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.

Is this Run Length compression?
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

http://www.itu.int/plweb-cgi/fastweb?getdoc+view1+itudoc+6004+2++CCITT%20T.6

A good description is at

http://www.advent.co.uk/ccitt.html

(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

http://www.igd.fhg.de/icib/telecom/ccitt

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

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

http://home.earthlink.net/~ritter/tiff

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

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
COI GmbH / Abt. KOS3    Industriestr. 1-3    D-91072 Herzogenaurach
Tel: +49 (9132) 82-3433                        Fax: +49-9132-824959