Processing math: 33%

SLIDE

Realigns an NDF using a translation

Description:

The pixels of an NDF  are shifted by a given number of pixels along each pixel axis. The shift need not be an integer number of pixels, and pixel interpolation will be performed if necessary using the scheme selected by Parameter METHOD. The shifts to use are specified either by an absolute vector given by the ABS parameter or by the difference between a fiducial point and a standard object given by the FID and OBJ parameters respectively. In each case the co-ordinates are specified in the NDF’s pixel co-ordinate Frame.

Usage:

slide in out abs method

Parameters:

ABS( ) = _DOUBLE (Read)
Absolute shifts in pixels. The number of values supplied must match the number of pixel axes in the NDF. It is only used if STYPE="Absolute".
FID( ) = _DOUBLE (Read)
Position of the fiducial point in pixel co-ordinates. The number of values supplied must match the number of pixel axes in the NDF. It is only used if STYPE="Relative".

An object centred at the pixel co-ordinates given by Parameter OBJ in the input NDF will be centred at the pixel co-ordinates given by Parameter FID in the output NDF.

IN = NDF (Read)
The NDF to be translated.
METHOD = LITERAL (Read)
The interpolation method used to perform the translation. The following values are permitted:

  • "Nearest" –- Nearest-neighbour sampling.

  • "Linear" –- Linear interpolation.

  • "Sinc" –- Sum of surrounding pixels weighted using a one-dimensional sinc(πx) kernel.

  • "SincSinc" –- Sum of surrounding pixels weighted using a one-dimensional sinc(πx)sinc(kπx) kernel.

  • "SincCos" –- Sum of surrounding pixels weighted using a one-dimensional
    sinc(πx)cos kernel.

  • "SincGauss" –- Sum of surrounding pixels weighted using a one-dimensional sinc(πx)e−kx2 kernel.

  • "BlockAve" –- Block averaging over all pixels in the surrounding n-dimensional cube.

In the above, sinc(z) = sin(z)/z. Some of these schemes will require additional parameters to be supplied via the PARAMS parameter. A more-detailed discussion of these schemes is given in the “Sub-pixel Interpolation Schemes” section below. The initial default is "Linear". [current value]

OBJ = LITERAL (Read)
Position of the standard object in pixel co-ordinates. The number of values supplied must match the number of pixel axes in the NDF. It is only used if STYPE="Relative".

An object centred at the pixel co-ordinates given by Parameter OBJ in the input NDF will be centred at the pixel co-ordinates given by Parameter FID in the output NDF.

OUT = NDF (Write)
The translated NDF.
PARAMS( ) = _DOUBLE (Read)
Parameters required to control the resampling scheme. One or more values may be required to specify the exact resampling behaviour, according to the value of the METHOD parameter. See the section on “Sub-pixel Interpolation Schemes”.
STYPE = LITERAL (Read)
The sort of shift to be used. The choice is "Relative" or "Absolute". ["Absolute"]
TITLE = LITERAL (Read)
Title for the output NDF. A null (!) value will cause the input title to be used. [!]

Examples:

slide m31 m31_acc [3.2,2.3]
The pixels in the NDF m31 are shifted by 3.2 pixels in x and 2.3 pixels in y, and written to NDF m31_acc. Linear interpolation is used to produce the output data (and, if present, variance) array.
slide m31 m31_acc [3.2,2.3] nearest
The same as the previous example except that nearest-neighbour resampling is used. This will be somewhat faster, but may result in features shifted by up to half a pixel.
slide speca specb stype=rel fid=11.2 obj=11.7
The pixels in the NDF speca are shifted by 0.5 (i.e. 11.7 − 11.2) pixels and the output NDF is written as specb.
slide speca specb stype=abs abs=0.5
This does just the same as the previous example.

Sub-Pixel Interpolation Schemes

When performing the translation the pixels are resampled from the input grid to the output grid by default using linear interpolation. For many purposes this default scheme will be adequate, but for greater control over the resampling process the METHOD and PARAMS parameters can be used. Detailed discussion of the use of these parameters can be found in the “Sub-pixel Interpolation Schemes”  section of the AST_RESAMPLE documentation.

Notes:

Implementation Status:

Related Applications

KAPPA: REGRID, SQORST, WCSADD.