On 10/23/2010 08:50 PM, Jay Smith wrote:
One caution that should be mentioned if the OP decides to use the Gimp option Frank mentioned to generate thumbnails when saving JPG files. If the JPG is opened in gimp and then saved (even though "no changes" are made), the JPG will suffer some amount of quality loss even if the highest quality level is selected when saving. I run into a _lot_ of people who are simply not aware that _every_
time a JPG is saved, the quality is reduced.

This is definitely not true...  Try this:

- load an image in Gimp
- save it as JPEG, using whatever settings you fancy (note; even if the file is alreay a JPEG, it's important to save it here...). Let's call it "original.jpg".
- close image
- reload "original.jpg"
- do a local edit on it (for instance, small circle selection plus bucket fill) - save it with same settings (Gimp will normally reuse the ones used in the loaded image) as "edited.jpg"
- close image
- reload original.jpg
- load edited.jpg as a new layer
- put the "edited" layer in difference mode. You'll only see the bit of the image you replaced by a circle, the rest is black. - to see how black is the black, flatten the image and open the histogram dialog. - select any part of the image outside the edited circle. Under the histogram, if you use a minimum value above 0, the count is 0, in other words, the difference between the original and edited picture is 0 everywhere, and there is no loss.

I have Perl a script that runs Imagemagick multiple times against the same image (a white dot is added each round)(*). Then it uses imagemacick's compare to general the diff image. after two or three iterations the only thing in the diff image is the white dot added even with very low JPEG quality settings (the difference on the first iterations is due to imagemagick not using the same settings as the original image, in particular a different color subsampling)(this is why it was important to save the image from Gimp the first time above).

This is because the Jpeg algorithm is stable, and the decoded pixels get re-encoded in the very same values if encoding settings aren't changed.

Things are obviously different after a global change to the picture (contrast, white balance, etc...)

(*) I won't post it here but interested people can drop me a mail. It was written for Windows but will likely run under Linux with minor changes.

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