jtc@van-bc.wimsey.bc.ca (J.T. Conklin) (11/15/90)
In article <velasco.658597478@beowulf> 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
ricks@EE.MsState.Edu (Rick Schumeyer) (11/16/90)
>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. I believe there is an extension to the JPEG standard to allow lossless compression. I don't have the details in front of me, but I'm pretty sure that DPCM is used after the DCT if lossless is desired. -- Rick Schumeyer ricks@ee.msstate.edu Drawer EE, Mississippi State, MS, 39762. (601)325-3660
gregorya@cutmcvax.cutmcvax.cs.curtin.edu.au (Andrew Gregory KBS351) (04/24/91)
This is my first post, so here goes... A friend recently told me about a new graphics compression techinque called JPEG. My questions are: 1) What is it? 2) Is there source or an algorithm for it? 3) Is there a standard for it yet (like GIF)? My friend said it could do a lot better than GIF compression. Any information would be a great help. Cheers.
pete@saturn.ucsc.edu (Peter Hughes) (05/24/91)
I know this question has come up before, but it would be very useful to get any info available. Does anyone know what sort of public domain or commercial JPEG compressors/decompressors exist for unix boxes? I will gladly post a summary. pete@saturn.ucsc.edu