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May 1997

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1997.05.08 13:50 "CCITT Group 4 compression and 256 Colors", by Sanjay G Prabhudesai
1997.05.08 21:05 "Re: CCITT Group 4 compression and 256 Colors", by Gregory W Cook
1997.05.08 21:06 "Re: CCITT Group 4 compression and 256 Colors", by Daniel Mccoy

1997.05.08 21:05 "Re: CCITT Group 4 compression and 256 Colors", by Gregory W Cook

> Hello,
> 
> It's me again. I am stuck with a problem. In my previous mails, I had
> enquired about this problem and I had got quite a few responses. 
> However, I am facing a problem right now and I am in need of the solution
> urgently.
> 
> The problem is that when libtiff is used, and the image being created uses
> Group 4 compression, then it does not convert to TIFF file. It gives a
> message 
> "For Group 3/4 compression, bits per sample must be 1".
> However, bits/samples is 4/8 for color images as provided by the TIFF 6.0
> specs. Also, specs says that the Group 4 compression can be used only for
> bilevel, and not for the color images. In this situation, can I compress
> the image using Group 4 compression ? 

I've seen at least 4 TIFF mailing list replies to this question, all correct.
Perhaps there is a language barrier here, but the answer is still,
and always has been:

NO.

Aside from the fact that virtually no other TIFF reader would successfully
decompress it, the compression you get is horribly inefficient.

> Also, is there any way out on this perticular situation?

One more time:

Use either LZW or Deflate (zlib) compression.  The best results use a 
differencing filter before the compression. 

> Also, one of the responses received by me last time suggested that this is
> possible, but the compressed image size is very large compared to the
> original image size. But I feel that is NOT the better alternative. 
> 
> Please extend your help in this situation.
> 
> regards,
> 
> Sanjay Prabhudesai
> 

What is unclear here is why the images are compressed using Group 4 to
begin with.  The best solution would be to stop using Group 4 compression,
since it won't be very efficient anyway.  If for some reason you can't do
that, you may have to modify libtiff source code for your particular 
images.  Whatever you do, *don't* use a color image/G4 combination and
call it TIFF.

Greg Cook, gcook@ecn.purdue.edu
Video and Image Processing Laboratory (VIPER)
Purdue University
West Lafayette, Indiana, USA