Re: [Gimp-user] A Sad case of regression

[Ofnuts <ofnuts laposte net>]

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.