Shel Belinkoff mused: > > We had a discussion about this some time ago, and a couple of people (John > Francis comes to mind as one) suggested that this is not always the case, > and that degradation of image quality takes place only in certain > circumstances. My recollection is that if a JPEG has not been changed, but > just opened for vieweing and then saved, there is no image degradation.
Not true, in general. I may have been pointing out that the JPEG specification allowed for the theoretical possibility of rotations (in units of 90 degrees) without the necessity of re-quantising the image. But most image editing applications will re-save (using their own set of favourite parameters, which might well not match those chosen by whatever created the image originally), and introduce new degradations. Opening a JPEG file, and the saving it to a new destination file, is a really bad way to copy image files around :-( > I also seem to recall that if only a small area has been changed, such as > making an adjustment to one feature in the image, only that portion which > has been adjusted suffers. This seems like a good time to clear this up ... Again, not true in general. If making no changes introduces error, then just making changes to a small area in the image is unlikely to be able to do better. > Not all image viewers will "unpack" a JPEG when viewed. Photoshop does (at > least recent versions of it) but, for example, Irfan doesn't. Yes it does - if it didn't, you wouldn't be able to view the image. What Photoshop is doing (that Irfan apparently does not do) is telling you the size of the in-memory copy of the image. No image application changes the on-disk version of the file (which I what I guess you are thinking of when you talk about unpacking a JPEG file). If it were necessary to modify the file, you'd have serious difficulties trying to view files stored on read-only media such as a CD! > > Shel > > > > [Original Message] > > From: Steve Jolly <[EMAIL PROTECTED]> > > > > Yes, with the important difference that the jpeg packing process > > degrades the image quality each time you save it. The advantage of this > > is that it makes the file sizes much smaller than a "lossless" > > compression format such as compressed TIF. > > > > S > > > > Jens Bladt wrote: > > > So, a Jpeg is like a zip file, packing itself when saved, unpacking when > > > opended? > >
