1999.09.14 02:34 "G4 decode on Mac", by Christopher Thomas

1999.09.14 07:53 "Re: G4 decode on Mac", by Klaus Bartz

Hi Christopher,

Would appreciate any help regarding compiling libtiff on Mac. Upon compile looks for "windows.h" header,?? Specifically interested in decompressing the Group 4 fax algorithm.

  1. Is this Run Length compression?
  2. Does it RLE per line?

It is a 2 dimensional compression with coding deltas for short position changes related to the last line; at longer position changes it codes with "G3", that is a Run Length compression coding with a modified Huffman code for the runs.

For confussing G4 is named CCITT T.6 and G3 is named CCITT T.4 now. For more confussing CCITT is renamed to ITU-T ( since 1992 ??). You can buy the official definiton from the ITU


A good description is at


(But there is a little error; there is no V(0) in front of a line; that V(0) should be one pixel after the line. :-) )

You can also look at


But there are no picture as in the original where you can see the "last part of scan line".

  1. Besides libtiff is there any generic C code for decompression?

The G3/G4 code in libtiff is very good - not for reading, else for working. The G4 decoder is able to recover ( if there is a pure white line ). Which other decoder do it?

The code is original from Frank Cringle's viewfax program, do not know the current "home" of it...

Look to


for others. But the page have more than one dead links...

  1. Are fax machines really this smart?

Fax machines uses most G3_2Dim because recovering of G4 is offen not possible.

Dr. Klaus Bartz                                     bartzkau@ina.de
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