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.
It would be possible to do that in theory, but the software responsible for saving the new JPEG would have to have access to the lossy compression decisions that were made when the old JPEG was created. The BBC has developed a way of doing this for MPEG streams (http://www.broadcastpapers.com/sigdis/Snell&WilcoxMoleTechnology03.htm), but AFAIK there's nothing similar available for JPEG. Such a technology would inevitably mean larger file sizes, and a new (probably incompatible) file format.
John Francis wrote:
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.
Recent versions of Paint Shop Pro support lossless JPEG rotatation, FWIW.
S
