2000.10.05 03:58 "Re: Varying BitsPerSample", by Sam Leffler
I would really like to see some of these (in particular the MINSAMPLEVALUE and MAXSAMPLEVALUE) handled on a per-sample in libtiff. It would also be nice if the types of each sample could be controlled, though I think this would result in datasets that work properly with very few packages. It might be quite difficult to even get the libtiff tools programs to support this fully.
In short, I am willing to work with you to make libtiff handle this better, but I think it unwise to generate files like this if you actually want them to work with any other software.
I too would be interested in more educated opinions.
The documentation clearly states (or so I recall) that BitsPerSample must be the same for each sample. This was a conscious design decision based on my experiences--that the vast majority of usage had each sample the same width and that it wasn't worthwhile adding the complexity of varying bits per sample.
You will find a couple other TIFF tags likewise constrained by the library. All are/should be documented (I'm certain the manual pages state limitations and the HTML documentation should as well).