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Date: Mon, 24 Jan 2005 22:36:53 -0800
From: Lee Howard <faxguy@howardsilvan.com>
To: tiff@remotesensing.org
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Subject: [Tiff] converting tags
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X-UIDL: 51f1b0eb41389edfce5c568ccf1949c7

Hello.

HylaFAX uses the TIFFTAG_FAXRECVPARAMS tag.  The tag was secured by SGI 
specifically for HylaFAX.

FaxRecvParams holds encoded information describing the fax session 
parameters that existed when the fax page was received.

Currently FaxRecvParams is typed as uint32.  The problem is, though, 
that per Table 2/T.30 (the description of all possible fax session 
parameters, specified by the sender in a "DCS" signal) there are more 
than 110 possible bits.  So basically the "encoding" is just a 
compression of sorts of that 110+ bits into 24-32 bits, and we have to 
be rather selective about how we do it, and sooner or later we're 
inevitably going to run out of bits.  It is rather difficult to 
continually update and revise HylaFAX's "encoding" function for 
FaxRecvParams every time we add more support for various things on 
Table 2/T.30, and we'd like to simply store the DCS signal raw into 
FaxRecvParams rather than compressing/encoding it.

However, I don't know how easy it is to change TIFFTAG_FAXRECVPARAMS 
from uint32 to char* in libtiff.  The bigger issue, however, is that 
we'll need to be able to successfully TIFFGetField(tif, 
TIFFTAG_FAXRECVPARAMS, ...) on old TIFFs and retreive the data, at 
which point we can make use of it.

I'm not a TIFF tag expert, but I assume that the uint32 tag is stored 
as 4 bytes.

Maybe we should just use a different tag instead?

Thanks.

Lee.
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