2004.04.15 13:33 "Re: [Tiff] Large TIFF files", by Frank Warmerdam
Are there people currently active to define a "64 bit tiff" proposal? Is there a list of proposed alternatives?
I don't think there is anyone actively working on this. However, I propose that we re-initiate a discussion on this topic here on the list. This has been percolating in the back of my mind for several years. My current thinking is that we should:
- Introduce a new data type TIFF_LONGLONG = 14 which is a 64 bit integer value.
- Allow as a non-core extention the option of having the strip offsets or tile offsets as TIFF_LONGLONG.
- leave everything else the same.
This approach has a number of downsides compared to a complete 64-bit conversion. Primarily that only image tiles and strips can be managed beyond the 4GB barrier. This makes places the onus on applications wanting to write large files to ensure that all other data items (the directory, and all references data objects like color table, description and so forth) are written out in the first 4GB of the file.
On the plus side, it would be a relatively modest effort to modify existing TIFF libraries to support such an approach - for reading at least.
Another approach of course would be to alter all offsets to be 64 bit which would be much more general, but require more substantial changes to supporting libraries. It would also pretty much require a new magic code and we would not really be within our rights to call the result TIFF.
Which ever approach is taken, I would intend to produce a libtiff that supported both the new "big" format and the existing 32bit format. Ideally, libtiff would produce old style files normally, and switch over to producing the big format for large images.
The other issue that arises is whether we should actively try to get Adobe to lead a new TIFF specification or just produce something ourselves as a community. While it would be nice to have Adobe's involvement, I am not keen on waiting for them.
One thing I would really like is some indication of what other TIFF implementations exist out there beyond libtiff, and if their developers would be interested in working cooperatively on a 64 bit TIFF standard. I know that folks like LEADtools must have their own. Adobe likely does. In the remote sensing field I know most vendors use libtiff (or hacked variations thereof) but I think some folks (Intergraph?) have their own TIFF libraries. It would be nice to get some non-libtiff folks involved in a 64 bit TIFF specification effort. Perhaps especially the folks who produce the scanners that need to produce big files.
By the way, I would forsee a 64 bit TIFF specification (which I imagine we would have to call something different - perhaps BTIFF - Big Tagged Image File Format) being developed essentially only defining low level structural details, with all higher level concepts (tag lists, values and so on) defined by reference to the TIFF 6 specification.
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