2003.02.19 15:47 "Re: Extended TIFF-Tags", by Frank Warmerdam
could you please tell me, if there is already a predefined TIFF-Type for UINT64 or QWORD. This is an 8-BYTE-Integer. I have to save those data and would like to use compatible type-specifications in the tags.
Another question: We have to save images and files which are so large, that offsets may blow up DWORD-limits. So I am thinking about a TIF-derivative using "TIFF-Tags" providing 8-Bytes per offset. Consequently more data can theoretically be saved directly in the tags. My approach would be, to apply TIFF 6.0 rules:
b) Write values directly into the Tag if and only if there is room.
I don't want to bother you with the second question, which is anyhow off-topic. But it would be helpful to me if you could give me some hints on how to procede. May be, a format like that already exists.
The data type of pixels in a TIFF file are defined via two values in the file, the bits per sample, and the sample format. In theory you should be able to write TIFF files with a bits per sample of 64, and a sample format of unsigned integer. However, I suspect you will find few software packages that will support this data type. I am not sure if libtiff is able to read or write pixels of this type, but I would appreciate feedback on fixes to libtiff to make it work.
On the issue of support a 64 bit data type in tags and so that large files (larger than can be addressed by 32bit unsigned ints), this is an area that has seen lots of discussion in the past.
The short summary seems to be that there aren't any practical ways of supporting TIFF files larger than 4GB without altering the format in a way that makes it fundamentally incompatible with existing libraries and applications. Suggested approaches include the addition of a new 64bit offset format (essentially a TIFF64 format that wouldn't work with any existing applications), or using external means such as super-tiling (break your large file into a set of related TIFF files.
At this time I am not too interested in hacking libtiff to support a TIFF64 format.
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