Message-Id: <mailto:199408270627.BAA18766@library.wustl.edu> Date: Fri, 26 Aug 1994 09:39:05 -0700 From: Joshua Yeidel <mailto:yeidel@TOMAR.ACCS.WSU.EDU> Subject: Re: What is JPEG?? (lossy/lossless---vital issues.) Knowing To: Multiple recipients of list IMAGELIB <mailto:IMAGELIB@ARIZVM1.BITNET>
One further note on JPEG compression:We are storing (*not* archiving for 500 years :-) images using "very good" (qulaity˙0) or "excellent" (quality 3.99) JPEG compression (via Macintosh Quicktime). We get an average of 5:1 compression on our images. I have to blow them up to 4x *and* spend some time carefully inspecting to detect any differences (though perhaps I am not as perceptive as some folks).
However, we have found that decompressing an image, modifying it, then recompressing it, then decompressing (to display) sometimes produces artifacts (ghosts, odd spots) in an image. This is especially true of images with hard, artificial edges (e.g., type). Modifications that produce artifacts include simple cropping. Therefore, we always keep a copy of the never-compressed scan on tape, in case we need a reliably-modifiable image.
Joshua Yeidel, Learning Systems | mailto:yeidel@tomar.accs.wsu.edu Information Technology | All standard disclaimers apply Washington State University | "Believe it if you need it, Pullman, WA 99164-1222 | or leave it if you dare..." 509/335-0441 | -- Robert Hunter