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September 2004
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2004.09.17 11:43 "Re: BigTIFF Tag Value Count issue", by Joris Van Damme
> > Option 0: stick with 4 byte tag count members (alignment, tag data <= 4 gig, > > Frank's choice) > > Option 1: make it an 8 byte tag count (no alignment, tag data > 4 gig > > allowed) > > Option 2, courtesy of Bob: make it 8 byte tag count, add 4 padding bytes > > (alignment, tag data > 4 gig allowed) > > There is another option to get 8 byte alignment, so instead of 4 padding > bytes they are used for extending the first 2 fields => aligning every field > to 32 bit. This might be easier to handle by processors [less complex > masking] > > Offset - Datatype - Value > -------------------------- > 0 - ULONG - Tag identifying code > 4 - ULONG - Datatype of tag data > 8 - Unsigned Long - Number of values > 8 - 4 bytes - Tag data or offset Rob, Very valid fourth option, I think. It was a good idea to instert the strucure's sortoff table view again (except, of course, that I mentioned this structure for classic TIFF, in BigTIFF the last member is 8 bytes long). So allow me to do the same for all options sofar ** Option 0: stick with 4 byte tag count members ** (Alignment, tag data <=4 gig, Frank's choice) 0 - U16 - Tag identifying code 2 - U16 - Datatype of tag data 4 - U32 - Number of values 8 - U64 - Tag data or offset 16 ** Option 1: make it an 8 byte tag count ** (No alignment, tag data > 4 gig allowed, no votes sofar) 0 - U16 - Tag identifying code 2 - U16 - Datatype of tag data 4 - U64 - Number of values 12 - U64 - Tag data or offset 20 ** Option 2: make it 8 byte tag count, add 4 padding bytes ** (Alignment, tag data > 4 gig allowed, Bob's choice) 0 - U16 - Tag identifying code 2 - U16 - Datatype of tag data 4 - 4 bytes padding - Unused 8 - U64 - Number of values 16 - U64 - Tag data or offset 24 ** Option 3: make it 8 byte tag count, extend previous 2 members to 4 bytes ** (Alignment, tag data > 4 gig allowed) 0 - U32 - Tag identifying code 4 - U32 - Datatype of tag data 8 - U64 - Number of values 16 - U64 - Tag data or offset 24 Rob, am I to understand you like option 3 best, or were you merely signalling the possibility? Joris Van Damme info@awaresystems.be http://www.awaresystems.be Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html