Thursday, July 10, 2008
Update on BigTIFF
Aware Systems' BigTIFF page has just had a significant update. Previously it had strong warnings that the design was still in flux. It now states:
Whilst BigTIFF is not yet an actual official standard, there were quite a few people involved with developing the BigTIFF draft specification, including people from Adobe, and there have been no changes since that time. There are at least two independent implementations (our own AWare Systems AsTiff, and LibTiff since version 4.0). Furthermore, BigTIFF is a small and quite natural progression from TIFF. It seems highly unlikely that an organization with some apparent position of authority (i.e. a standards organization or Adobe) will produce a document on their own that contradicts this draft specification at this stage. We thus feel we can safely recommend adoption of BigTIFF now.
This seems to be more a matter of finally declaring it done (after nudging by me and several other people on the TIFF list) rather than any flurry of recent activity.
The formal description of BigTIFF -- to the extent we can call it "formal" -- is found here. Some issues, notably what MIME type to use, are still open.
There has been talk on the TIFF discussion list of sticking with "image/tiff". However, doing that would contradict IETF RFC 3302, which defines the type in terms of TIFF 6.0. The changes to BigTIFF from TIFF 6.0 are minor in a certain sense, but no TIFF reader following the TIFF 6.0 definition will accept a BigTIFF file. Registering "image/tiff-big" would require publication of a formal standard, and that isn't likely to happen. My own suggestion is informally adopting "image/x-tiff-big".
Anyway, congratulations to all those who have brought BigTIFF to this stage!
Related link: LibTIFF. Beware of imitations such as www dot libtiff dot org!
Thursday, April 10, 2008
The state of BigTIFF
A post by Bob Friesenhahn to the TIFF mailing list says: "The sponsored BigTIFF effort has come to completion. Libtiff4 is now in a more gradual 'maintenance' state which will hopefully lead to a stable release."
I hope this maintenance state isn't a euphemism for stagnation.
Related links:
Thursday, July 05, 2007
Is BigTIFF TIFF?
An argumentenergetic discussion is developing on the TIFF mailing list on whether BigTIFF is a new version of TIFF or a new file format. There are good reasons for calling it a new format.
At the most obvious level, no existing application to read TIFF files can read any BigTIFF file. The TIFF specification says that bytes 2-3 contain "An arbitrary but carefully chosen number (42) that further identifies the file as a TIFF file." BigTIFF files contain the value 43 in this location. The TIFF spec says that bytes 4-7 contain "The offset (in bytes) of the first IFD." BigTIFF uses these locations for other purposes, and places the offset of the first IFD starting at byte 8. In addition, a tag structure is 12 bytes long in TIFF; it's 20 bytes in BigTIFF, to accommodate larger counts and offsets.
The BigTIFF proposal calls the value at offset 2 a "version number." But according to the TIFF specification, it isn't a version but a fixed identifier. Putting a different number there is a declaration that a file isn't TIFF.
File formats do change over time, and inevitably some files based on newer versions will not be readable by software based on older versions. But it's reasonable to expect that the newer files be recognizable as incompatible or broken instances of the format. When the format's signature information, other than the part which identifies the version, changes, it's no longer the same format.
BigTIFF does have a lot in common with TIFF, and it should be possible to modify a well-written TIFF reader to read BigTIFF without too much trouble. All tag values and definitions from TIFF are retained.
It's unfortunate that TIFF wasn't designed to include version information. The only way to tell a TIFF 6 file from a TIFF 4 file is by the features (tags and data types) it uses. This puts BigTIFF in a difficult situation. But from the standpoint of format identification, it makes more sense to call BigTIFF a new format derived from TIFF than to call it TIFF, and to give it its own MIME type and file extension(s).
The decision is actually in the hands of Adobe, which hasn't shown much interest in the TIFF standard in a long time. If Adobe does nothing, then the new format isn't TIFF, since it's fundamentally different from any Adobe-approved specification. Otherwise the decision is legally in Adobe's hands.
