[iaufwg] FITS conventions

[Francois Ochsenbein]

Hi Doug,

Sure, but NAXIS=3 with BITPIX=8 and NAXIS1=4 does exactly the
same thing, with the advantage that the associated metadata
can explain the content. There is not FITS keyword to explain
the content of the array -- in which keyword would you specify the
RGB (or RGBE -- BTW what is the meaning of the 'E'?) 
Some systems use BTYPE (as an analogy with CTYPEn) but it's not 
a standard FITS keyword. And assuming a 32-bit array, is the red 
component represented by the higher or lower byte ?

Simply spelling out these conventions and proposing an unambiguous
scheme would help a lot the developpers --- if the FITS-WG does not do it, 
all possible conventions will co-exist, with inevitable resulting confusions...

--Francois

>
>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
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