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When saving JPG images with Pixelmator or Photoshop, I select a quality from the scale 1–100 that gives optimum file-size.

Later on, I sometimes take these images and crop them a little more (not always possible to find the originals), but when re-saving I can no longer know what quality I saved them with.*

Are there any Mac programs that will reveal the quality that the JPG was saved at?

* Perhaps this is not the right way of going about it, because it seems that, for example, if I'm already saving a file that was saved lossy at 59 quality previously, and then go ahead and I save it again (even at 59 or 99), it will loose even more quality? Correct?

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For the side question, yes, saving a jpg as a jpg compresses it again, losing more quality each time. Try to always keep the original and try to never save a jpg as a jpg. Topical side note: since each time a Facebook image is uploaded, it gets jpg-compressed, it's possible to guesstimate how many generations of friends any one of those red/pink equals sign icons has been through by looking at the jpg compression; as a way of estimating the spread of a viral campaign. –  user568458 Apr 3 '13 at 9:51
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You can't. Once you save out the JPG, it's now a new 'original' image. It has no 'memory' of the original to figure out how much it was compressed. –  DA01 Apr 3 '13 at 21:44
    
I'm still trying to figure out how that information would benefit you? You can see just by looking at the image if it has good enough quality. –  Joonas Apr 3 '13 at 22:46

3 Answers 3

When saving images as .jpeg you always lose information. The dialog basically asks you how much information you would like to lose in favor of smaller size on disk (1 = most loss, 100 = least loss). There is no way to tell what you originally selected and the only use would be to have a history of your workflow because this loss is irrecoverably applied to the image when saving.

Your assumption is right; every time you open the compressed image and save it again (with anything else but setting the quality value to 100) you will lose more information.

Because of this I would encourage you to archive all the original pictures in full size and a lossless format.

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You can not tell the exact settings which you applied on a jpg. As DA01 said there is no memory of that... but there is the fingerprint.

There are programs that analize the chunks of data to estimate the compression.

There is one program I know but it is not for mac.

http://www.mediachance.com/digicam/jpgq.htm

In that same page it is a good explanation on re-compression.

Some other programs (like gimp) have an option to try to gess the compression (analize this fingerprints) and use the same settings.

But you should try to have a workflow to minimize this re-compressions.

1) Save your original files in a safe place.

2) Use a losless compression format in the process, like PSD.

3) You can use jpg again on the output, but if you need to make some changes, go back to your psd files. Jpg should be only a final output file format.

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Yes! You can do this from command line (using the Terminal app) with ImageMagick.

After you install ImageMagick, navigate to the directory where your picture is located and run the following command:

identify -verbose yourimage.jpg | grep -i quality

Where yourimage.jpg is the name of the image.

And you should get the value which indicates the image quality. 0 means the lowest quality and 100 means the highest.

How to install ImageMagick on a Mac

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