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