I've noticed a loss in quality when I import digital photos from my camera and view/save in Preview. The files are smaller than the original, and there is a slight, albeit visually noticable, loss in quality. Is this part of the same issue? CCG
> ---------- > From: owner-macgroup at erdos.math.louisville.edu on behalf of Jerry > Yeager > Reply To: macgroup at erdos.math.louisville.edu > Sent: Monday, March 28, 2005 4:06 PM > To: macgroup at erdos.math.louisville.edu > Subject: Re: MacGroup: lossless JPEG??? [bcc][faked-from] > > There has been a lot of talk lately about improvements in the jpeg > compression ideas being used (Allume -- makers of StuffIt -- has some > new stuff out that is supposed to shrink jpeg files up to 30% without > any loss of quality) and a lot of speculation about how Apple is > somehow reading the scheme used to compress the original imported jpeg, > then using the exact same scheme to compress the newly saved image > (this is supposed to maintain quality longer, through more levels of > edits --- how Apple does this is also supposed to be a closely guarded > secret -- at least according to the tin-hat-wearing folks, a bit > doubtful since if Apple can get this info out of an arbitrary jpeg > image, so can anyone else.) > > Not being a big iPhoto user (Go GIMP! Go PhotoShop!), I have only > followed those discussions out of idle curiosity. > > But in terms of how iPhoto does do some of its wonders, you need to be > careful here. Apple cheats. (Reeeeaaalllllyyyy big toothy grin!!!!!) > If you import a jpeg photo, make changes, quit, come back make changes, > quit et.c etc. the saved photo looks really good. Okay so far. But > there is a menu option that says "Revert to Original". Choosing this > takes you all the way back to the original imported image. So what > Apple is doing is keeping a history of your editing for the photo and > applying that to the original and showing you that. (Also if you take a > gander through the iPhoto library you will see lots of data files > popping up for your albums, especially after you start editing). > > On the other hand, if you edit a photo, then Export that, then import > it back in, make changes, export it and import it again, etc. then you > will begin to see image quality degradation within a few generations > (using the jpeg format) that do not show up for the same edits on the > one that stays inside of iPhoto the whole time. > > One thing that is creeping into the lexicon is the idea of what > "lossless" means. With these new attempts to make jpeg last longer > (instead of switching to the better jpeg2000 -- for jpeg compression) > there is a lot of talk of thing being lossless in image quality, but > not being a bit-for-bit identical copy. Hmm, it does sound like the > words are going to be altered here soon. > > Shoot RAW NEFs, it's just better (smile). > > Jerry > > > > On Mar 28, 2005, at 11:00 AM, John Robinson wrote: > > > 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> > > > > > ----------------------------------- > Someday, I will come up with a clever signature line. I am not sure if > I will use it or not, but I will come up with one. > > > > | 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> > > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.math.louisville.edu/pipermail/macgroup/attachments/20050328/84d472d7/attachment.html
