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TheWriter
12-31-2003, 03:54 AM
When saving a jpeg back to a jpeg format, if I keep the quality on the highest, I assume there will still be some quality degradation.

Seems to be a few arugements on this matter, would like to see some technical input. Thanks.

halo
12-31-2003, 05:58 PM
ok...heres the latest which has been debated at adobe's forums.

If you open a jpeg and make no changes and keep it at the same compression when it was opened it wont degrade the image.
If you change anything then that new area will be recompressed, if you drop the quality then you will see recompression.

Bear in mind that even colour changes will be considered a change, as will crops and resizes, so to be safe, keep an uncompressed copy :)

EricChadwick
01-02-2004, 03:35 PM
I would test it with a recursive Action, see what happens to the image when you save it overtop itself again and again, say 30 times.

You could easily add some more variables to the action, like a single type of change (i.e. rotate canvas) then resave.

In the image browser XNView, there's a command for Lossless Jpeg Conversions... which don't affect image quality. You can mirror and/or rotate. Perhaps Photoshop supports the same?

dg
01-03-2004, 10:26 AM
Hi,

That is a debate without end, some people says that a lossless resave in Jpeg is possible but I don't know about that, maybe with simples transforms like rotate canvas or something, but I think that for more intensive alterations, like CC and stuff you will have a recompression over your already compressed format leading to some degradation in your file, so I agree with halo :)

For longer intensive works use a uncompressed format like PSD, TIFF or TGA and just save as JPEG to do your output.

See ya!

Arctos
01-12-2004, 04:54 AM
I have taken the time to do some tests a while back. (using pshop 6 i think)
with JPG quality set to 12, i saved and resaved a few different JPGs 25 or 30 times. Then i compared the first and last.
You can do a comparison with photopshop:

drag the first and last generation into the same pshop file. (hold down shift to keep them both centered over each other) Then set the blending mode of whatever layer is on top to "difference."

You will most likely get a nearly black image. With difference mode, any pure black pixels are the same between both layers. White pixels are opposite colors.
So the darker the color the smaller the change between the two layers.

You may have to throw a levels adjustment layer on top to exagerate the contrast to be able to see the "different" pixels.

After doing this i concluded the max quality JPG was perfectly safe for files that i was done editing.

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