I believe you are not correct, here. I have read in several accounts, both from people who have tried this experimentally and from people who understand the theory of JPEG compression far better than I do, that opening a JPEG, and then resaving it without making any changes to the file, at the same compression ratio as it was originally saved, does not further degrade the image. The degradation happens when the file is first saved as a JPEG, and further degradation is likely to happen if you make changes to the contents of the file and then resave it, but it won't degrade if you merely open the file, make no changes, resave it, open it again, and resave it making no changes, no matter how many times you repeat this process. (Though why you would want to open a JPEG and then resave it unchanged, at the same compression ratio, rather than simply closing it, is an open question.)
- David -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Robert Meier Sent: Friday, February 08, 2002 9:31 PM To: [EMAIL PROTECTED] Subject: [filmscanners] Re: JPEG Lossless mirror? --- "Pat Cullinan, jr." <[EMAIL PROTECTED]> wrote: > I had been a believer in the proposition that multiple jpeg saves > would > degrade an image, but after reading a notice to the contrary in one > of the > trade mags, I did my own trials and now I save and resave jpegs which > aren't even maximum quality without any qualms. The trade magazine is wrong at least for the following common scenario. If you save a picture as jpeg in PS, close the image, reload the image and save the image again in jpeg you will lose data. The difference noise like and very small. For a normal picture you won't see any difference. Also it might be the additional loss gets smaller and smaller with many additional savings (without editing) upto a point where there is no change anymore. I have no mathematical proof for that, though. Now if you start with an image in PS, edit it, save it, edit it, save it, etc. you are not losing any data. The reason is that PS only writes the compressed image to a file but keeps the uncompressed image in memory. So it does not compress it and then reload the compressed image back into memory. In the later case you would lose data with each save and it would be awfully slow. One thing I wonder is if it is possible to do a lossless flipping of an image that has not a multiple of 8 pixels in the direction you flip it. Does anybody know about that? Robert __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com ---------------------------------------------------------------------------- ------------ Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body ---------------------------------------------------------------------------------------- Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
