Comments on: About me http://shuttersounds.thedailynathan.com Photographic adoxography at its finest Tue, 30 Aug 2011 16:45:28 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: Nathan Yan http://shuttersounds.thedailynathan.com/about/comment-page-1/#comment-25906 Nathan Yan Tue, 30 Aug 2011 16:45:28 +0000 http://nyan.photodecal.org/photoblog/?page_id=2#comment-25906 Eric, By definition, JPEG is a lossy compression scheme, meaning it decreases the file size by throwing away the data. This is as opposed to lossless compression schemes such as ZIP, which typically do something like reducing redundantly repeated data to shrink the file size. Because JPEG is going to throw away data whenever you save, it *will* decrease in quality (as measured by pixel value differences from the original). You've noticed in your own tests, since the brightness values in your subsequent images have changed from the original. For the most part, however, image degradation at Photoshop's JPEG 12 setting is negligible - you could save over an image 20 times and you will still have something that more or less matches the original, certainly to the naked eye (as opposed to mathematically calculating the differences). Eric,

By definition, JPEG is a lossy compression scheme, meaning it decreases the file size by throwing away the data. This is as opposed to lossless compression schemes such as ZIP, which typically do something like reducing redundantly repeated data to shrink the file size. Because JPEG is going to throw away data whenever you save, it *will* decrease in quality (as measured by pixel value differences from the original). You’ve noticed in your own tests, since the brightness values in your subsequent images have changed from the original.

For the most part, however, image degradation at Photoshop’s JPEG 12 setting is negligible – you could save over an image 20 times and you will still have something that more or less matches the original, certainly to the naked eye (as opposed to mathematically calculating the differences).

]]> By: Eric Westling http://shuttersounds.thedailynathan.com/about/comment-page-1/#comment-25904 Eric Westling Tue, 30 Aug 2011 15:58:46 +0000 http://nyan.photodecal.org/photoblog/?page_id=2#comment-25904 Nathan, I noticed your comment on PetaPixel, "Saving JPEG's" (last year). I am looking for a definitive source to back up my contention that… "Saving JPEGs at J12 does not re-compress" [using PS-E 2.0, it is FAST!] After the initial enhancement treatment, and saving, (my version "B") I have loaded and saved 5 or 6 times (simulating many changes of mind?!) And have only been able to detect a very slight loss of luminance, which is easily restored with 2 or 3 points on the brightness slider. Does my (above) contention stand? Sources, references?? Eric Westling Houston, TX ----------------------------------- Nathan,
I noticed your comment on PetaPixel, “Saving JPEG’s” (last year).
I am looking for a definitive source to back up my contention that…
“Saving JPEGs at J12 does not re-compress” [using PS-E 2.0, it is FAST!]
After the initial enhancement treatment, and saving, (my version “B”)
I have loaded and saved 5 or 6 times (simulating many changes of mind?!)
And have only been able to detect a very slight loss of luminance,
which is easily restored with 2 or 3 points on the brightness slider.
Does my (above) contention stand?
Sources, references??

Eric Westling
Houston, TX
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