2005.05.02 18:48 "Re: [Tiff] Color TIFF/JPEG compression questions", by Bob Friesenhahn
So, I think that tiffinfo might be playing correctly here (since it agrees with tiffdump, and I don't see any obvious problems with the code.)
We should check that we are talking about the exact same quad-jpeg.tif. I know for a fact that AsTiffTagViewer shows exactly what is there, and, unlike LibTiff, that stuff isn't burdoned with an actual image decoder part and a subsampling-fixup scheme and such... So if AsTiffTagViewer doesn't mention the tag, and tiffinfo does, I know for a fact that tiffinfo must be wrong.
Tiffinfo seems to dump information the way libtiff would handle it (e.g. includes some defaults). Tiffdump should be used to produce a raw dump of TIFF information.
A newer version of AsTiffTagViewer that isn't published yet, also supports hex viewing of just about any structure in a TIFF file. With this version I was able to triple-check, and sure enough, the tag count of this IFD shows as 14 in a hex dump that is merely a hex dump and cannot possibly be wrong. 14 is the number of tags shown by both versions of AsTiffTagViewer. So tiffinfo *does* add the default or fixed-up or whatever values of YCbCrSubsampling even though the tag is *not* there.
I am still eagerly awaiting the release of this software. :-)
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/