1998.11.11 16:06 "RE: Four-gig limit?", by Ed Grissom

d_smith -

I asked a similar question on this list a couple of months ago (in a message entitled "Large images (>2gb & >4gb)" )

I got very little response.

The scanners that we manufacture will also get close to the 4GB limit right now, and since we always include reduced resolution "overviews" in our images, this takes them over the limit. Currently we disable creation of files that would go over the limit.

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There are basically two routes to go, and both require adding to the TIFF spec to allow 64-bit data types and offsets:

One route is to have the initial IFD offset be 8 bytes instead of the current 4. This will break all existing TIFF readers, and will probably require a version number change (from the current "42") to something else.

The other route is to always include a initial image that is less than 2 GB that current viewers can understand. This could either be a reduced resolution version of the big image, or a warning page with text that says "This image is >4 GB and cannot be read by this software". Then, a private tag in the first image could point to an hidden IFD that uses the 64-bit offsets to access the big image. The method could be codified into a Tiff Technical Note that could be stored at archive sites so that anyone could create / read these images.

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The private tag idea comes from Niles Ritter, and the warning page idea was suggested by Michel Plungjan.

I'd be glad to hear from anyone else that is running into this problem, and send you my proposal for using the private tag method.

I have yet to hear anything from Adobe about their plans for TIFF 7.0, but this problem should surely be addressed in any revision to the TIFF spec. Adobe's web page actually sounds like they are giving up maintenance of the spec and point you to this mailing list for active development...

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ed grissom
egrissom@ingr.com