Guillermo Espertino writes:
 > The same image exported as jpeg with the same quality factor (let's take 
 > 75% as an example)

Where did you get that percent sign from? GIMP doesn't show any
percent sign. The quality value is not a percentage of anything. You
should just treat it as a number on a nonlinear scale between 1 and
100. (With the usable range being between 70 and 95, say.)

(What would it be linear to? File size? Perceived quality, as if that
could even be quantized?)

It is not necessarily related to corresponding scales in other
software at all. The only thing one can know is that the higher the
number is, the less compression artefacts one should see (and the
bigger the file will be).

 > Both programs calculate the compression ratio differently.

That is no surprise, as they use different JPEG software. You shoukd
not think of the JPEG quality value as some "compression ratio" or any
other "ratio". It is just a number whose exact meaning you will have
to check from the libjpeg sources in GIMP's case. It is just a
coincidence that both programs in this case happen to use a scale from
1 to 100.


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