lsst::meas::astrom::SipForwardTransform Class Reference

A transform that maps pixel coordinates to intermediate world coordinates according to the SIP convention. More...

#include <SipTransform.h>

Inheritance diagram for lsst::meas::astrom::SipForwardTransform:

Public Member Functions
	SipForwardTransform (geom::Point2D const &pixelOrigin, geom::LinearTransform const &cdMatrix, PolynomialTransform const &forwardSipPoly)
	Construct a SipForwardTransform from its components.
	SipForwardTransform (SipForwardTransform const &other)=default
	SipForwardTransform (SipForwardTransform &&other)=default
SipForwardTransform &	operator= (SipForwardTransform const &other)=default
SipForwardTransform &	operator= (SipForwardTransform &&other)=default
void	swap (SipForwardTransform &other)
geom::AffineTransform	linearize (geom::Point2D const &in) const
	Return an approximate affine transform at the given point.
geom::Point2D	operator() (geom::Point2D const &uv) const
	Apply the transform to a point.
SipForwardTransform	transformPixels (geom::AffineTransform const &s) const
	Return a new forward SIP transform that includes a transformation of the pixel coordinate system by the given affine transform.
geom::Point2D const &	getPixelOrigin () const
	Return the pixel origin (CRPIX, but zero-indexed) of the transform.
geom::LinearTransform const &	getCdMatrix () const
	Return the CD matrix of the transform.
PolynomialTransform const &	getPoly () const
	Return the polynomial component of the transform (A,B) or (AP,BP).

Static Public Member Functions
static SipForwardTransform	convert (PolynomialTransform const &poly, geom::Point2D const &pixelOrigin, geom::LinearTransform const &cdMatrix)
	Convert a PolynomialTransform to an equivalent SipForwardTransform.
static SipForwardTransform	convert (ScaledPolynomialTransform const &scaled, geom::Point2D const &pixelOrigin, geom::LinearTransform const &cdMatrix)
	Convert a ScaledPolynomialTransform to an equivalent SipForwardTransform.
static SipForwardTransform	convert (ScaledPolynomialTransform const &scaled)
	Convert a ScaledPolynomialTransform to an equivalent SipForwardTransform.

Protected Member Functions
void	swap (SipTransformBase &other)
void	transformPixelsInPlace (geom::AffineTransform const &s)

Protected Attributes
geom::Point2D	_pixelOrigin
geom::LinearTransform	_cdMatrix
PolynomialTransform	_poly

Detailed Description

A transform that maps pixel coordinates to intermediate world coordinates according to the SIP convention.

The SIP forward transform is defined as

\[ \left[\begin{array}{ c } x \\ y \end{array}\right] = \mathbf{Z} \left[\begin{array}{ c } (u - u_0) + {\displaystyle\sum_{p,q}^{2 \le p + q \le N}} \mathrm{A}_{p,q} (u-u_0)^p (v-v_0)^q \\ (v - v_0) + {\displaystyle\sum_{p,q}^{2 \le p + q \le N}} \mathrm{B}_{p,q} (u-u_0)^p (v-v_0)^q \end{array}\right] \]

where

• \((u,v)\) are pixel coordinates (zero-indexed).
• \((x,y)\) are "intermediate world coordinates" – the result of applying the gnomonic (TAN) projection at sky origin CRVAL to sky coordinates).
• \(\mathbf{Z}\) is the \(2 \times 2\) linear transform ( \(\mathrm{CD}\)) matrix.
• \((u_0,v_0)\) is the pixel origin \(\mathrm{CRPIX}\) (but zero-indexed; the FITS standard is 1-indexed).
• \(\mathrm{A}\), \(\mathrm{B}\) are the polynomial coefficients of the forward transform.

The SIP convention encourages (but does not require) nulling the zeroth- and first-order elements of \(\mathrm{A}\) and \(\mathrm{B}\), which ensures the representation of a given transform is unique. This also makes fitting a SIP transform to data a nonlinear operation, as well as making the conversion from standard polynomial transforms to SIP form impossible in general. Accordingly, this class does not attempt to null low-order polynomial terms at all when converting from other transforms.

SipForwardTransform instances should be confined to a single thread.

Definition at line 136 of file SipTransform.h.

Constructor & Destructor Documentation

SipForwardTransform() [1/3]

lsst::meas::astrom::SipForwardTransform::SipForwardTransform ( geom::Point2D const & pixelOrigin, geom::LinearTransform const & cdMatrix, PolynomialTransform const & forwardSipPoly )

inline

Construct a SipForwardTransform from its components.

Parameters

[in]	pixelOrigin	CRPIX \((u_0,v_0)\) (zero-indexed).
[in]	cdMatrix	CD matrix \(Z\)
[in]	forwardSipPoly	Polynomial transform \((A,B)\)

Definition at line 175 of file SipTransform.h.

            : SipTransformBase(pixelOrigin, cdMatrix, forwardSipPoly) {}

SipForwardTransform() [2/3]

lsst::meas::astrom::SipForwardTransform::SipForwardTransform ( SipForwardTransform const & other )

default

SipForwardTransform() [3/3]

lsst::meas::astrom::SipForwardTransform::SipForwardTransform ( SipForwardTransform && other )

default

Member Function Documentation

convert() [1/3]

SipForwardTransform lsst::meas::astrom::SipForwardTransform::convert ( PolynomialTransform const & poly, geom::Point2D const & pixelOrigin, geom::LinearTransform const & cdMatrix )

static

Convert a PolynomialTransform to an equivalent SipForwardTransform.

Parameters

[in]	poly	PolynomialTransform to convert.
[in]	pixelOrigin	CRPIX \((u_0,v_0)\) (zero-indexed)
[in]	cdMatrix	CD matrix \(Z\)

Definition at line 52 of file SipTransform.cc.

                                                                                      {
    auto forwardSipPoly = compose(geom::AffineTransform(cdMatrix.inverted()),
                                  compose(poly, geom::AffineTransform(geom::Extent2D(pixelOrigin))));
    // Subtracting 1 here accounts for the extra terms outside the sum in the
    // transform definition (see class docs) - note that you can fold those
    // terms into the sum by adding 1 from the A_10 and B_01 terms.
    forwardSipPoly._xCoeffs(1, 0) -= 1;
    forwardSipPoly._yCoeffs(0, 1) -= 1;
    return SipForwardTransform(pixelOrigin, cdMatrix, forwardSipPoly);
}

convert() [2/3]

SipForwardTransform lsst::meas::astrom::SipForwardTransform::convert ( ScaledPolynomialTransform const & scaled )

static

Convert a ScaledPolynomialTransform to an equivalent SipForwardTransform.

The pixel origin CRPIX and CD matrix are defined to reproduce the translation and linear transformation in the ScaledPolynomialTransform's input and output scalings (respectively).

Definition at line 79 of file SipTransform.cc.

                                                                                        {
    geom::Point2D pixelOrigin(-scaled.getOutputScalingInverse().getTranslation());
    geom::LinearTransform cdMatrix(scaled.getInputScaling().getLinear().inverted());
    return convert(scaled, pixelOrigin, cdMatrix);
}

convert() [3/3]

SipForwardTransform lsst::meas::astrom::SipForwardTransform::convert ( ScaledPolynomialTransform const & scaled, geom::Point2D const & pixelOrigin, geom::LinearTransform const & cdMatrix )

static

Convert a ScaledPolynomialTransform to an equivalent SipForwardTransform.

Parameters

[in]	scaled	ScaledPolynomialTransform to convert.
[in]	pixelOrigin	CRPIX \((u_0,v_0)\) (zero-indexed)
[in]	cdMatrix	CD matrix \(Z\)

Definition at line 65 of file SipTransform.cc.

                                                                                      {
    auto forwardSipPoly =
            compose(geom::AffineTransform(cdMatrix.inverted()) * scaled.getOutputScalingInverse(),
                    compose(scaled.getPoly(),
                            scaled.getInputScaling() * geom::AffineTransform(geom::Extent2D(pixelOrigin))));
    // Account for the terms outside the sum in the definition (see comment
    // earlier in the file for more explanation).
    forwardSipPoly._xCoeffs(1, 0) -= 1;
    forwardSipPoly._yCoeffs(0, 1) -= 1;
    return SipForwardTransform(pixelOrigin, cdMatrix, forwardSipPoly);
}

getCdMatrix()

geom::LinearTransform const & lsst::meas::astrom::SipTransformBase::getCdMatrix ( ) const

inlineinherited

Return the CD matrix of the transform.

Definition at line 56 of file SipTransform.h.

{ return _cdMatrix; }

getPixelOrigin()

geom::Point2D const & lsst::meas::astrom::SipTransformBase::getPixelOrigin ( ) const

inlineinherited

Return the pixel origin (CRPIX, but zero-indexed) of the transform.

Definition at line 51 of file SipTransform.h.

{ return _pixelOrigin; }

getPoly()

PolynomialTransform const & lsst::meas::astrom::SipTransformBase::getPoly ( ) const

inlineinherited

Return the polynomial component of the transform (A,B) or (AP,BP).

Definition at line 61 of file SipTransform.h.

{ return _poly; }

linearize()

geom::AffineTransform lsst::meas::astrom::SipForwardTransform::linearize

(

geom::Point2D const &

in

)

const

Return an approximate affine transform at the given point.

Definition at line 85 of file SipTransform.cc.

                                                                              {
    geom::AffineTransform tail(-geom::Extent2D(getPixelOrigin()));
    return geom::AffineTransform(_cdMatrix) * (geom::AffineTransform() + _poly.linearize(tail(in))) * tail;
}

operator()()

geom::Point2D lsst::meas::astrom::SipForwardTransform::operator()

(

geom::Point2D const &

uv

)

const

Apply the transform to a point.

Definition at line 90 of file SipTransform.cc.

                                                                       {
    geom::Point2D duv(uv - geom::Extent2D(getPixelOrigin()));
    return getCdMatrix()(geom::Extent2D(duv) + getPoly()(duv));
}

operator=() [1/2]

SipForwardTransform & lsst::meas::astrom::SipForwardTransform::operator= ( SipForwardTransform && other )

default

operator=() [2/2]

SipForwardTransform & lsst::meas::astrom::SipForwardTransform::operator= ( SipForwardTransform const & other )

default

swap() [1/2]

void lsst::meas::astrom::SipForwardTransform::swap ( SipForwardTransform & other )

inline

Definition at line 187 of file SipTransform.h.

{ SipTransformBase::swap(other); }

swap() [2/2]

void lsst::meas::astrom::SipTransformBase::swap ( SipTransformBase & other )

inlineprotectedinherited

Definition at line 85 of file SipTransform.h.

                                       {
        std::swap(_pixelOrigin, other._pixelOrigin);
        std::swap(_cdMatrix, other._cdMatrix);
        _poly.swap(other._poly);
    }

transformPixels()

SipForwardTransform lsst::meas::astrom::SipForwardTransform::transformPixels

(

geom::AffineTransform const &

s

)

const

Return a new forward SIP transform that includes a transformation of the pixel coordinate system by the given affine transform.

Definition at line 95 of file SipTransform.cc.

                                                                                           {
    SipForwardTransform result(*this);
    result.transformPixelsInPlace(s);
    return result;
}

transformPixelsInPlace()

void lsst::meas::astrom::SipTransformBase::transformPixelsInPlace ( geom::AffineTransform const & s )

protectedinherited

Definition at line 40 of file SipTransform.cc.

                                                                          {
    // The implementation for transformPixels is identical for
    // SipForwardTransform and SipReverseTransform.  That's pretty obvious for
    // the pixel origin and CD matrix, which are the same in both cases, but
    // it wasn't obvious to me until I did the math that the polynomial
    // transforms are composed with the affine transform the same way.
    auto sInv = s.inverted();
    _pixelOrigin = s.getLinear()(_pixelOrigin - sInv.getTranslation());
    _cdMatrix = _cdMatrix * sInv.getLinear();
    _poly = compose(s.getLinear(), compose(getPoly(), sInv.getLinear()));
}

Member Data Documentation

_cdMatrix

geom::LinearTransform lsst::meas::astrom::SipTransformBase::_cdMatrix

protectedinherited

Definition at line 94 of file SipTransform.h.

_pixelOrigin

geom::Point2D lsst::meas::astrom::SipTransformBase::_pixelOrigin

protectedinherited

Definition at line 93 of file SipTransform.h.

_poly

PolynomialTransform lsst::meas::astrom::SipTransformBase::_poly

protectedinherited

Definition at line 95 of file SipTransform.h.

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The documentation for this class was generated from the following files:

• /j/snowflake/release/lsstsw/stack/lsst-scipipe-8.0.0/Linux64/meas_astrom/gff1a9f87cc+86cf3d8bc9/include/lsst/meas/astrom/SipTransform.h
• /j/snowflake/release/lsstsw/stack/lsst-scipipe-8.0.0/Linux64/meas_astrom/gff1a9f87cc+86cf3d8bc9/src/SipTransform.cc