2004.09.16 19:12 "[Tiff] BigTIFF Tag Value Count issue", by Joris Van Damme

2004.09.19 16:43 "Re: [Tiff] BigTIFF Tag Value Count issue", by Joris Van Damme

Chris and I have been discussing BigTIFF, as the current Wiki specifies, and we've come to a subject that we both would like more comments and opinions on.

The thing that is now up for debate is the third member, the 'Count' member, the member that specifies the number of values. We are unsure as to whether we really want to keep it 4 bytes, or extend this member to 8 bytes, as well.

In order to try and help ensuring a final conclusion, here is a overview of the founded opinions sofar. I've decided to even eliminate discussions about alignment from this overview, since, by now, it is totally clear that the two issues are not related, because we can/could add pad bytes wherever we want or extend other members or whatever. That also eliminates the original argument pro 4 bytes that I gave in the very first mail, and further narrowes down the discussion quite a bit.

I would now like to add an opinion of my own. First off, the more I think about it, the more the original argument pro 8 byte tag counts seems to make sense. It is logical for a 64 bit offset variant of TIFF to acknowledge the need for breaking the 4 gig boundary when it comes to opaque data. Secondly, while I acknowledge the need for BigTIFF to be as little as possible different from TIFF classic, so as to extend current libraries as easy as possible, and so as to extend the specification as painlessly as possible, it seems to me that the nature of the extension is such that we also need it to be future-proof. I fear that in five or maybe even less years from now, we might start to regret it, if we should now prefer a 4 byte tag count.

Chris asked at one time 'Can you think of a situation that needs an 8 byte count?'. I would like to turn that around, and answer to that we learned from the past such situations are bound to arise, probably sooner rather then later, even if it is rarely thinkable five years earlier. Therefore, if we cannot come up with a legit reason to limit things to 4 byte tag counts, we should eliminate the limitation in BigTIFF, now that we have the chance to do so relatively painlessly.

I tried to install an 'advisory voting' tag in this thread. I would now like to add my own vote, pro 8 byte tag counts. I would like to add that, in the event there would be no further discussion, I'm hoping Frank, Chris and Steve make the final decision at their earliest convinience. I really need this, preferably sooner rather then later. I do want to make sure that I comply with the future spec, preferably without many additional discussion about god, the universe, the maining of life, and the continued adventures of shell extension building.

Joris Van Damme
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