1997.05.09 15:27 "To ALL TIFF MAILING LIST MEMBERS", by Sanjay G. Prabhudesai
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.