2011.09.28 09:30 "Re: [Tiff] Write Exif data to Tiff file", by Joris Van Damme

Phil,

One particular datatype is IFD offset, and that's how the IFD list eventually turns into an IFD tree.

Regretfully this is not the case. offsets unfortunately  IFD use the LONG datatype. IFD-offset datatype is mentioned  An in the PageMaker 6.0 technical notes, but to my knowledge this is not part of the official TIFF specification (but it it should be!!), and it is certainly not in widespread use.

Of course you are correct. I was simplifying things, in order to get the message through: we would not need any specific hacks for any IFD or any tag, if we had the proper generic IFD/tag handling, so we should at least consider adding the latter, or at the very least be aware of the fact that any specific hacks are 'a bit off' and be conscious about this when making decisions. More specifically, we should certainly not go and invent additional IFD/tag replacement structures, like Bob's BLOB, convoluting the bare essence of TIFF that LibTIFF never was able to support with alternatives that we'll never be able to fully support and that come with design flaws on top of it all.

That being said, I personally do regard the 'technotes' as part of the specification, actually. We're all promoting new-style JPEG-in-TIFF, and that too is a 'technote'. We've build on from the notion of IFD offset datatype in designing BigTIFF, so we do at least acknowledge its existence. Might as well go a bit further and promote its use a bit. But as you say, we will always have to add a little tag-specific knowledge in our code in this area, so as to auto-reinterpret some 'long' values as 'IFD offset' values for certain known tags, that is absolutely correct.

Best regards,

Joris Van Damme
AWare Systems
info@awaresystems.be
http://www.awaresystems.be/