What do the buttons do in digikam's face recognition UI?

Question

A while back, there was a question on digikam's mailing list that unfortunately never got answered. I'd like to ask that question here.

Paraphrasing, I'd like to know what happens when, after running face detection and recognition, I go through the faces in a person's tag and click the '-' (minus) button. What I'd expect to happen is that that face does not appear under that person's tag anymore but under 'Unknown', but that doesn't seem to be happening.

I've clicked that button quite a few times and the faces never turn up again anywere, even if I re-scan my entire collection. So, in addition to what that button does (and how it differs from the 'Not a face' button), I'd also like to know how I can label the faces that I've told the application didn't belong to the people it thought they did.

Answer 1

This answer refers to digiKam 7.3.0 (the behaviour may have been different in the past).

All the buttons have tooltips which give some insight on what they are doing. Note, however, that the minus-button behaves differently depending on which view you are in:

• When browsing Unknown faces: The tooltip says "Mark this face as ignored". When clicking it, the face disappears from the Unknown section and instead appears in the Ignored section (so you can go there and revert your decision later).

  So clicking this button acknowledges that the area indeed shows a face, just one you don't care about.

• When browsing Unconfirmed faces: I cannot double-check this right now but I think in the Unconfirmed section the minus-button does not ignore the face but just puts it back in the Unknown pool. I.e. it tells the software that this is not the person it thought it is.

• When viewing a photo: When viewing the full photo, the minus-button on the face regions in that photo is used to delete the face region completely. There does not seem to be a way to mark a face as ignored in this view.

The red button in the top-right corner always has the same function (tooltip: "If this is not a face, click to delete it"). It is used to remove a face area completely and should be used for cases when something was wrongly detected as a face. As far as I know there is no easy way to undo this.

Update for digiKam 7.5.0: I just upgraded to 7.5.0 and found that there was some change. In both the Unconfirmed view and when viewing full images, there is a third buttons now (the one with the arrow):

It can be used to mark faces as ignored (i.e. same as the minus-button in the Unknown section).

Answer 2

From my testing, these are the button functions:

1 plus/minus icon

The plus/minus icon is merely an informative icon (albeit an poor one, in my opinion) of whether that photo is selected. If you select one or more photos and hover over one of them, the icon instead shows a minus icon. Otherwise, hovering over the photo shows a plus button. Oddly, clicking exactly on the plus icon removes your other selections and just selects that photo, rather than adding it to your selection. So it really isn't a special clickable icon.

Much easier to see is the bright blue border around the selected photos.

4 check mark

The check mark button obviously sets the tag for the recognized face.

2 and 3, x icon and minus button

The minus button and the x icon both seem to do the same thing in that they both remove the detected face data from that portion of the photo. Even if the program has a suggestion in the unconfirmed section, if you click the minus button, it doesn't simply remove the suggestion and keep it in unknown; rather, it actively removes the face data from the photo (you'd have to go back to the photo manually and re-add the face along with the correct tag).

The x icon also removes the face data.

I don't know if the backend algorithm defines a difference between thumbnails where you click the x icon vs the minus button. There's no hover text for the minus button, but when you hover over the x icon, it says "If this is not a face, click to reject it."

The minus button also rejects it, but it's possible that the x icon is for thumbnails that aren't actually a face (so the algorithm can learn from its mistakes), and the minus button is simply used to delete the face data from someone you don't care about (person in the crowd, picture of a painting, your estranged lover, etc) without causing the algorithm to think it made a mistake in detecting a face.

That's one possibility. The other is that the x and minus button simply do the same thing.

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Another piece of evidence that the x icon and minus button do the same thing is in another section of the app. When you view a whole photo with faces detected but not verified, it shows boxes around all the faces. You can then choose to Confirm or Remove the tag.

If the x icon and minus button did different things, you'd think the functionality would be replicated in the section where you view the entire photo. Since the developers didn't add that, it seems likely that the x icon is superfluous and is mainly there because some people expect a close icon.