Re-saving JPEG while keeping quality

Re-saving JPEG while keeping quality Sometimes I use XnView's* "batch conversion" tool to downsize a bunch of JPEGs (to maximum of 1600x1200). But some of them are equal or smaller than that and no resizing is done.
Asked by: Guest \| Views: 228

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Guest [Entry]

"Yes, there will be loss of quality, even at 100%. That said, is it likely to be all but unnoticeable immediately. If you have Photoshop you can easily confirm the loss of quality:

Load up the original and the resaved image as two layers in the same image.
Change the top one's display mode to Difference.
Select All and then Copy Merged (Ctrl+A and Ctrl+Shift+C by default).
Paste the result in as a new layer.
Using the Magic Wand tool, (Tolerance 0, Contiguous unchecked, Antialias unchecked), select an all-black pixel (RGB 0, 0, 0).

If all pixels are selected (which won't happen), the two images are identical. Otherwise the unselected areas have been modified.

If you resave the same JPEG a few times it will become much more noticeable."