[iaufwg] FITS conventions

[Doug Tody]

On Fri, 10 Feb 2006, Francois Ochsenbein wrote:
> The two approaches for storing RGB images in FITS files are, either to
> store one color as the main 2D matrix and other colors in FITS extensions,
> or to use a 3D 'matrix' with a convention specifying which 'axis' contains
> the color information (e.g. via CTYPE='RGB' as proposed many years ago).

Another possibility (probably the most natural one) would be to store the
RGB value as the value of the pixel, as is done in most graphics systems.
Hence you have a 32 (or 64!) bit integer image containing RGB, RGBE, YCC,
etc., as the color-encoded pixel value, with 8-16 bits per color element.

I think this is what someone storing color images in FITS would prefer,
as the data can easily be processed with standard graphics software by
merely passing arrays of data without conversion.

This would seem to be more a characteristic of the observable than of the
coordinate system.  - Doug




> I know that several discussions occured about these alternatives and the
> usage of 'pseudo-dimensions', but a recommendation of the FITS authorities
> on how to write color images in FITS would avoid to see a proliferation of
> contradictory conventions about this issue.
>
> One immediate help could be to add to the FITS documents a list of
> the "official" CTYPEs . This list should contain, in addition to
> the WCS conventions, at least the "STOKES" convention which is
> widely used, and the "RGB" convention which was discussed many times.
>
> Does this sound impossible ?
>
> Cheers, francois
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> Francois Ochsenbein       ------       Observatoire Astronomique de Strasbourg
>   11, rue de l'Universite F-67000 STRASBOURG       Phone: +33-(0)390 24 24 29
> Email: francois at astro.u-strasbg.fr   (France)         Fax: +33-(0)390 24 24 17
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