[fitsbits] Spectral FITS -- encoding extraction area

[Doug Tody]

The "pixel mask" facility used in IRAF and some other projects does exactly
this.  Bill (Pence) - do you know what happened to that paper we did on
compression in FITS some years back, which included a discussion of pixel
masks?  Pixel masks are good for outlining irregular areas and can be
expressed in a compressed from in a binary table.

The purpose of the region stuff in VO is mainly to outline regions on the
sky in world coordinates.  This gets compiled into more efficient formats
such as HTM or pixel mask for use by software.  This approach is better
for working in "world coordinate" space; the pixel mask approach may be
preferable for describing masks such as extraction masks, exposure masks,
etc.  at the level of an instrument or detector.     Doug


On Thu, 17 Feb 2005, Walter Jaffe wrote:

> For encoding irregular extraction areas, I recommand NOT using
> a list of polygon vertices, but rather an image, which is much
> more general and intuitive.  So in one FITS file you could
> include both your spectra and a "finding chart" image, say of
> integer values=0,1,2,3.  Pixels where chart values=1 were included
> in spectrum 1, etc.  A more complicated variation is where pixels are
> included in spectra with various unequal weights.  Then I would
> use a 3-d image with the weights for each spectra in one plane of
> the image.
>
>
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