Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!cis.ohio-state.edu!ucbvax!agate!stanford.edu!neon.Stanford.EDU!minakami From: minakami@neon.Stanford.EDU (Michael K. Minakami) Newsgroups: comp.compression Subject: Re: JPEG compression errors ?! Message-ID: <1991Jun5.033410.27129@neon.Stanford.EDU> Date: 5 Jun 91 03:34:10 GMT References: Organization: Computer Science Department, Stanford University, Ca , USA Lines: 37 In article d88-jwa@byse.nada.kth.se (Jon W{tte) writes: >.ipmce.su (Leonid A. Broukhis) writes: > > I have some problems when converting GIF files to JPEG format > (using Image Alchemy 1.4 under MS-DOS). > >Why can't people get it; conversion of GIF to JPEG is a really >stupid thing to do. JPEG works better (gives smaller output with >better output) if applied to the original 24bit RGB source than >if applied to the already-quantified 8-bit GIF images. This is >because of "unnatural" noise and high frequency generated by the >quantification process. Perhaps it has never occurred to anyone that people do not have the original >8-bit images. There is also no guarantee that the original would be >8 bit. Leonid's criticism would perhaps be better if he applied it to dithered gif images; these do generally contain high-frequncy data that would not be preserved in JPEG conversion. However my experience has been that at high quality levels, even dithered images maintain are not distorted much. Second, Image Alchemy, at least in the early versions, had a problem with conversion with high qualities. The resulting image would appear blocky; definitely not of high quality. I'm not sure if this has been fixed in subsequent releases. A final qustion: Has anyone tried going from 8-bit gif -> jpeg -> 24-bit gif? Specifically, is the 24-bit gif of higher quality than the 8-bit gif given that jpeg "extrapolates" a 24-bit representation from the 8-bit representation? --Mike -- ------------------------------------------------------------------------------- Michael Minakami | These thoughts are my own, not that anyone's Computer Science/Psychology | tried claiming them. Stanford University |