[AstroPy] Parameter 'origin' in WCS-methods

Thomas Robitaille thomas.robitaille@gmail....
Fri May 8 06:32:47 CDT 2009


Hi Martin,

The various ways to call the pix2sky or sky2pix methods are

wcs.wcs_pix2sky(x,y,0)
wcs.wcs_pix2sky(x,y,1)
wcs.wcs_pix2sky(xy,0)
wcs.wcs_pix2sky(xy,1)

where x and y are 1D numpy arrays and xy is a 2D array. The API is  
described here: http://stsdas.stsci.edu/astrolib/pywcs-1.4-4.3_api/

In the FITS standard, the bottom left pixel is defined as (1,1)  
whereas numpy arrays are zero-based, so the bottom left pixel in the  
image is (0,0). Basically, the origin argument specifies whether you  
want to assume the bottom left pixel is (1,1) (origin=1) or (0,0)  
(origin=0). Does this make any sense?

Cheers,

Thomas

On 8 May 2009, at 07:07, Martin Kuemmel wrote:

> Hi all,
>
> I installed the newest version of pywcs (v1.4-4.3). The 'test.py' in
> the tarball complains since there is no parameter 'origin' given
> in the various methods (e.g. wcs_pix2sky(origin, *args)).
>
> The 'test.py' in the repository works, good.
>
> But what's the meaning of this parameter, anyway??
> It's an integer, and learning-by-doing tells me that it matters
> whether it is a 0 or a 1? Is there some documentation somewhere?
>
> Cheers,
> Martin
>
> -- 
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