Class SipForwardTransform

• Defined in File SipTransform.h

Inheritance Relationships

Base Type

• public lsst::meas::astrom::SipTransformBase (Class SipTransformBase)

Class Documentation

class SipForwardTransform : public lsst::meas::astrom::SipTransformBase

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

The SIP forward transform is defined as

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

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.

Public Functions

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

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 $(A,B)$

SipForwardTransform(SipForwardTransform const &other)

SipForwardTransform(SipForwardTransform &&other)

SipForwardTransform &operator=(SipForwardTransform const &other)

SipForwardTransform &operator=(SipForwardTransform &&other)

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.

Public Static Functions

static SipForwardTransform convert(PolynomialTransform const &poly, geom::Point2D const &pixelOrigin, geom::LinearTransform const &cdMatrix)

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 $Z$

static SipForwardTransform convert(ScaledPolynomialTransform const &scaled, geom::Point2D const &pixelOrigin, geom::LinearTransform const &cdMatrix)

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 $Z$

static SipForwardTransform convert(ScaledPolynomialTransform const &scaled)

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).