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Overlapping tag data

I don't think I've seen this mentioned before. An issue with JHOVE is relevant to TI/A. The TIFF spec requires that "No data should be referenced from more than one place." That is, two different tags shouldn't point to the same data. I think this should be an explicit requirement of TI/A, since it isn't easy to notice and requires some special checking (which JHOVE hasn't done up to now). This may have been mentioned before. If so, sorry for committing the very violation I'm trying to avoid. :)

Comments

#1

Totally agree with Gary, reusing tags offsets shouldn't be allowed in TI/A.

In addition, DPF Manager is able to detect and report this TIFF baseline 6.0 specification violation. Rules id bl-g-4 (reuse offsets) and bl-g-5 (Offset and length not overlapping other tags) from DPF-Manager TIFF baseline implementation document.

Sorry for this XML document link, we are working on a "Human readable" version of the DPF Manager baseline implementation. 

#2

This is a very interesting issue. Basically I totally agree with both of you. The condition is stated in General Requirements in Section 7 Additional Baseline TIFF Requirements.

As this Section says, the mentioned characteristics apply for all Baseline TIFF files. But it is not only about Duplicate Pointers. There is much more: It is also about options, defaults, multiple subfiles, extra components, new field types, pixel types. et cetera.

There is also a paragraph about required fields. All these statements are relevant for TI/A. I would not pick out a single one.

There are two possibilities: either we relate to TIFF 6.0 baseline and state that all sections including Section 7 is mandatory for TI/A or we take Section 7 into the TI/A recommendation

What do you suggest?