DNG files are themselves TIFF files, with the compression method as a speciality. There are many other aspects relating to a "negative" image, which are not present in normal TIFF files, but the format is still TIFF.
You mentioned the excellent compression capabilities of the DNG format. The compression method used in the "standard DNG" is the one of the original lossless JPEG encoding, which is a pretty useless method on its own, it has been effectively dropped already. However, it turned out to be an excellent method for the non-demosaiced data if used abusively. You would not be very happy with this method if you used it on demosaiced data.
Well, i found the PNG compression to be the best for 16bit files. I have an old archive of scans exclusively in PNG format.
Even TIFF w- ZIP compression doesn't come close to the PNG compression. Unfortunately Lightroom (and some other apps) don't support PNG. But i need an archiving format for my PNG collection that uses high compression. So DNG seems to be the only choice.
What i have not mentioned in my original post is the following. I have found a workaround to convert PNG into DNG. It's madness (almost 4000 images and not enough disk space!), but it can be automated.
1) convert PNG to TIFF
2) convert TIFF to NEF in Nikon Capture NX 2
3) import the NEFs in Lightroom 3 beta and convert to DNG
So, obviously a TIFF can be converted into DNG if it's converted to NEF first. Of course, it remains a non-RAW file.
My concern is about the future compatibility of these DNG files i produced with the above method. Is it possible that i get problems in the future? Or is a DNG always safe, even if the NEF was not a real NEF?
It's a very important issue for me, and i'll greatly appreciate any further insight.
PS: Gabor, you said "You would not be very happy with this method if you used it on demosaiced data." I can only say, on demosaiced data it's almost as good as PNG. Good enough for me, and much better than the best TIFF compression (on 16bit anyway).
Do you mind uploading such a NEF (i.e. converted from TIFF) and the DNG converted from that NEF? If you don't have web space, pls use yousendit.com
OK, it seems there is a problem, at least in LR3 beta. The white balance sliders have no influence on the picture - use of this function freezes the history. The gray picker always messes up the colors. No need to search for more incompatibillities. My method simply doesn't work.
I need to find another solution.