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To: "Adrian Phillips (Tandem User)" <adrian.phillips@dnmi.no>
Cc: tiff@sgi.sgi.com
Subject: Re: TIFF photometric question 
In-Reply-To: Your message of "Wed, 10 Aug 1994 11:40:57 CDT."
             <9408101141.ZM2453@freeze.oslo.dnmi.no> 
Date: Wed, 10 Aug 1994 10:37:05 PDT
From: Sam Leffler <sam@flake.asd.sgi.com>
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    To:  tiff@sgi.sgi.com
    Subject:  TIFF photometric question
    Date: Wed, 10 Aug 1994 11:40:57 CDT
    From:  "Adrian Phillips (Tandem User)" <adrian.phillips@dnmi.no>

    We are converting a raw group 3 fax message embedded in another to postscript
    holding the messages on disk in compressed (g3 or g4) format . We use pbm's
    g3topbm to convert to pbm, pnmtotiff to convert to tiff, and then tiff2ps to
    convert to postscript for printing.

Why not just use fax2tiff?
    
    After the pnmtotiff, the tiff images have photometric min-is-black - the images
    we're originally scanned in with white background and black foreground. When
    the last version of tiff2ps is used the images come out thus, but if the new
    version 3.3 is used, the images come out reversed. I interpret min-is-black to
    mean that this reversing should take place (TIFF spec says
    WhiteIsZero/BlackIsZero) - is this correct ?
    
Sounds like pnmtotiff is setting the Photometric tag incorrectly.  It
should be MinIsWhite instead of MinIsBlack.  The change between v3.2 and
v3.3 was to eliminate the use of the Photometric tag in the G3+G4 decoders.
This is according to the spec (though contrary to my intuition, but that's
neither here nor there).  Basically G3/G4-encoded data is defined to be
MinIsWhite (i.e. you the run codes are inherently encoded to mean white or 
black independent of any Photometric tag value).  In v3.2 the library
interpreted the run codes according to the value of the Photometric tag
while in v3.3 it always assigns a 0/1 value according to the CCITT spec.

Your can switch to fax2tiff or change pnmtotiff to emit a different
Photometric tag value.

	Sam




