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February 2008
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2008.02.01 08:05 "Re: TIFF T.30 vs TIFF Class F", by Joris Van Damme
Bob, Bob Friesenhahn wrote: > It seems that the T.30 FAX specification says to send the > most-significant bits first while TIFF Class F > (http://palimpsest.stanford.edu/bytopic/imaging/std/tiff-f.html) > recommends (requires?) sending the least-significant bits first. > > I have people in both camps claiming that their specification is the > right one to use and that it can't work any other way. > > Can someone please educate me in the ways of FAX and what the best way > is to address this situation? I don't remember a T.30, only a T.4 and T.6. But I assume we're likely refering to the same thing with 'FAX specification'. The compression scheme specification specifies a lot of stuff that is instead specified on the IFD level in TIFF. Most obvious example is color and dimensions: the FAX specification says some stuff about how wide images are, and what is white and black, whilst in TIFF this is determined by the IFD ImageWidth and IFD PhotometricInterpretation tags and can be anything. The same does indeed also apply to bit order, if I remember correctly. Thus, in TIFF either bit order is correct, and this applies to any compression scheme and is always to be indicated by the FillOrder tag. The general rule that applies, is that the TIFF specification is the higher priority. From the FAX specification we need to read only and exactly what is the compression scheme and does fit in TIFF without contradicting or overwriting TIFF. Otherwise FAX compression would end up a mess similar to the OJPEG mess, with tag meaning depending on compressed data properties, at which point all TIFF hell breaks loose. I've elaborated on the general rule and some specific applications, with regard to FAX compression in TIFF, in this mailing list before, attempting to help get the record straight. Please see http://www.asmail.be/msg0054894761.html. In my experience, most people comply with this point of view, I've seen very little of a second camp and instead, fortunately, much good interchange. Best regards, 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