[PRODIG] JPEG resaves

[Martin Evening]

To answer those who tried out the suggested test, it was to demonstrate that yes, if you repeat save a JPEG it will degrade each time, but perhaps not always by as much as you have been led to think. I am not for a minute suggesting that JPEG be used as an editing format, but there people do often assume that the image degradation is a constant with every resave situation. I believe that it is not the number of resaves that is the issue, but how much the image structure gets changed between each resave you do.

In the suggested test, you are not adjusting the image data with global tonal corrections or cropping it or altering the JPEG compression settings. However, if you were to crop the image by so much as a row of pixels, resave, add a row of pixels, resave etc... then you would see a swifter degradation. Many years ago someone on a Photoshop mailing list reported how he had kept resaving a JPEG over and over again and couldn't see any worsening of the image. It was pointed out that in a real world situation, a user would more likely open a JPEG, and make a curves adjustment, resave. The same user might reopen and crop the picture this time. That sort of workflow will lead to disaster. For example, if I have a folder of JPEGs where I need to add some metadata via a Photoshop action, I am not going to worry myself unduly about an extra single resave in those circumstances, especially if a low amount of compression was applied initially

Martin Evening Photography <www.martinevening.com>
Co-listowner ProDIG discussion list <http://www.prodig.org>

Author of Adobe Photoshop 7.0 for Photographers
<www.photoshopforphotographers.com>

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