo->cd[1] = -_wcsInfo->cd[1];
        _wcsInfo->cd[3] = -_wcsInfo->cd[3];
        _wcsInfo->crpix[1] = -_wcsInfo->crpix[1]+dimensions.getY();
    }

    // tells libwcs to invalidate cached data, since transformation has been modified
    _wcsInfo->flag = 0;
}

void Wcs::rotateImageBy90(int nQuarter, afwGeom::Extent2I dimensions) const {
    assert(_wcsInfo);

    while (nQuarter < 0 ) {
        nQuarter += 4;
    }

    
    int const naxis = _wcsInfo->naxis;

    //If naxis != 2, I'm not sure if any of what follows is correct
    assert(naxis == 2);
    double a = _wcsInfo->cd[0];
    double b = _wcsInfo->cd[1];
    double c = _wcsInfo->cd[2];
    double d = _wcsInfo->cd[3];
    double crpx = _wcsInfo->crpix[0];
    double crpy = _wcsInfo->crpix[1];
    switch (nQuarter%4) {
        case 0:
            break;
        case 1:
            _wcsInfo->cd[0] = -b;
            _wcsInfo->cd[1] = a;
            _wcsInfo->cd[2] = -d;
            _wcsInfo->cd[3] = c;
            _wcsInfo->crpix[0] = -crpy + dimensions.getY();
            _wcsInfo->crpix[1] = crpx;
            break;
        case 2:
            _wcsInfo->cd[0] = -a;
            _wcsInfo->cd[1] = -b;
            _wcsInfo->cd[2] = -c;
            _wcsInfo->cd[3] = -d;
            _wcsInfo->crpix[0] = -crpx + dimensions.getX();
            _wcsInfo->crpix[1] = -crpy + dimensions.getY();
            break;
        case 3:
            _wcsInfo->cd[0] = b;
            _wcsInfo->cd[1] = -a;
            _wcsInfo->cd[2] = d;
            _wcsInfo->cd[3] = -c;
            _wcsInfo->crpix[0] = crpy;
            _wcsInfo->crpix[1] = -crpx + dimensions.getX();
            break;
    }

    // tells libwcs to invalidate cached data, since transformation has been modified
    _wcsInfo->flag = 0;
}

PropertyList::Ptr Wcs::getFitsMetadata() const {
    return lsst::afw::formatters::WcsFormatter::generatePropertySet(*this);
}

bool Wcs::isFlipped() const {
    // We calculate the determinant of the CD (i.e the rotation and scaling)
    // matrix. If this determinant is positive, then the image can be rotated
    // to a position where increasing the right ascension and declination
    // increases the horizontal and vertical pixel position. In this case the
    // image is flipped.
    assert(_wcsInfo);
    double det = (_wcsInfo->cd[0] * _wcsInfo->cd[3]) - (_wcsInfo->cd[1] * _wcsInfo->cd[2]);

    if (det == 0) {
        throw(LSST_EXCEPT(except::RuntimeError, "Wcs CD matrix is singular"));
    }

    return (det > 0);
}

static double square(double x) {
    return x*x;
}

double Wcs::pixArea(GeomPoint pix00     
                   ) const {
    //
    // Figure out the (0, 0), (0, 1), and (1, 0) ra/dec coordinates of the corners of a square drawn in pixel
    // It'd be better to centre the square at sky00, but that would involve another conversion between sky and
    // pixel coordinates so I didn't bother
    //
    const double side = 1;             // length of the square's sides in pixels

    // Work in 3-space to avoid RA wrapping and pole issues.
    afwGeom::Point3D v0 = pixelToSky(pix00)->getVector();

    // Step by "side" in x and y pixel directions...
    GeomPoint px(pix00);
    GeomPoint py(pix00);
    px.shift(afwGeom::Extent2D(side, 0));
    py.shift(afwGeom::Extent2D(0, side));
    // Push the points through the WCS, and find difference in 3-space.
    afwGeom::Extent3D dx = pixelToSky(px)->getVector() - v0;
    afwGeom::Extent3D dy = pixelToSky(py)->getVector() - v0;

    // Compute |cross product| = area of parallelogram with sides dx,dy
    // FIXME -- this is slightly incorrect -- it's making the small-angle
    // approximation, taking the distance *through* the unit sphere
    // rather than over its surface.
    // This is in units of ~radians^2
    double area = sqrt(square(dx[1]*dy[2] - dx[2]*dy[1]) +
                       square(dx[2]*dy[0] - dx[0]*dy[2]) +
                       square(dx[0]*dy[1] - dx[1]*dy[0]));

    return area / square(side) * square(180. / afwGeom::PI);
}

afwGeom::Angle Wcs::pixelScale() const {
    return sqrt(pixArea(getPixelOrigin())) * afwGeom::degrees;
}

/*
 * Worker routine for skyToPixel
 */
GeomPoint Wcs::skyToPixelImpl(afwGeom::Angle sky1, // RA (or, more generally, longitude)
                              afwGeom::Angle sky2  // Dec (or latitude)
                             ) const {
    assert(_wcsInfo);

    double skyTmp[2];
    double imgcrd[2];
    double phi, theta;
    double pixTmp[2];
    /*
     printf("_skyCoordsReversed: %c\n", (_skyCoordsReversed ? 'T' : 'F'));
     printf("wcsinfo.lat: %i,  lng: %i\n", _wcsInfo->lat, _wcsInfo->lng);
     */
    // WCSLib is smart enough to notice and handle crazy SDSS CTYPE1 = DEC--TAN,
    // by recording the indices of the long and lat coordinates.
    skyTmp[_wcsInfo->lng] = sky1.asDegrees();
    skyTmp[_wcsInfo->lat] = sky2.asDegrees();

    int stat[1];
    int status = 0;
    status = wcss2p(_wcsInfo, 1, 2, skyTmp, &phi, &theta, imgcrd, pixTmp, stat);
    if (status == 9) {
        throw LSST_EXCEPT(except::DomainError,
            (boost::format("sky coordinates %s, %s degrees is not valid for this WCS")
             % sky1.asDegrees() % sky2.asDegrees()
             ).str()
        );
    }
    if (status > 0) {
        throw LSST_EXCEPT(except::RuntimeError,
            (boost::format("Error: wcslib returned a status code of %d at sky %s, %s deg: %s") %
            status % sky1.asDegrees() % sky2.asDegrees() % wcs_errmsg[status]).str());
    }

    // wcslib assumes 1-indexed coords
    return afwGeom::Point2D(pixTmp[0] + lsst::afw::image::PixelZeroPos + fitsToLsstPixels,
                            pixTmp[1] + lsst::afw::image::PixelZeroPos + fitsToLsstPixels); 
}

GeomPoint Wcs::skyToPixel(lsst::afw::coord::Coord const & coord) const {
    afwCoord::Coord::Ptr sky = convertCoordToSky(coord);
    return skyToPixelImpl(sky->getLongitude(), sky->getLatitude());
}


afwCoord::Coord::Ptr
Wcs::convertCoordToSky(afwCoord::Coord const & coord) const {
    return coord.convert(_coordSystem);
}

GeomPoint Wcs::skyToPixel(afwGeom::Angle sky1, afwGeom::Angle sky2) const {
    return skyToPixelImpl(sky1, sky2);
}

GeomPoint Wcs::skyToIntermediateWorldCoord(lsst::afw::coord::Coord const & coord) const {
    assert(_wcsInfo);

    afwCoord::Coord::Ptr sky = convertCoordToSky(coord);
    double skyTmp[2];
    double imgcrd[2];
    double phi, theta;
    double pixTmp[2];

    /*
     printf("skyToIWC: _coordSystem = %i\n", (int)_coordSystem);
     printf("coord (%.3f, %.3f)\n", coord->getLongitude().asDegrees(), coord->getLatitude().asDegrees());
     printf("->sky (%.3f, %.3f)\n", sky->getLongitude().asDegrees(), sky->getLatitude().asDegrees());
     */

    skyTmp[_wcsInfo->lng] = sky->getLongitude().asDegrees();
    skyTmp[_wcsInfo->lat] = sky->getLatitude() .asDegrees();

    //Estimate pixel coordinates
    int stat[1];
    int status = 0;
    imgcrd[0] = imgcrd[1] = -1e6;
    /*
     printf("  skyTmp[] = (%.3f, %.3f)\n", skyTmp[0], skyTmp[1]);
     printf("  _wcsInfo->lng,lat = %i, %i\n", _wcsInfo->lng, _wcsInfo->lat);
     */
    status = wcss2p(_wcsInfo, 1, 2, skyTmp, &phi, &theta, imgcrd, pixTmp, stat);
    if (status > 0) {
        throw LSST_EXCEPT(except::RuntimeError,
            (boost::format("Error: wcslib returned a status code of %d at sky %s, %s deg: %s") %
            status % skyTmp[0] % skyTmp[1] % wcs_errmsg[status]).str());
    }
    /*
     printf("->iwc (%.3f, %.3f)\n", imgcrd[0], imgcrd[1]);
     printf("-> pix (%.2f, %.2f)\n", pixTmp[0], pixTmp[1]);
     afwCoord::Coord::Ptr crval = getSkyOrigin();
     printf("(crval is (%.3f, %.3f))\n", crval->getLongitude().asDegrees(), crval->getLatitude().asDegrees());
     */
    return GeomPoint(imgcrd[0], imgcrd[1]); 
}

/*
 * Worker routine for pixelToSky
 */
void
Wcs::pixelToSkyImpl(double pixel1, double pixel2, afwGeom::Angle skyTmp[2]) const
{
    assert(_wcsInfo);
    
    // wcslib assumes 1-indexed coordinates
    double pixTmp[2] = { pixel1 - lsst::afw::image::PixelZeroPos + lsstToFitsPixels,
                         pixel2 - lsst::afw::image::PixelZeroPos + lsstToFitsPixels}; 
    double imgcrd[2];
    double phi, theta;
    
    double sky[2];
    int status = 0;
    status = wcsp2s(_wcsInfo, 1, 2, pixTmp, imgcrd, &phi, &theta, sky, &status);
    if (status > 0) {
        throw LSST_EXCEPT(except::RuntimeError,
            (boost::format("Error: wcslib returned a status code of %d at pixel %s, %s: %s") %
            status % pixel1 % pixel2 % wcs_errmsg[status]).str());
    }
    // FIXME -- _wcsInfo.lat, _wcsInfo.lng ?
    skyTmp[0] = sky[0] * afwGeom::degrees;
    skyTmp[1] = sky[1] * afwGeom::degrees;
}

CoordPtr Wcs::pixelToSky(GeomPoint const & pixel) const {
    return pixelToSky(pixel.getX(), pixel.getY());
}

CoordPtr Wcs::pixelToSky(double pixel1, double pixel2) const {
    assert(_wcsInfo);

    afwGeom::Angle skyTmp[2];
    pixelToSkyImpl(pixel1, pixel2, skyTmp);
    return makeCorrectCoord(skyTmp[0], skyTmp[1]);
}

void Wcs::pixelToSky(double pixel1, double pixel2, afwGeom::Angle& sky1, afwGeom::Angle& sky2) const {
    afwGeom::Angle skyTmp[2];
    // HACK -- we shouldn't need to initialize these -- pixelToSkyImpl() sets them unless an
    // exception is thrown -- but be safe.
    skyTmp[0] = 0. * afwGeom::radians;
    skyTmp[1] = 0. * afwGeom::radians;
    pixelToSkyImpl(pixel1, pixel2, skyTmp);
    sky1 = skyTmp[0];
    sky2 = skyTmp[1];
}

CoordPtr Wcs::makeCorrectCoord(lsst::afw::geom::Angle sky0, lsst::afw::geom::Angle sky1) const {

    //Construct a coord object of the correct type
    int const ncompare = 4;                       // we only care about type's first 4 chars
    char *type = _wcsInfo->ctype[0];
    char *radesys = _wcsInfo->radesys;
    double equinox = _wcsInfo->equinox;

    if (strncmp(type, "RA--", ncompare) == 0) { // Our default.  If it's often something else, consider
        ;                                       // using an tr1::unordered_map
        if(strcmp(radesys, "ICRS") == 0) {
            return afwCoord::makeCoord(afwCoord::ICRS, sky0, sky1);
        }
        if(strcmp(radesys, "FK5") == 0) {
            return afwCoord::makeCoord(afwCoord::FK5, sky0, sky1, equinox);
        } else {   
            throw LSST_EXCEPT(except::RuntimeError,
                              (boost::format("Can't create Coord object: Unrecognised radesys %s") %
                               radesys).str());
        }

    } else if (strncmp(type, "GLON", ncompare) == 0) {
        return afwCoord::makeCoord(afwCoord::GALACTIC, sky0, sky1);   
    } else if (strncmp(type, "ELON", ncompare) == 0) {
        return afwCoord::makeCoord(afwCoord::ECLIPTIC, sky0, sky1, equinox);
    } else if (strncmp(type, "DEC-", ncompare) == 0) {
        //check for the case where the ctypes are swapped. Note how sky0 and sky1 are swapped as well

        //Our default
        if(strcmp(radesys, "ICRS") == 0) {
            return afwCoord::makeCoord(afwCoord::ICRS, sky1, sky0);
        }
        if(strcmp(radesys, "FK5") == 0) {
            return afwCoord::makeCoord(afwCoord::FK5, sky1, sky0, equinox);
        } else {   
            throw LSST_EXCEPT(except::RuntimeError,
                              (boost::format("Can't create Coord object: Unrecognised radesys %s") %
                               radesys).str());
        }
    } else if (strncmp(type, "GLAT", ncompare) == 0) {
        return afwCoord::makeCoord(afwCoord::GALACTIC, sky1, sky0);   
    } else if (strncmp(type, "ELAT", ncompare) == 0) {
        return afwCoord::makeCoord(afwCoord::ECLIPTIC, sky1, sky0, equinox);
    } else {
    //Give up in disgust
        throw LSST_EXCEPT(except::RuntimeError,
                          (boost::format("Can't create Coord object: Unrecognised sys %s") %
                           type).str());
    }
    
    //Can't get here
    assert(0);
}


lsst::afw::geom::AffineTransform Wcs::linearizePixelToSky(
    lsst::afw::coord::Coord const & coord,
    lsst::afw::geom::AngleUnit skyUnit
) const {
    return linearizePixelToSkyInternal(skyToPixel(coord), coord, skyUnit);
}
lsst::afw::geom::AffineTransform Wcs::linearizePixelToSky(
    GeomPoint const & pix,
    lsst::afw::geom::AngleUnit skyUnit
) const {
    return linearizePixelToSkyInternal(pix, *pixelToSky(pix), skyUnit);
}

/*
 * Implementation for the overloaded public linearizePixelToSky methods, requiring both a pixel coordinate
 * and the corresponding sky coordinate.
 */
lsst::afw::geom::AffineTransform Wcs::linearizePixelToSkyInternal(
    GeomPoint const & pix00,
    lsst::afw::coord::Coord const & coord,
    lsst::afw::geom::AngleUnit skyUnit
) const {
    //
    // Figure out the (0, 0), (0, 1), and (1, 0) ra/dec coordinates of the corners of a square drawn in pixel
    // It'd be better to centre the square at sky00, but that would involve another conversion between sky and
    // pixel coordinates so I didn't bother
    //
    const double side = 10;             // length of the square's sides in pixels
    GeomPoint const sky00 = coord.getPosition(skyUnit);
    typedef std::pair<lsst::afw::geom::Angle, lsst::afw::geom::Angle> AngleAngle;
    AngleAngle const dsky10 = coord.getTangentPlaneOffset(*pixelToSky(pix00 + afwGeom::Extent2D(side, 0)));
    AngleAngle const dsky01 = coord.getTangentPlaneOffset(*pixelToSky(pix00 + afwGeom::Extent2D(0, side)));

    Eigen::Matrix2d m;
    m(0, 0) = dsky10.first.asAngularUnits(skyUnit)/side;
    m(0, 1) = dsky01.first.asAngularUnits(skyUnit)/side;
    m(1, 0) = dsky10.second.asAngularUnits(skyUnit)/side;
    m(1, 1) = dsky01.second.asAngularUnits(skyUnit)/side;

    Eigen::Vector2d sky00v;
    sky00v << sky00.getX(), sky00.getY();
    Eigen::Vector2d pix00v;
    pix00v << pix00.getX(), pix00.getY();
    //return lsst::afw::geom::AffineTransform(m, lsst::afw::geom::Extent2D(sky00v - m * pix00v));
    return lsst::afw::geom::AffineTransform(m, (sky00v - m * pix00v));
}

lsst::afw::geom::AffineTransform Wcs::linearizeSkyToPixel(
    lsst::afw::coord::Coord const & coord,
    lsst::afw::geom::AngleUnit skyUnit
) const {
    return linearizeSkyToPixelInternal(skyToPixel(coord), coord, skyUnit);
}

lsst::afw::geom::AffineTransform Wcs::linearizeSkyToPixel(
    GeomPoint const & pix,
    lsst::afw::geom::AngleUnit skyUnit
) const {
    return linearizeSkyToPixelInternal(pix, *pixelToSky(pix), skyUnit);
}

lsst::afw::geom::AffineTransform Wcs::linearizeSkyToPixelInternal(
    GeomPoint const & pix00,
    lsst::afw::coord::Coord const & coord,
    lsst::afw::geom::AngleUnit skyUnit
) const {
    lsst::afw::geom::AffineTransform inverse = linearizePixelToSkyInternal(pix00, coord, skyUnit);
    return inverse.invert();
}

lsst::afw::geom::LinearTransform Wcs::getLinearTransform() const
{
    return lsst::afw::geom::LinearTransform(getCDMatrix());
}

// -------- table-based persistence -------------------------------------------------------------------------

namespace lsst { namespace afw { namespace image {

class WcsFactory : public table::io::PersistableFactory {
public:

    explicit WcsFactory(std::string const & name) : table::io::PersistableFactory(name) {}

    virtual PTR(table::io::Persistable) read(
        InputArchive const & archive,
        CatalogVector const & catalogs
    ) const;

};

namespace {

// Read-only singleton struct containing the schema and keys that a simple Wcs is mapped
// to in record persistence.
struct WcsPersistenceHelper : private boost::noncopyable {
    table::Schema schema;
    table::Key< table::Point<double> > crval;
    table::Key< table::Point<double> > crpix;
    table::Key< table::Array<double> > cd;
    table::Key<std::string> ctype1;
    table::Key<std::string> ctype2;
    table::Key<double> equinox;
    table::Key<std::string> radesys;
    table::Key<std::string> cunit1;
    table::Key<std::string> cunit2;

    static WcsPersistenceHelper const & get() {
        static WcsPersistenceHelper instance;
        return instance;
    };

private:
    WcsPersistenceHelper() :
        schema(),
        crval(schema.addField< table::Point<double> >("crval", "celestial reference point")),
        crpix(schema.addField< table::Point<double> >("crpix", "pixel reference point")),
        cd(schema.addField< table::Array<double> >(
               "cd", "linear transform matrix, ordered (1_1, 2_1, 1_2, 2_2)", 4)),
        ctype1(schema.addField< std::string >("ctype1", "coordinate type", 72)),
        ctype2(schema.addField< std::string >("ctype2", "coordinate type", 72)),
        equinox(schema.addField< double >("equinox", "equinox of coordinates")),
        radesys(schema.addField< std::string >("radesys", "coordinate system for equinox", 72)),
        cunit1(schema.addField< std::string >("cunit1", "coordinate units", 72)),
        cunit2(schema.addField< std::string >("cunit2", "coordinate units", 72))
    {
        schema.getCitizen().markPersistent();
    }
};

std::string getWcsPersistenceName() { return "Wcs"; }

WcsFactory registration(getWcsPersistenceName());

} // anonymous

std::string Wcs::getPersistenceName() const { return getWcsPersistenceName(); }

std::string Wcs::getPythonModule() const { return "lsst.afw.image"; }

void Wcs::write(OutputArchiveHandle & handle) const {
    WcsPersistenceHelper const & keys = WcsPersistenceHelper::get();
    afw::table::BaseCatalog catalog = handle.makeCatalog(keys.schema);
    PTR(afw::table::BaseRecord) record = catalog.addNew();
    record->set(keys.crval, getSkyOrigin()->getPosition(afw::geom::degrees));
    record->set(keys.crpix, getPixelOrigin());
    Eigen::Matrix2d cdIn = getCDMatrix();
    Eigen::Map<Eigen::Matrix2d> cdOut((*record)[keys.cd].getData());
    cdOut = cdIn;
    record->set(keys.ctype1, std::string(_wcsInfo[0].ctype[0]));
    record->set(keys.ctype2, std::string(_wcsInfo[0].ctype[1]));
    record->set(keys.equinox, _wcsInfo[0].equinox);
    record->set(keys.radesys, std::string(_wcsInfo[0].radesys));
    record->set(keys.cunit1, std::string(_wcsInfo[0].cunit[0]));
    record->set(keys.cunit2, std::string(_wcsInfo[0].cunit[1]));
    handle.saveCatalog(catalog);
}

bool Wcs::isPersistable() const {
    if (_wcsInfo[0].naxis != 2) return false;
    if (std::strcmp(_wcsInfo[0].cunit[0], "deg") != 0) return false;
    if (std::strcmp(_wcsInfo[0].cunit[1], "deg") != 0) return false;
    return true;
}

Wcs::Wcs(afw::table::BaseRecord const & record) :
    daf::base::Citizen(typeid(this)),
    _wcsInfo(NULL),
    _nWcsInfo(0),
    _relax(0),
    _wcsfixCtrl(0),
    _wcshdrCtrl(0),
    _nReject(0),
    _coordSystem(static_cast<afw::coord::CoordSystem>(-1))
{
    WcsPersistenceHelper const & keys = WcsPersistenceHelper::get();
    if (!record.getSchema().contains(keys.schema)) {
        throw LSST_EXCEPT(
            afw::table::io::MalformedArchiveError,
            "Incorrect schema for Wcs persistence"
        );
    }
    _setWcslibParams();
    Eigen::Matrix2d cd = Eigen::Map<Eigen::Matrix2d const>(record[keys.cd].getData());
    initWcsLib(
        record.get(keys.crval), record.get(keys.crpix), cd,
        record.get(keys.ctype1), record.get(keys.ctype2),
        record.get(keys.equinox), record.get(keys.radesys),
        record.get(keys.cunit1), record.get(keys.cunit2)
    );
    _initWcs();
}

PTR(table::io::Persistable)
WcsFactory::read(InputArchive const & inputs, CatalogVector const & catalogs) const {
    WcsPersistenceHelper const & keys = WcsPersistenceHelper::get();
    LSST_ARCHIVE_ASSERT(catalogs.size() >= 1u);
    LSST_ARCHIVE_ASSERT(catalogs.front().size() == 1u);
    LSST_ARCHIVE_ASSERT(catalogs.front().getSchema() == keys.schema);
    PTR(Wcs) result(new Wcs(catalogs.front().front()));
    return result;
}

}}} // namespace lsst::afw::image

// ----------------------------------------------------------------------------------------------------------

//
//Mutators
//


void Wcs::shiftReferencePixel(double dx, double dy) {
    assert(_wcsInfo);
    _wcsInfo->crpix[0] += dx;
    _wcsInfo->crpix[1] += dy;

    // tells libwcs to invalidate cached data, since transformation has been modified
    _wcsInfo->flag = 0;
}

/************************************************************************************************************/
/*
 * Now WCSA, pixel coordinates, but allowing for X0 and Y0
 */
namespace lsst {
namespace afw {
namespace image {

namespace detail {

lsst::daf::base::PropertyList::Ptr
createTrivialWcsAsPropertySet(std::string const& wcsName, 
                              int const x0,               
                              int const y0                
                             ) {
    lsst::daf::base::PropertyList::Ptr wcsMetaData(new lsst::daf::base::PropertyList);

    wcsMetaData->set("CRVAL1" + wcsName, x0, "Column pixel of Reference Pixel");
    wcsMetaData->set("CRVAL2" + wcsName, y0, "Row pixel of Reference Pixel");
    wcsMetaData->set("CRPIX1" + wcsName, 1, "Column Pixel Coordinate of Reference");
    wcsMetaData->set("CRPIX2" + wcsName, 1, "Row Pixel Coordinate of Reference");
    wcsMetaData->set("CTYPE1" + wcsName, "LINEAR", "Type of projection");
    wcsMetaData->set("CTYPE2" + wcsName, "LINEAR", "Type of projection");
    wcsMetaData->set("CUNIT1" + wcsName, "PIXEL", "Column unit");
    wcsMetaData->set("CUNIT2" + wcsName, "PIXEL", "Row unit");

    return wcsMetaData;
}
afwGeom::Point2I getImageXY0FromMetadata(std::string const& wcsName,            
                                      lsst::daf::base::PropertySet *metadata 
                                     ) {
        
    int x0 = 0;                         // Our value of X0
    int y0 = 0;                         // Our value of Y0
    
    try {
        //
        // Only use WCS if CRPIX[12] == 1 and CRVAL[12] is present
        //
        if (metadata->getAsDouble("CRPIX1" + wcsName) == 1 &&
            metadata->getAsDouble("CRPIX2" + wcsName) == 1) {
            
            x0 = metadata->getAsInt("CRVAL1" + wcsName);
            y0 = metadata->getAsInt("CRVAL2" + wcsName);
            //
            // OK, we've got it.  Remove it from the header
            //
            metadata->remove("CRVAL1" + wcsName);
            metadata->remove("CRVAL2" + wcsName);
            metadata->remove("CRPIX1" + wcsName);
            metadata->remove("CRPIX2" + wcsName);
            metadata->remove("CTYPE1" + wcsName);
            metadata->remove("CTYPE1" + wcsName);
            metadata->remove("CUNIT1" + wcsName);
            metadata->remove("CUNIT2" + wcsName);
        }
    } catch(lsst::pex::exceptions::NotFoundError &) {
        ;                               // OK, not present
    }

    return afwGeom::Point2I(x0, y0);
}

int stripWcsKeywords(PTR(lsst::daf::base::PropertySet) const& metadata, 
                     CONST_PTR(Wcs) const& wcs                          
                    )
{
    PTR(lsst::daf::base::PropertySet) wcsMetadata = wcs->getFitsMetadata();
    std::vector<std::string> paramNames = wcsMetadata->paramNames();
    paramNames.push_back("CDELT1");
    paramNames.push_back("CDELT2");
    paramNames.push_back("LTV1");
    paramNames.push_back("LTV2");
    paramNames.push_back("PC001001");
    paramNames.push_back("PC001002");
    paramNames.push_back("PC002001");
    paramNames.push_back("PC002002");
    for (std::vector<std::string>::const_iterator ptr = paramNames.begin(); ptr != paramNames.end(); ++ptr) {
        metadata->remove(*ptr);
    }

    return 0;                           // would be ncard if remove returned a status
}


}}}}



// -------------------------------------------------------------------------------------------------
//
// XYTransformFromWcsPair


XYTransformFromWcsPair::XYTransformFromWcsPair(CONST_PTR(Wcs) dst, CONST_PTR(Wcs) src)
    : XYTransform(), _dst(dst), _src(src)
{ }


PTR(afwGeom::XYTransform) XYTransformFromWcsPair::clone() const
{
    return boost::make_shared<XYTransformFromWcsPair>(_dst->clone(), _src->clone());
}


afwGeom::Point2D XYTransformFromWcsPair::forwardTransform(Point2D const &pixel) const
{
    //
    // TODO there is an alternate version of pixelToSky() which is designated for the 
    // "knowledgeable user in need of performance".  This is probably better, but first I need 
    // to understand exactly which checks are needed (e.g. I think we need to check by hand 
    // that both Wcs's use the same celestial coordinate system)
    //
    PTR(afw::coord::Coord) x = _src->pixelToSky(pixel);
    return _dst->skyToPixel(*x);
}

afwGeom::Point2D XYTransformFromWcsPair::reverseTransform(Point2D const &pixel) const
{
    PTR(afw::coord::Coord) x = _dst->pixelToSky(pixel);
    return _src->skyToPixel(*x);
}

PTR(afwGeom::XYTransform) XYTransformFromWcsPair::invert() const
{
    // just swap src, dst
    return boost::make_shared<XYTransformFromWcsPair> (_src, _dst);
}