On 06/15/2013 07:53 PM, Grue wrote:
Wow, just wow. Here are the facts: every time you "save" your image as JPEG, you lose information. It is by design a lossy image format that uses an algorithm to conserve your disk space via throwing away some "insignificant" information (which works well for photos, but ruins many other types of images). Worse yet, if you edit a JPEG image and resave it, you lose even more information. This results in very noticeable artifacts in the image. And GIMP actually tries to prevent you from this destructive workflow, yet you keep doing it anyway, and you're complaining about GIMP instead of your own ineptitude. Please, if you work with images, learn about image formats and how they work. The eyes of people who look at your images will thank you later.

Well, not really... This is what everyone thinks/is told but in practice, if you only do local editing and save the image back with the exact same JPEG quality settings, the "blocks" which no changed pixels very quickly end up producing the very same data as their source in the JPEG file in every editing cycle, so the image as a whole doesn't degrade after the two or three first editing sessions. I have a script somewhere that repeatedly edits and saves an image with ImageMagick to demonstrate this, This is is even used in image forensics: to find the edited spots in a JPEG image you have been given, save it again, then compare the two images. Chances are that the encoding of the edited parts hasn't "settled down" yet and will produce slightly different values when saved, so the two images will show minute differences in the edited places.
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