Where is the JPEG Quality Stored?

[poldi]

In Image->Information->Compression, you can read the compression degree of the loaded JPG-file. (Mostly)
Where is this information stored, and is it possible to filter or sort by this degree?

[Bhikkhu Pesala]

I might be wrong, but don't think the information is stored in the file. I assume that it is calculated (estimated) when opening the file.

[poldi]

And do you see any possibilty to filter for this cirterium or just sort files for it? Im am interested to filter for files above or below a special compression rate.

[Bhikkhu Pesala]

Unless the information is stored in the file, how could the file explorer sort them by compression rate.

[j7n]

At the start of a JPEG file there is a series of 64 values for each color channel called a quantization table. They indicate the divisor applied to frequency components before they are stored and thus their accuracy. Any value in this table can vary somewhat independently, but usually they all grow simultaneously with worsening qualiuty. The standard JPEG library provides 100 steps by which the quantization tables are scaled. To obtain the overall quality factor, the program attempts to find the closest match between the set of quantization values stored in the file and those provided by the library.

In a sense, the quality is stored. The quantizers have to be retrieved to restore the image close to the original. But it consists of multiple numbers like a line chart with several points. The overall curve goes up and down and the area below it represents the quality.

This could theoretically be done in other programs, but I don't know any that have this feature.