dolf@fwi.uva.nl (Dolf Starreveld) (05/07/90)
twhlai@watdragon.waterloo.edu (Tony Lai) writes: > The May 1990 issue of Byte mentions the proposed Joint Photographic Experts > Group (JPEG) standard compression algorithm, which Byte says is capable of > achieving a 10:1 compression ratio on video images. Does anyone know > anything about how the algorithm works? Yes: The algorithm takes as input a 3-component color space. Usually this is YUV, but the algorithm does not specify this. Next a DCT (Discrete Cosine Transform), something similar to a Fourier transform, is performed on each 8x8 block of the each component. This transforms the information contained in the image to the color spatial frequency domain. In the next step, the coefficients so obtained are quantized. This is done using the formula: Quantized Coeff = (Coeff +/- Q/2) / Q In this formula, Q is a frequency dependetn quantization value, choosen with the properties of the human eye in mind. (Things are not so simple as it sounds, but this will have to do). This reduces the dynamic ranges of coefficients, but throws away information. In the third step the 64 quantized coefficients are order accoding to increasing spatial frequenty. Hopefully the properties of the image and the quantization have introduced long runs of zeros in this ordering. This and the reduced dynamic range of the coefficients is used to perform a modified Huffman coding. This is the compression algorithm described in simpliefied form. Decompression performs basically all steps in reverse. Compression ratios of 1:10 can be achieved without visible loss of quality. This is very much dependent on the image itself however. Some images with large planes of equal colors can get much higher compression ratios with the same imge quality. As an example: At Storm Technology we have an image scanned in from a slide. It is a 32-bit quickdraw image. The original is 4.5 Mb, the compressed file is just 115K. There is almost no visible quality loss. For more information contact: Storm Technology 220 California Avenue Suite 101 Palo Alto, CA 94306 Ph: (415) 322-0506 Fax: (415) 494-2760 --dolf -- Dolf Starreveld Phone: +31 20 592 5056/5022 (FAX: 5155), TELEX: 10262 HEF NL EMAIL: dolf@fwi.uva.nl (dolf%fwi.uva.nl@hp4nl.nluug.nl) SNAIL: Dept. of Math. and Computing Science, University of Amsterdam, Kruislaan 409, NL-1098 SJ Amsterdam, The Netherlands