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October 1999

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1999.10.28 10:31 "Stupid TIFF question: tags in ascending order", by Robert Vesterman
1999.10.29 03:27 "Re: Stupid TIFF question: tags in ascending order", by Daniel Mccoy
1999.10.29 04:18 "Re: Stupid TIFF question: tags in ascending order", by Tom Lane
1999.10.29 07:15 "Re: Stupid TIFF question: tags in ascending order", by Rainer Wiesenfarth
1999.10.29 09:23 "Re: Stupid TIFF question: tags in ascending order", by Robert Vesterman

1999.10.29 04:18 "Re: Stupid TIFF question: tags in ascending order", by Tom Lane

rvesterman@attmail.com (Robert Vesterman) writes:
> Stupid question: does this mean strictly ascending order? In other
> words, is it legal to have the same tag twice (or more) in a single
> IFD?

AFAICS, the assumption is that a single IFD may have at most one
instance of a particular tag.  I can't find any text to that effect
in the spec either.  But it would obviously be bogus to have, say,
two different Compression tags in one IFD.  There is definitely no
text saying "use the first one" or "use the last one", so I think a
reader is entirely justified in rejecting such a file as incorrect.

There are some tags such as ImageDescription where you could possibly
cope with multiple instances (say by concatenating the contents, in
the case of ImageDescription).  But in the absence of any explicit
text in the spec saying that that's what should happen, I can't fault
a reader for rejecting multiple instances of noncritical tags, either.
I don't see any standard tags where a lot would be gained by allowing
this, so on the whole the answer seems clear: no duplicate tags.

There are a *lot* of things that the TIFF 6.0 spec doesn't spell out
as explicitly as I'd like.  This is one.  If Adobe ever gives up
their TIFF-suffocation project, maybe there will be a new edition
that's a little bit better written...

                        regards, tom lane