[fitsbits] start of Public Comment Period on the INHERIT convention

[Mark Taylor]

Dear fitsbits,

The proposed INHERIT convention looks a bit questionable to me.

The FITSBITS discussion digest referenced on the inherit
convention registration page
(http://fits.gsfc.nasa.gov/registry/inherit/fitsbits.txt)
expresses several concerns which don't seem to me to have been
conclusively addressed, especially in the context of adopting
this convention as part of the standard itself.  I don't however
have the leisure or energy to participate in a re-run of those
arguments now.

The main worry I have is the possibility of INHERITed keywords
that make a significant difference to the way that an inheriting
HDU is processed.  For instance if a PHDU has a set of TFORMn
keywords that are INHERITed by subsequent BINTABLE extensions
(which themselves lack those keywords), then the file will look
legal to a reader which is aware of INHERIT but illegal to one
which is not.  In that context, Lucio's comment:

On Mon, 22 Jun 2015, Lucio Chiappetti wrote:

> INHERIT       : 1 keyword, honour it or not

seems to be understating its impact.  The same applies to a lesser
extent to TTYPEn and other T*n keywords.

(You might argue that the inclusion of TFORMn in the Mandatory
list of sec 7.3.1 precludes omitting it from a BINTABLE extension -
though "the exact merging mechanism is left up to the application"
looks to allow that at least as an option - but that doesn't
apply for the reserved keywords TTYPEn et al.).

The comment about BSCALE and BZERO suggests (? I think, though I'm
not sure) that the intention here is *not* to encourage inheritance
of 'structural' keywords like the T*n BINTABLE ones.
However, on my reading of the proposed text this kind of thing
is not actually prohibited, and software following the standard's
"should" prescription concerning INHERIT would have to merge
PHDU T*n keywords marked as inherited in this way.

Is the proposal in fact intended to permit and/or encourage
inheritance of 'structural' keywords like BINTABLE TFORMn and TTYPEn?
I feel at the very least this should be clarified in the text.
(And I still might not like it even then.)

Mark

On Fri, 19 Jun 2015, Lucio Chiappetti wrote:

> ANNOUNCEMENT:  START OF FORMAL PUBLIC COMMENT PERIOD
> 
> This is to announce the official start of a 3-week formal Public Comment 
> Period on the incorporation of so called header inheritance (for short 
> INHERIT) convention in the FITS Standard.
> 
> This is part of a process to incorporate the most useful, widely used 
> registered, and simple conventions (which are valid FITS constructs) into 
> the official definition of the standard.
> 
> This convention allows to specify whether keywords common to primary and 
> extension HDUs can be written only once in the primary HDU and inherited 
> by the extensions.
> 
> The proposed text consists just
> 
> - in the ADDITION of a several new paragraphs at the end of 4.4.2.6 after 
>   blank keywords (hilighted in brick red colour)
> 
>    and has been prepared by a technical team including L.Chiappetti, 
>    W.Pence, A.Dobrzycki, R.A.Shaw and W.Thompson (main editor 
>    L.Chiappetti).
> 
> - If the proposal is approved a section H.3 will be added to Appendix H 
>   describing the update, and the keyword will be listed in Appendix C
> 
> The proposed draft text is available at 
> http://sax.iasf-milano.inaf.it/~lucio/FITS/Conventions/inherit-upd2.pdf
> 
> Supporting material is provided in the FITS Convention Registry at the 
> following URL http://fits.gsfc.nasa.gov/registry/inherit.html
> 
> For general considerations on the Public Comment Period, the IAUFEC review 
> and background information on the FITS approval process please refer to 
> the announcement of Public Comment Period for CONTINUE issued earilier 
> today.
> 
> Please review the text carefully and post any comments, criticisms, or 
> suggestions on the FITSBITS mailing list, separately for each convention. 
> ==================================================================
> 
> The Public Comment Period starts today 19 June 2015 and will last formally 
> for 3 weeks until July 9 included.
> 
> -- 
> Lucio Chiappetti - INAF/IASF - via Bassini 15 - I-20133 Milano (Italy)
> Current IAU FWG chair   
> 
> _______________________________________________
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> 

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/