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Date: Thu, 8 May 97 14:06 PDT
From: mccoy@pixar.com (Daniel McCoy)
To: tiff@sgi.com, sanjayp@gsslco.co.in
Subject: Re:  CCITT Group 4 compression and 256 Colors
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Look, 
everybody on the list is telling you that trying to compress
a color image with Group 4 compression is a BAD idea.

The software is kind enough to tell you that it's a BAD idea.

There are plenty of other compression schemes available for
color images.  I, too, would suggest you use one.

But if you are hell bent on compressing your 8-bit image with Group 4
you have to either convert it to a black and white image or
tell the library that it is eight times wider and set bitspersample to 1.

Sheesh!   Enough already.

Dan McCoy

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

