[fitsbits] Start of the 'INHERIT' Public Comment Period

[William Pence]

Rob Seaman wrote:

> 4) Yes, but that boat has sailed.  The community has been on a course to
> deal with inheritance since this note from the image extension paper:
>
>     "Although allowed, it is recommended that the primary header does
> not set the keyword NAXIS=0, since it would not make sense to extend a
> non-existing image with another image."
>
> FITS is either going to tie the contents of separate HDUs together
> semantically or not.  The community eagerly - and widely - adopted the
> notion of the primacy of the primary HDU - likely before the words above
> were published.  Implicit here is that the primary header of an empty
> HDU is often used for information that applies to the entire file. 

Maybe I'm missing your point, but I don't see how that paper can be 
interpreted as an endorsement of the inherit convention.  In that 
sentence you quote, and elsewhere in the paper, they make it clear that 
they do not recommend appending an image extension to a null primary 
array; instead they think the primary array should be filled first, and 
then only append more image extensions if the primary array is already 
occupied.  This is contrary to the inherit convention, which requires 
that the primary array be empty to avoid confusion about whether the 
keywords in the primary array should be interpreted as applying globally 
to the following extensions or not.

Some might suggest that with the abundance of low cost disk space that 
is now available, the inherit convention is trying to fix a 
non-problem.  The amount of diskspace that is saved by not duplicating 
the keywords in every extension is rather insignificant in most cases 
and doesn't warrant the extra software complexity in supporting the 
inherit convention..  There are no doubt some pathological cases where 
the size of the headers could dominate the size of the whole file, but 
in those cases there may be alternate ways to pack the data more 
efficiently (e.g. pack the separate image extension data into vectors in 
rows of a single binary table extension).  

Bill Pence