2004.11.13 12:52 "[Tiff] EXIF tags", by Andrey Kiselev

2004.11.13 13:30 "Re: [Tiff] EXIF tags", by Joris Van Damme


I'm wondering whether the EXIF tags are registered as private tags by Adobe? Or should I interpret that tags in the namespace limited by the EXIF SubIFD only?

There's three different cases.

  1. Some EXIF tags are documented to occur in both image IFDs and EXIF IFDs. Such tags, in regular image IFDs, certainly fall into the category of 'private tags'. Since EXIF is not obscure or anything, I expect them to be registered with Adobe, most probably.
  2. The EXIF IFD tag itself can only occur in a regular image IFD, not in an EXIF IFD, of course. Apart from that, it's just like the other EXIF tags that occur in the image IFD, it's a private tag, and I expect it to be registered.
  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.

I've been planning to add a section to the TIFF Tag Directory over at http://www.awaresystems.be/imaging/tiff/tifftags.html, labelled 'Private IFD Tags', as opposed to 'Private Tags', with, for now, a single subsection 'EXIF Tags', with an enumeration of those tags that live inside the 'EXIF IFD namespace'... but I didn't find the time yet.

Joris Van Damme
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