Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!decwrl!netcomsv!marcos
From: marcos@netcom.COM (Marcos H. Woehrmann)
Newsgroups: comp.compression
Subject: Re: JPEG compression
Keywords: JPEG image compression query
Message-ID: <1991May30.005920.24914@netcom.COM>
Date: 30 May 91 00:59:20 GMT
References: <1991May26.231855.20247@athena.mit.edu>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 21

>
> An interesting observation about JPEG (or at least the particular implementation of it which I was using) was that it is the only compression algorithm I know of where decompression takes more CPU time than the compression did.  This may have been a fluke> , but it was true for all images I tried and the difference was a factor of two and not just a few percent.
> 

The reason that the decompression times are worse then the compression times
is that to convert from a JPEG image to a GIF image you have to do not
only the DCT and other JPEG stuff, but also colour quantization and
dithering (this is because JPEG is a true colour format and gif is a paletted
format).  If you convert from true colour -> jpeg -> true colour, the
conversion is virtually symmetric.

marcos

-- 
Marcos H. Woehrmann    {claris|apple}!netcom!marcos  |  marcos@netcom.COM

 Oh, I'm sure you've heard it all before, but remember it's not what you
 say, it's how you say it, and how much you're paid to do so, and besides
 who's listening anyway.  No one, that's who, because it's all been said
 and done and done and done and done to death.  Let's talk about art,
 said the fool to the idiot.  --Lydia Lunch