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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.

Is any image quality lost during the process? (Even if I choose JPEG 100% quality?)

Edit: I was asking about those images which are already smaller than the limit, and are not being resized, just re-compressed.

* My Linux laptop is toast, can't do any imagemagick tricks.

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2 Answers

up vote 2 down vote accepted

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 dozen times it will become much more noticeable.

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Any sort of resizing that is not 1:1 aspect ratio will affect the quality, however, if you do not change the size or any properties, a 100% quality export / save should not affect quality.

However, I have never used the tool you are using. What I said is just general experience from using Photoshop and other photo editing software.

(Also, you can always use Imagemagick on Windows!)

Lastly a bit of general advise, Jpeg is not really a good format for anything professional, you should use a lossless format and then just use Jpeg for the distribution ... Although, for every day stuff, Jpeg is more than sufficient.

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Yup. Related question for image downsizing only: superuser.com/questions/74611/… –  John T Jan 24 '10 at 22:13
    
Uh, Molly, cropping is not the same as resizing. –  John T Jan 24 '10 at 22:54
    
i know, but according to Wil, any sort resizing (or cropping/exporting) will affect the quality. cropping or rotating does not necessarily affect the quality. –  Molly7244 Jan 24 '10 at 23:01
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My question was only about images which are being re-compressed (at 100%) without any other changes. Sorry for not putting that in the question :( Also, I don't have much choice regarding file format, and it's just for personal use. –  grawity Jan 25 '10 at 13:47
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@Molly - Rotating affects quality if you rotate on another angle than a 90° multiple, simply. Because like for resizing, there will be interpolation or melting required. –  Gnoupi Jan 25 '10 at 13:50
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