Message-Id: <mailto:199607072319.SAA13181@library.wustl.edu> Date: Sun, 7 Jul 1996 18:29:32 -0500 From: Kurt Foss <mailto:kfoss@JOURNALISM.WISC.EDU> Subject: Re: Lossy compression files larger? To: Multiple recipients of list IMAGELIB
I suspect potential changes in file size would be the lesser concern over increasingly degraded image quality with repeated JPEG compression *if* the level of compression or other content-impacting factors were successively changed. Repeated saving of JPEG files at the original compression ratio seem to have no impact on size or image degradation from the occasional tests I've seen done by Storm Technology folks a few years ago. What would be the reason for successive compressions in the situations you're postulating? Seems it's always good to have a non-JPEGed original for situations where varying compression ratios might be required.rgds ~ Kurt
---At 3:33 PM -0500 7/7/96, Paula Berinstein wrote:
> A couple of months ago I asked if anyone knew why image files compressed > repeatedly with lossy compression methods will eventually get larger > rather than smaller. I think I may have found the answer, which I'd like > to share. > > In their write-up of the PCX format in *Encyclopedia of Graphics File > Formats, Second Edition,* James D. Murray and William vanRyper state, "It > is possible for an image to be so complex that the PCX compression scheme > actually causes the data to increase in size after compression." (page 663) > > I suspect that this is what I had heard about JPEG as well, though I > still remember something about repeated compressions.
-- <!-- MARK YOUR CALENDARS --> 8th NPPA Electronic Photojournalism Workshop Sept. 27 - Oct. 4, 1996, San Antonio, TX For latest news: Send a message to <mailto:epw8info@journalism.wisc.edu> *To register, contact Cindi Christie, co-chair, at <mailto:cindic@ccnet.com> >--- KURT FOSS <! -- ~k --> U_of Wisconsin-Madison * School of Journalism and Mass Comm. 5020 Vilas Communication Hall * 821 Univ. Ave. * Madison, WI 53706 Email: mailto:kfoss@journalism.wisc.edu * kfoss@emrg.com * CIS: 70541,1040 Phone: W 608.263.3391 or .4080 * FAX 262.1361 * H 271.1210 mailto:ONline@UW-Madison <http://www.journalism.wisc.edu/> Technology Editor, EPW8 <http://sunsite.unc.edu/nppa/epw8home.html> <!-- # -->