Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uwm.edu!linac!mp.cs.niu.edu!ux1.cso.uiuc.edu!uicbert.eecs.uic.edu!eddins From: eddins@uicbert.eecs.uic.edu (Steve Eddins) Newsgroups: comp.compression Subject: Re: JPEG compression errors ?! Message-ID: <1991Jun11.144958.4839@uicbert.eecs.uic.edu> Date: 11 Jun 91 14:49:58 GMT References: <1991Jun4.223719.2958@qualcomm.com> <4022@sixhub.UUCP> <1991Jun11.021223.29209@nntp-server.caltech.edu> Organization: EECS Dept., University of Illinois at Chicago Lines: 27 madler@nntp-server.caltech.edu (Mark Adler) writes: >The talk about JPEG being acceptable or not seems to imply that there >is a such a thing as "the" JPEG. There isn't. You can set the >compression level (roughly) by varying the quantization parameters >from no quantization (lossless decompression, up to round-off errors >in the DCT), to a great deal of quantization, the latter resulting >it really awful looking, blocky decompression (at about the 30:1 to >50:1 level). Setting these parameters is still rather a black art, >but up to a point, you have control over the resulting quality of the >decompressed images. >Mark Adler >madler@tybalt.caltech.edu A good summary, to which I would add the following comment: if lossless compression is what you want, the JPEG draft standard includes a lossless method based on DPCM and entropy coding (Huffman or arithmetic). This part of the standard does *not* use the DCT and incurs no round-off errors. According to the draft, you can expect 2:1 compression with the lossless coder for "typical" scenes (whatever that means). -- Steve Eddins eddins@brazil.eecs.uic.edu (312) 996-5771 FAX: (312) 413-0024 University of Illinois at Chicago, EECS Dept., M/C 154, 1120 SEO Bldg, Box 4348, Chicago, IL 60680