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.

Thanks! Eric