Lee, Thanks, as usual for a great explanation, but just for your info. my eyes glaze over on most anything you say as you speak so far above my capabilities, but boy do I ever learn from you!!
John R. On Mar 28, 2005, at 10:54 AM, Lee Larson wrote: > On Mar 27, 2005, at 9:22 PM, Bill King puzzled: > >> I just read an interesting article posted on MacSurfer from the >> Syracuse Post-Standard newspaper. It concerned the ability of iPhoto >> 5 to repetitively save a picture using JPG compression and apparently >> losslessly. >> >> I would love to hear for any graphics compression experts about the >> authenticity of this technique... > > I'm not a graphics expert, but I think I can explain what's going on. > > The JPEG scheme is actually a whole collection of different > compression techniques all lumped into one standard. The most common > one chosen is what's called "discrete cosine transform" (DCT) > compression. Within the DCT algorithm, you can choose the amount of > information to be retained. (It's just the value of a constant in the > formula.) The more information that's retained, the larger is the size > of the compressed file. It's possible to choose lossless DCT > compression, at the expense of almost no compression for complicated > images. > > You can see this in programs like Canvas, which has a slider control > to select the quality of the output image. Sliding it over to 100% > results in a big file with no quality loss. > > The JPEG scheme also includes a lossless algorithm called entropy > encoding which I believe is less often used than the DCT. > > I don't know which scheme Apple uses, but It's probably one of those > two. > > As a postscript to this, let me note that I've had disagreements with > so-called experts about this. They claimed that JPEG is an inherently > lossy format while I countered that it need not be. Most "experts" > don't really understand the capabilities of JPEG because most programs > don't take advantage of JPEG's real capabilities. I bring up the DCT > and their eyes glaze over. | The next meeting of the Louisville Computer Society will | be March 22. The LCS Web page is <http://www.kymac.org>. | List posting address: <mailto:macgroup at erdos.math.louisville.edu> | List Web page: <http://erdos.math.louisville.edu/macgroup>
