[fitsbits] Any bad pixel mask convention?

[Arnold Rots]

Chandra data products contain bad pixel files that use the (local)
Region convention to indicate the location of groups of bad pixels.

  - Arnold

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Wed, Dec 4, 2013 at 3:58 PM, Rob Seaman <seaman at noao.edu> wrote:

> Hi Richard,
>
> > Is there something like a bad pixel mask FITS standard that stores bad
> pixels
> > with a boolen value or perhaps a float where 1=good, NaN=bad in FITS
> images?
> > Should one use a BLANK value combined with a byte or integer image or is
> > it best to invent a private ASCII format for this purpose?
>
> I guess I'm not entirely sure what's being requested here.  Are you
> looking for a way to represent a separate full size array of flags, one per
> pixel?  Or a way to embed special values within a particular data array
> itself?  I guess a "private ASCII format" would be something like a pixel
> list concept?
>
> As others have said there isn't really a mask standard in FITS since the
> concept is too generalized, e.g., support for MEFs and world coordinates
> and other features would have to be factored in.  However, the IRAF pixel
> list format was created to efficiently represent per-pixel masks, and this
> algorithm is supported by FPACK:
>
>         http://heasarc.nasa.gov/fitsio/fpack/fpackguide.pdf
>
> If the general concept of a mask is a separate array of per-pixel integer
> flags mapped one-to-one with one or more data arrays in an MEF, then the
> default FPACK Rice algorithm is pretty efficient itself.  Just create an
> integer array the same size as your data array(s) and fill it with whatever
> values make sense.  Copy over the WCS and other metadata as needed from
> your data arrays.  NOAO uses zeroes for good pixels and non-zero for bad.
>  Perhaps use different values to represent different kinds of bad - which
> is something that will vary from project to project.  Compress it using
> FPACK and it will get dramatically smaller since such a file will be very
> low entropy.  Try the -p flag to compare PLIO versus Rice for your
> application, both speed and size.
>
> Without compression of some sort such a mask will be the same size as data
> image of the same BITPIX.  But Rice will squeeze a 32-bit image into the
> same output size as a 16-bit image containing the same pixel values.
>
> Rob
>
>
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