Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!van-bc!jtc From: jtc@van-bc.wimsey.bc.ca (J.T. Conklin) Newsgroups: comp.graphics Subject: JPEG (was Re: Next machine as animation platform) Message-ID: <342@van-bc.wimsey.bc.ca> Date: 14 Nov 90 18:59:50 GMT References: > Organization: UniFax Communications Inc., Vancouver, B.C., Canada Lines: 37 Summary: In article velasco@beowulf.ucsd.edu (Gabriel Velasco) writes: >From: Kim_Orumchian@NeXT.COM >>Images that are compressed and played back by a >>NeXTdimension are not going to look as good as those produced frame by >>frame on a dedicated video production system. > >Why? This is not a flame. I am really wondering where the deficiency is. Is >it in the hardware? The software? The compression algorithm? The original >poster was talking about producing the sequences frame by frame. >I am not at all familiar with NeXTdimension's JPEG compressed images. What >type of algorithm do they use? Is it delta modulation? Run length encoding? >Send only changes from one frame to the next? Selective color mapping? In short, JPEG is a lossy compression algorithm. That means that some information is discarded when an image is encoded, and the reconstructed image is different than the source image. For a lot of images, "close" is good enough, so lossy compression is a win. The following is from Larry Press's "Compuvision or Teleputer" article in the September '90 CACM which describes JPEG in a bit more detail. The JPEG compression algorithm runs in two steps, a discreet cosine transform on a sliding 8x8 pixel window followed by Huffman encoding. The first stage discards some information, so reproductions are not exact. Color information is sampled at a lower rate than lumanance information (since color changes are more gradual in natural images than lumanance changes), and high-frequency color information, which the human eye is relatively insensitive, is discarded. --jtc -- J.T. Conklin UniFax Communications Inc. ...!{uunet,ubc-cs}!van-bc!jtc, jtc@wimsey.bc.ca