jjoshua@remus.rutgers.edu (J. Joshua) (12/05/90)
I am looking for a compresion algorithm that has a good trade off between speed and compression % leaning more towards speed. I would like the algorith to work in as real-time as possible. Any suggestions? Any code? Thanks, JOn. -- ________ | | This Messsage A Service By: | | Closed Captioned Jon Joshua | | For the Hearing Impaired jjoshua@remus.rutgers.edu `----. .-' |/ #include <whittyComment.h>
jtc@van-bc.wimsey.bc.ca (J.T. Conklin) (12/05/90)
In article <Dec.4.19.32.13.1990.16026@remus.rutgers.edu> jjoshua@remus.rutgers.edu (J. Joshua) writes: >I am looking for a compresion algorithm that has a good trade off >between speed and compression % leaning more towards speed. I would >like the algorith to work in as real-time as possible. Any >suggestions? Any code? This is a classic example of an incomplete specification. To satisifactorily answer this question, the nature of the data to be compressed, acceptable time/space tradeoffs, etc. is unknown. It would be irresponsible for anyone to suggest a compression algorithm when there is nothing to go on. --jtc -- J.T. Conklin Toolsmith, Language Lawyer UniFax Communications Inc. ...!{uunet,ubc-cs}!van-bc!jtc, jtc@wimsey.bc.ca
jjoshua@remus.rutgers.edu (J. Joshua) (12/05/90)
xIn article <774@van-bc.wimsey.bc.ca> jtc@van-bc.wimsey.bc.ca (J.T. Conklin) writes: > It would be irresponsible for anyone to suggest a compression > algorithm when there is nothing to go on. > > --jtc if it help you to know... i want to compress a byte stream as it goes from a hard disk to a floppy disk. ie a backup program. this will run on a macintosh and i would like to acheive a good compression ratio without too much cost in time. now can you suggest an algorithm? JOn. -- ________ | | This Messsage A Service By: | | Closed Captioned Jon Joshua | | For the Hearing Impaired jjoshua@remus.rutgers.edu `----. .-' |/ #include <whittyComment.h>
jtc@van-bc.wimsey.bc.ca (J.T. Conklin) (12/06/90)
In article <Dec.5.00.30.55.1990.3826@remus.rutgers.edu> jjoshua@remus.rutgers.edu (J. Joshua) writes: >if it help you to know... i want to compress a byte stream as it goes >from a hard disk to a floppy disk. ie a backup program. this will >run on a macintosh and i would like to acheive a good compression >ratio without too much cost in time. > >now can you suggest an algorithm? I usually recommend against compressing backups, as it is almost impossible to recover anything if even a single bit is changed. For your application, it will be imporatant to design in error recovery. So you probably don't want to compress all of the files together in one stream, but rather compress each file individually. This way, a damaged archive will only effect the one file where the damage occurs. Storing files individually has the added advantage that each file can be compressed using a different encoding method, as no one method is approprate for every different file type. You might want to consider adding error detection and correction codes to the compressed data. This will expand the data a bit, but when combined with compression it should be a net gain. I'd start by supporting two compression methods: no compression at all, and LZW encoding. LZW is a good compression method, especially for text and uncompressed bilevel graphic images. You want a "no compression" compression methods for the files that exibit "negative compression". i.e. they get bigger when compressed. For a sample LZW implementation, look at the compress source code. It should be availiable on your machine, as compress is used by the news software to compress news batches. --jtc -- J.T. Conklin Toolsmith, Language Lawyer UniFax Communications Inc. ...!{uunet,ubc-cs}!van-bc!jtc, jtc@wimsey.bc.ca
lalibert@bcarh188.bnr.ca (Luc Laliberte) (12/07/90)
I will agree that LZW is an excellent compression algorithm. The original article by Welch is in the June 1984 issue of IEEE Computer. However, the algorithm was later patented by Unisys (#4,558,302) because he did the work on company time. Check recent issues of Dr. Dobbs Journal for a discussion of this topic. Eric Ball