I need to edit some B&W TIFF files, which are stored using TIFF Class F
Group 4 fax compression (2D coding, ITU-T T.6 standard).  GIMP can open
these just fine, though they wind up being grayscale images.  I can
convert them to indexed black & white images.  When I try to save them
as TIFF, I am given several compression choices, of which G4 is not
listed.  However, it appears that if I simply select "None", it does try
to do what I want.  But I get the error messages "TIFF Failed a scanline
write on row 0" and "Bits/sample must be 1 for Group 3/4 encoding/decoding".

Is there some other step I'm missing here, in order to force the image
to be one bit/sample?

I tried to save as a PNM and was disappointed to find that it wrote
the file in PGM format rather than PBM.  With a suitable chain of
netpbm tools I can convert that to PBM then to TIFF Class F Group 4, but
it would be much easier if I could save in that format directly from
Gimp, especially since I want to do it in Script-Fu.

I'm using Gimp 1.2.3 as supplied in the Red Hat 8.0 distribution.


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