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When I save a JPG file with GIMP, I can adjust the quality I save it at, from 0-100 (I use 89). It seems like I've used an app to see what this number was on saved file but if I did I can't for the life of me figure out what it was. Any suggestions as to what to use?

BinaryMisfit
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Nathaniel
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    Just to make sure that it is known: the quality setting of different applications is not comparable, in general: http://www.faqs.org/faqs/jpeg-faq/part1/section-5.html. Both GIMP and ImageMagick should use the IJG quality scale, though. – Michael Schumacher Sep 29 '15 at 12:36

6 Answers6

37

To add to Arjan's answer:

ImageMagick's identify appears to actually look inside the JPEG image to guess the quality setting used to encode it.

ImageMagick's source code (cheer for free software :-)) contains the lines:

/*
  Determine the JPEG compression quality from the quantization tables.
*/
sum=0;
for (i=0; i < NUM_QUANT_TBLS; i++)
{
  if (jpeg_info.quant_tbl_ptrs[i] != NULL)
    for (j=0; j < DCTSIZE2; j++)
      sum+=jpeg_info.quant_tbl_ptrs[i]->quantval[j];

(coders/jpeg.c, line 843ff. in my recent version of ImageMagick's source code).

I don't know enough about JPEG to really understand, but it appears to do something like described in this article:

Determine the JPEG quality factor by using Visual C# .NET (link dead as of Januar 2018; copy on archive.org from 2015)

So yes, identify can actually determine the quality setting of a JPEG just from the compressed file alone (though the result may not always be completely accurate).

sleske
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    Whoa. Very nice of you to check the source code. Cool. – Nathaniel Jan 04 '10 at 20:17
  • @Nathaniel, can you please select this answer as being the accepted one, instead of mine? Thanks! (I cannot delete mine as long as it's accepted.) – Arjan Mar 24 '17 at 14:22
  • It is searching for the JPEG quantization table that best accounts for how the compressed bitstream looks. – jbarlow Apr 19 '18 at 04:05
  • Constants (jpeglib.h): `NUM_QUANT_TBLS` = `4`. `DCTSIZE2` = `64`. `quantval[i]` just gets the number at given position. – user136036 Feb 25 '20 at 23:58
26

Once saved, you cannot tell the quality anymore.

(Setting the quality while saving just tells the software how much loss you find acceptable, but once saved: what's lost is lost. You'd need a human to say if something looks nice.)

Hmmm, I guess I was wrong. I still think the above is correct, but ImageMagick's identify proves me wrong? 

identify -verbose myimage.jpg

Image: myimage.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Class: DirectClass
  Geometry: 358x240+0+0
  Resolution: 300x300
  [...]
  Compression: JPEG
  Quality: 90
  Orientation: Undefined
  [...]

I don't know how the image in my test was saved, but it does not have any EXIF data. Could the quality still be stored in the image?

Arjan
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    Can't you experiment converting to different qualities? I find it hard to believe, unless ImageMagick stores some private data in the jpg (so this might not work with other packages). – harrymc Oct 29 '09 at 20:37
  • Interesting. I'll wait to see how this pans out. – Nathaniel Oct 29 '09 at 21:10
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    +1 Yes imagemagick works. I can repeatedly change the jpeg quality and use identify to see the change. This works if I use convert (another imagemagick untility) or another tool like MS Photo Editor. – DaveParillo Oct 29 '09 at 22:20
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    ImageMagick is doing something different. It gives an estimate, rather than reading what your original software did. Your original, now-crossed-out, answer is really more correct. See @sleske's answer. – mattdm Mar 24 '17 at 13:14
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    **ImageMagick returns a quality assessment number, but that doesn't mean it's correct.** See http://www.faqs.org/faqs/jpeg-faq/part1/section-5.html from the jpeg creators themselves, and https://photo.stackexchange.com/questions/88167/is-it-possible-to-find-out-what-compression-ratio-was-used-for-a-particular-jpeg – matt wilkie Feb 18 '21 at 23:11
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As Arjan metioned identify -verbose myimage.jpg will do it. As imagemagick is a CLI tool, it may be useful for scripting. The approach identify -verbose myimage.jpg | grep ... is preety slow. I recommend using IM like this

identify -format '%Q' myimage.jpg

It is massively faster.

ManuelSchneid3r
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4

JPEGsnoop is a nice alternative to ImageMagick's identify. The download is quite small and is available in portable format.

After processing a jpg, you will find the "Approx quality factor" under the DQT marker.

idmadj
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2

Picasa 3 has the properties pane which shows the jpeg quality but it is an abandonware at the moment. Picasa 3 Dick Masterson

Uğur Gümüşhan
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1

With ImageMagick++ library it's easy:

Image magick_image( pathname );
size_t compressionFactor = magick_image.quality(); // 0..100
Valeriy Van
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