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Old Nov 10, 2005, 3:50 PM   #1
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I've read a couple of forum posts that say opening and closing a JPEG file can cause additional lossy compression. This is without re-saving, just opening and closing. Can anyone confirm if this is true? And if it is, is it software related? For example, do some applications handle this better than others? Thanks.
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Old Nov 10, 2005, 6:24 PM   #2
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What I've heard is pretty much that JPEG degradation only occurs if you resave the image. Just opening and closing without modifying the image and resaving has no effect.
If someone has good evidence to the contrary, I'd be interested in seeing it, as there does seem to be some controversy.
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Old Nov 10, 2005, 7:56 PM   #3
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You are correct. If you just open and close the file, there is no loss. If you resave the file, there is further loss. You can try it for yourself. Open a picture file and then save it to another file name. compare the file sizes and you'll see the second file is smaller in megabytes because it has been further compressed.
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Old Nov 10, 2005, 7:59 PM   #4
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If you just open and close the file, you make no changes to it. With no changes, there cannot be degredation.

Of course it is possible that some viewing software does change the file (should be counted as malware if it does). Make a note of the size anddate/time before and after as a check if you are worried about that possibility.
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Old Nov 14, 2005, 2:21 PM   #5
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Thanks for your replies. That's what I always assumed but after reading two posts on other forums, I thought I'd ask.

Got another question. If you re-save a medium quality JPEG as high quality, the file size increases. If I understand this right, you can't add any detail that wasn't there in the first place. So what is being added to drive the file size up?
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Old Nov 14, 2005, 7:31 PM   #6
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B1ue wrote:
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Got another question. If you re-save a medium quality JPEG as high quality, the file size increases. If I understand this right, you can't add any detail that wasn't there in the first place. So what is being added to drive the file size up?
Nothing is being added. You are just adding fewer compression artifacts to the resaved image by saving at a higher quality. Compression artifacts from the first compression are often enhanced with further compression. Your best bet is to save in an uncompressed format like TIFF, but if you must save in JPG use the highest quality. Multiple lower quality saves really mess up an image.

Someone said on one of the forums that he could take a SHQ JPG from his camera and recompress it 20 times at Photoshop quality 12 and you still wouldn't pick up artifacts because of the low initial compression from his camera. I gave that a hearty "bull" and tried it. I have a camera that puts out a very large (high quality) JPG file for the image size. I was very careful to close out the previous image and open the saved one before saving again at quality 12. After 20 saves the image was hardly different from the original.

If your camera saves at a fairly high quality and you resave at the highest JPG quality it appears there isn't much damage to the image. Had I done the saves even at quality 8 the image would have been a disaster after 20 saves.

Keep in mind that if you hit "Save" 20 times while working on an image you really have only one compression – the last. Each time you are just overwriting the previously saved image. If you want to try multiple saves to see what deterioration you get at different compression levels be sure to close out the image each time and reopen the one you just recompressed.


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