2007.07.03 18:37 "[Tiff] BigTIFF extension?", by Phil Harvey

2007.07.05 14:14 "Re: [Tiff] Re: Tiff Digest, Vol 38, Issue 12", by Gary McGath

Gary, John,

  1. BigTIFF is TIFF.

No it's not :-) It's not in the TIFF specification


Awkward snipping here. That makes it appear that that was my quote, which it wasn't.

By that token, any new extension of a file format is a different file format, since it is not covered in the previous specification, since it is a new extension. That's ridiculous. We know TIFF for its tags, specific tags, free combination of encoding properties, multi-page support in IFD tree structure, extra sample support, different sampleformats, private data through private tags, etc. None of that changed in BigTIFF. Much attention went to keeping all binary structures as same as possible. BigTIFF is TIFF. It's not covered in TIFF 6.0, because it's new, well, hey, OF COURSE!

There is a difference between changes which extend or revise a file format and those which diverge from its fundamental assumptions. TIFF defines a particular header not just for 6.0, but for _all_ its versions. The value at offset 2 is not a "version number," as the BigTIFF proposal falsely claims, but part of what makes a file TIFF. Aside from this value, the contents from offset 4-7 is the offset of the first IFD in TIFF, and is used for a different purpose in BigTIFF.

Some makers of file formats, particularly the proprietary ones, pay no attention to the requirements of previous specifications when coming up with new versions, or may even break compatibility just to make people upgrade their software. Word i is very different from Word j. But that's a poor example to follow.

Formats have to be upgraded periodically, and it's strongly desirable that their design allow for this, so that readers can determine that something is an instance of format X but is based on a version or contains features that the reader doesn't support. Unfortunately, TIFF simply doesn't do this. But when a new format breaks the fundamental assumptions of an existing format, then it has to be considered a different format, however much it owes to the old one.

Stick with reality. What do programmers/users know about TIFF, what does it stand for? Any part of that actually changed?

Compatibility with all existing software.

If you want to play it really official, and want to ignore the de facto reality, you'll have to wait before you decide if BigTIFF is TIFF, because it's not clear at this point of the specification owner will integrate BigTIFF or not.

Agreed. I suspect we're never going to agree on whether BigTIFF is "really" TIFF; and legally at least, it's ultimately Adobe's decision.

Gary McGath
Digital Library Software Engineer
Harvard University Library Office for Information Systems