: How to determine what quality a JPG was saved as? 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

@Jamie315

In your specific situation (cropping jpegs you don't have originals of) there are specialized tools to crop jpeg files without recompressing them. See this question: How to crop a JPG without recompressing?

Taken from jpegclub.org/losslessapps.html, here is a list of Mac programs that can do lossless jpeg edits:



PhotoLine [Computerinsel] (Windows, macOS) - Image processing for Windows and macOS, supports JPEG 9
exifiron [Marko Mäkelä] (Windows, Macintosh, C/POSIX) - Orientation correction and optimization of EXIF JPEG images, includes Exif patch
Cameraid [Juri Munkki] (Macintosh) - Digital photography utility package
GraphicConverter [Lemke Software] (Macintosh) - Shareware graphic converter
ACDSee [ACD Systems] (Windows, Macintosh) - Picture viewer and digital imaging software
Shutterfly [Shutterfly, Inc.] (Windows, Macintosh) - Online photo album uploader software
MaPiVi [Martin Herrmann] (Linux, Unix/X11, Macintosh, Windows, Perl/Tk) - Martin's Picture Viewer and Organizer
Photo Viewer [PhotoParade] (Windows, Macintosh) - Digital photo viewing and managing

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@Hamaas979

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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@Jessie844

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 analyze the chunks of data to estimate the compression.

There is one program I know but it is not for mac.
www.mediachance.com/digicam/jpgq.htm
On that same page, it is a good explanation on re-compression.

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

But you should try to have a workflow to minimize the need for this re-compression.

1) Save your original files in a safe place.

2) Use a lossless 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.



I must add that there is not just one algorithm to compress JPG images, but several subsampling methods and this give different quality-compression ratios. This program gives you a glimpse of two.

Also, different programs use different "scales" so you probably need to reinterpret the data.

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@Samaraweera207

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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