"Transposed" and "Transversed" are very technical terms, and it's hard to understand what they mean in this context. I actually have no idea myself! Could we consider replacing these terms with plainer language?

Thats somewhat ambigous - flipped along the vertical axis, or top/bottom flipped?

Thats mirroring/flipping along the diagonals. You get the same effect when you mirror vertically and rotate +- 90 degrees.

I don't think there is a less technical term.

Clockwise or CCW?

Actually I don't know, I just copied the code from Dolphin into here.
Seems to be a common term though according to https://docs.microsoft.com/en-us/uwp/api/windows.storage.fileproperties.photoorientation
GIMP describes Flip vertically as top/bottom, see https://docs.gimp.org/2.10/en/gimp-layer-flip-vertical.html

CCW, see https://docs.microsoft.com/en-us/uwp/api/windows.storage.fileproperties.photoorientation.

Of course I can look this up. But this information is not available when the user looks at the string.

Probably we should ask what is the target audience here? The average user only wants the image to be displayed correctly, i.e. apply the transform and be done with it. This IMHO also implies never showing this value by default.

For power users, it may be better to also show the numerical value, so they can look up whats meant.

Otherwise, it is not only necessary to mention CW or CCW, but also if this is the transform which has been applied to the image, or the inverse transform required to show the image correctly.

How about using the descriptions from D15503, which are also consistent with digiKam and Gwenview? The best solution is probably to make D15503 work, but this is out of scope for this revision. And a text is still better than a plain number.

IMHO thats not better at all - what does "top, left" refer to? And whats the difference to "left, top"?

Every orientation save "normal" and "rotated 180" is ambiguous without further description. It is of no interest for most users, even serious photographers, as it is a technical implementation detail. Creation times, aperture, exposure time ... are useful for users.

So we should hide it by default, and for the 0.1% interested in it we can give a description they can lookup themselves, e.g. "Rotated 90° CCW (6)"

Phew, I didn't anticipate that these strings are that controversial since I'm basically copying existing behavior.

I'm fine with adding "CCW" (I think the number should not be shown, it can still be queried separately after all) if that is sufficient for you. Hiding this information is not the responsibility of KFileMetaData.
Besides image size and date, orientation is the only property for images that can be shown in Dolphin currently, so I suspect there are actually users of this.

If we need more fundamental discussion, I'd like to defer that to another revision, since this is currently a drop-in behavior for Dolphin and Baloo-Widgets and copies the status quo. I'd like to land this revision as a starting point to get the infrastructure done and improve upon afterwards. There are many more properties that need special formatting, especially for exif data.

I think this should be "44.1 kHz". Insert the correct expected value and make it QEXPECT_FAIL?

dito, "128 kb/s"

for 180°, CCW is irrelevant.

That was a localization issue.

I'm going to fix this shortly after, so I don't bother

That was pretty stupid :)

I don't think using a static default constructed KFormat is a good idea ...

Why is midnight an invalid time?

I think this would be better written as d->formatAsString = &FormatStrings::toStringFunction; unconditionally on the top (like d->shouldBeIndexed = true), and d->formatAsString = &FormatStrings::joinStringListFunction; in the few matching case statements.

I think you should hand in a KFormat here if you want to avoid constructing a new one for each value.

Same with the previous discussing around the orientation values, I simply copied the code. My plan was to improve it later on, but that did not plan out as expected :)

I decided against that and used a local KFormat. Baloo-Widgets would need bigger changes otherwise as it handles everything on a per property basis, and currently already constructs a KFormat per property. It's also not used everywhere in the display functions. Others also seem to construct it locally, so I guess it's not expensive.

This is a private headers, and thus should have a _p infix.