2004.11.13 14:02 "Re: [Tiff] EXIF tags", by Andrey Kiselev

Joris,

3. Finally, a number of EXIF tags can only occur in an EXIF IFD. My understanding is that, as you say, we should regard them as 'private IFD tags' rather then private tags. It does indeed boil down to a namespace issue. In theory, the exact same code could occur in a regular image IFD and have a completely different meaning there.

The single paragraph in the specification that mentions this mechanism of private IFDs, says...

<quote from tiff spec>

If you need more than 10 tags, we suggest that you reserve a single private tag, define it as a LONG TIFF data type, and use its value as a pointer (offset) to a private IFD or other data structure of your choosing. Within that IFD, you can use whatever tags you want, since no one else will know that it is an IFD unless you tell them.

<end quote>

This seems to back up my understanding that private IFD tags live in their own namespace limited to the private IFD, and that they do not need to be registered, and that the same code can occur in a regular image IFD and denote a completely different tag there.

The reason why I'm suspecting they are registered are they numbers. EXIF specification declares three SubIFDs: EXIF, GPS and Interoperability directories. The latter two ones have tags numbered as 1,2,3,..., so they are definitely have local meaning only. But EXIF tags have suspicious numbers 36864, 40960, etc. For me that looks like they may be registered.

Of course, this issue has not significant importance, but it is interesting to clarify this anyway.

Andrey

Andrey V. Kiselev
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