JPG Lossless Transformations

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JPG Lossless Transformations

Post by idiamond »

Why does the JPG Lossless Transformation result in a file size that is considerably smaller (5-7%) in size? What's not in the new file that was in the old?
Posts: 124
Joined: Wed Oct 15, 2003 2:51 am

Re: JPEG lossless convert changes file-size

Post by crazytb »

Read this message. It has been posted some time ago, in "real" newsgroup.

From: "Ilya Razmanov"
Newsgroups: xnview.english
Subject: Re: JPEG lossless convert changes file-size
Date: Thu, 7 Aug 2003 16:08:36 +0400
Organization: !

"Markus Nerf" wrote in message news:bgh8jb$5t4$
> Hello xnview-fan's!
> If I rotate my pictures with the "JPEG lossless convert" the size of the
> files changes. In fact the files-size is lower. In my opinion this isn't a
> lossless operation. What happens?
> Thanks!
> Markus N.

First, lossless compression may eliminate some pixels at the edge that does
not form the "full" JPEG block, i.e., crop the image by a couple of pixels.
Second, the blocks may be more or less "compressible" in the other
direction - I'm too lazy to get into more details; let's just say that the
"lossy" part of transform would remain the same (i.e. no additional losses
appear) yet the "lossless" part of coding may change.