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Date: Fri, 09 May 1997 20:27:55 +0500
To: tiff@sgi.com
From: "Sanjay G. Prabhudesai" <sanjayp@gsslco.co.in>
Subject: To ALL TIFF MAILING LIST MEMBERS
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Hello Everybody,

Let me first apologise to all of you for not expressing myself clearly in
my mail I sent yesterday on "Group 4 compression and 256 Colors".

Prior to yesterday's mail, I had sent another mail seeking your expert
advise on suitability of using Group 4 compression for 256 colour images. I
got very educative responses from different people. Once again I thank all
of them for the help.

All the mails clearly indicated that it is not advisable to apply Group 4
compression for colour images as it could lead to expansion instead of
compression. I also recieved explanation for why it happens that way. All
the mails pointed to available alternatives. 

I have a customer who is bent on using Group 4 compression for reasons not
known to me. One of the responses I received a few days ago, indicated the
possibility of using the Group 4 compression with 256 colours, although it
would expand the image rather than compress. The following method was
suggested :

   Treat a 256 color palette image as a "bilevel" image       (multiplying
image width by 8 before applying group 4       compression) 

I was trying this approach. I wanted to figure out if TIFF files generated
in this manner could be understood by other readers available in the
market. I hoped to gain more insight, from this experiment, on the basis of
which I could further disscuss this matter with the customer.

While trying this approach using libTIFF writer I got the following error
message:

 "Bits per sample must be 1 for Group 3/4 encoding/decoding".

I could not figure out how to overcome this problem. Hence I posted another
mail. Unfortunately I did not express myself clearly in that mail.

Once again, I sincerely apologise for not expressing myself clearly and the
inconvenience caused to all of you. I thank you very much for all the
responses and being patient with me.

Regards,

Sanjay Prabhudesai



