Message-Id: <mailto:199408270550.AAA17857@library.wustl.edu> Date: Fri, 26 Aug 1994 15:08:21 -0700 From: Robert MacKimmie <mailto:rm@CALIFHISTSOC.ORG> Subject: Re: JPEG degradation!!! (lossy/lossless---One of the big and To: Multiple recipients of list IMAGELIB <mailto:IMAGELIB@ARIZVM1.BITNET>
Date: Fri, 26 Aug 1994 09:39:05 -0700 Joshua Yeidel, mailto:yeidel@tomar.accs.wsu.edu states:One further note on JPEG compression: >>>decompressing an image, modifying it, then recompressing it, then
decompressing (to display) sometimes produces artifacts (ghosts, odd spots) in an image.>>>Therefore, we always keep a copy of the never-compressed scan on
tape, in case we need a reliably-modifiable image.Welcome to JPEG!!!!! Even SOME JPEG compression will degrade the image. I was fooled into thinking JPEG didn't impact the quality, then when backing up the files I had to compress the contents of the hard drive and the JPEG images weren't the same afterwards.
Hence, whenever I am comparing the "lossy" effects of JPEG, I always compress, then decompress the image, in order to see the real impact on the quality. I had an intern scan nearly 500 images saving them in JPEG, only to find the impact and degradation of JPEG after the scan project was completed. That is why those of us actually a year or so into this digitizing business should be vocal about the fine points!!!! There can be major "mission critical" errors in methodology because of things that don't seem apparent at the outset.
Collaboration is the name of the game in the ninties...
Watch out for JPEG---Strongly consider "lossless" compression.
Joshua, thanks for your note, and to all others, sorry about the bandwidth, my assumption is that this kind of information will save someone alot of heartache and possibly preserve the stability of their professional position by avoiding the elephant traps.
Robert MacKimmie California Historical Society, San Francisco. mailto:rm@califhistsoc.org