Identify with format switch reports erroneous JPEG quality if actual value is undefined about imagemagick6

[fmw42]

Best that I can see from identify and from exiftool, no quality value was recorded in the file. It may have been compressed at quality 50% but it was not recorded. So IM will assume its default quality of 92.

Someone else can double check my assessment.

[bitsgalore]

@fmw42 You're probably right, but IM's current behavior makes it impossible to distinguish between JPEGs that were actually compressed at 92% quality, and JPEGs for which the quality is unknown, which isn't very helpful. Reporting some NaN value or even an empty string (I don't know if there are any IM conventions for this?) would be much more helpful. Knowing that IM cannot establish the quality is actually useful information, whereas getting some arbitrary value that's indistinguishable from a meaningful estimate isn't!

BTW you mention the absence of quality level info in the metadata. I noticed that as well. From the Fotoforensics explainer (tab "Estimating Quality) I understand it's quite rare for JPEGs to have this info in the metadata, and even if it's there, it's often unreliable. It's not entirely clear to me how IM establishes the quality; I suspect somewhere under the hood IM or some delegate library uses either the "Approximate Ratios" or "Approximate Quantization Tables" methods that are mentioned in the Fotoforensics piece. But from the code I can't quite figure out if this is indeed the case.

However, JPEGs created inside IM don't appear to contain quality-related metadata either. Nevertheless, "identify" is able to establish the quality!

A quick example. First I create a new JPEG with 40% compression quality:

convert -quality 40 wizard: wizard-40.jpg

The output of the following ExifTool command doesn't contain anything related to the quality level:

exiftool -X wizard-40.jpg

Despite this, using "identify":

identify -format '%Q\n' wizard-40.jpg

Result:

40

This is all slightly straying from the issue, which is really about the reporting. But since it popped up in the conversation, and I'd be doing some tests with that already, I might as well mention it in case it's of any use.

[fmw42]

If you define a quality and add it to the file properly then it will be in the meta data. Most of the JPGs that I have checked have a quality value. If no quality value is in the file, then you get 92. If you have a quality value in the file, then it could have 92, but it will show as a quality value in the file as opposed to no quality value.

An IM developer might comment further on this.

[bitsgalore]

A bit of further digging seems to confirm that IM actually determines the JPEG compression quality from the quantization tables, and not from some pre-defined metadata field:

ImageMagick6/coders/jpeg.c

Line 925 in bf9bc7f

Determine the JPEG compression quality from the quantization tables.