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September 2004

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2004.09.16 19:12 "BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.16 19:23 "Re: BigTIFF Tag Value Count issue", by Frank Warmerdam
2004.09.16 19:36 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.16 19:38 "Re: BigTIFF Tag Value Count issue", by Frank Warmerdam
2004.09.16 20:08 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 01:14 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.16 22:39 "Re: BigTIFF Tag Value Count issue", by Ian Ameline
2004.09.17 00:09 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 00:32 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 01:54 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 02:29 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 02:58 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 03:08 "Re: BigTIFF Tag Value Count issue", by Chris Cox
2004.09.17 04:20 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 04:39 "Re: BigTIFF Tag Value Count issue", by Chris Cox
2004.09.17 11:25 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 14:31 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 14:40 "Re: BigTIFF Tag Value Count issue", by Frank Warmerdam
2004.09.17 15:02 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 21:41 "Re: BigTIFF Tag Value Count issue", by Chris Cox
2004.09.17 14:53 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 15:06 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 21:36 "Re: BigTIFF Tag Value Count issue", by Chris Cox
2004.09.17 05:46 "Re: BigTIFF Tag Value Count issue", by Frank Warmerdam
2004.09.17 11:40 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 13:11 "Re: BigTIFF Tag Value Count issue", by Frank Warmerdam
2004.09.17 13:34 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 15:10 "Re: BigTIFF Tag Value Count issue", by Fernando Loygorri
2004.09.17 15:10 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 15:55 "Re: BigTIFF Tag Value Count issue", by Fernando Loygorri
2004.09.17 16:00 "Re: BigTIFF Tag Value Count issue", by Frank Warmerdam
2004.09.17 16:33 "Re: BigTIFF Tag Value Count issue", by Fernando Loygorri
2004.09.17 16:13 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 16:34 "Re: BigTIFF Tag Value Count issue", by Fernando Loygorri
2004.09.17 16:43 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 21:48 "Re: BigTIFF Tag Value Count issue", by Ross A Finlayson
2004.09.17 22:15 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn
2004.09.17 21:41 "Re: BigTIFF Tag Value Count issue", by Chris Cox
2004.09.17 11:38 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 06:35 "Re: BigTIFF Tag Value Count issue", by Rob Tillaart
2004.09.17 11:43 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 12:46 "Re: BigTIFF Tag Value Count issue", by Rob Tillaart
2004.09.17 12:55 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 13:20 "Re: BigTIFF Tag Value Count issue", by Rob Tillaart
2004.09.17 13:47 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.17 13:08 "Re: BigTIFF Tag Value Count issue", by Ross A Finlayson
2004.09.19 12:31 "Re: BigTIFF Tag Value Count issue", by Andrey Kiselev
2004.09.19 16:43 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
2004.09.20 16:18 "Re: BigTIFF Tag Value Count issue", by Ed Grissom
2004.09.20 22:05 "Re: BigTIFF Tag Value Count issue", by Steve Carlsen
2004.09.20 23:09 "Re: BigTIFF Tag Value Count issue", by Bob Friesenhahn

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

I originally wrote:
> 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.

- Joris, in the very first mail, wrote that one argument pro 8 bytes in simply
plain logic and consistency. Tag values of private tags are opaque to the
specification, are said to be 'anything'. It is logical for a file format that
is specifically designed to break the 4 gig boundary, to acknowledge that any
'anything' may need to break that boundary.

- Frank wrote he prefered a 4 byte tag count, basically because 8 byte tag
counts can lead to major trouble in current LibTiff design where tag values get
read and written in a single go.

- Bob, however, argued that 4 byte tag counts does not eliviate this problem.
It's just as easy to efficiently bomb LibTiff writing tag values of say even
merely 2 gig, which fits perfectly fine in a 4 byte tag count specification.
Limiting tag counts to 4 bytes may help in ignoring the LibTiff problem, it does
not help in making it go away.

- Rob wrote that he preferred 8 byte tag counts, since private tags could want
to store additional information about every single pixel. Whilst his example
included things that may be more suitably stored in extra channels, I can see
the general statement might hold for eg per-pixel variable size data.

- Andrey pointed out tiles can have be a single pixel wide and single pixel
high, yielding a number of tiles that equals the number of pixels in the image.
To store StripOffsets and StripByteCounts in that case (or far less extreme
variants), we need an 8 byte tag count.


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
info@awaresystems.be
http://www.awaresystems.be
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