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. -------------- next part -------------- A non-text attachment was scrubbed... Name: smime.p7s Type: application/pkcs7-signature Size: 2363 bytes Desc: not available Url : http://www.math.louisville.edu/pipermail/macgroup/attachments/20050328/f165d167/attachment.bin
