Scanning (for Faces/People) in Photos.app

I copied several Photo Libraries from other devices to my iMac.

When I open these Libraries and select People, I get a message that none of the photos have been scanned and that they will be scanned “when you’re not using the app.”

What does “when you’re not using the app” mean?

  1. Quit the Photos and it will somehow scan as a background process?
  2. Leave the Photos app open and scanning will occur based on unused CPU time (Is Photo scanning impeded by either (a) minimizing the Photos app window, or (b) BOINC running in the background?)
  3. Something else?

Does the background scanning apply to:

  1. All Libraries all the time?
  2. Only the “System Photo Library” (as specified in Photos > Preferences > General)?
  3. Something else?

I posed these questions on https://discussions.apple.com/ but no one seems to know the answers.

I’m hoping that the brain trust here can enlighten me.

Thank you for reading this.

What Photos will tell you is that it “will finish scanning faces when you quit Photos and your computer has power.”

But again…does that mean when the computer is turned off but plugged in, or when the computer is booted up but not being used, or something else?

It’s like some of the people I’m tracking always get their Faces scanned, and others are always pending.

So, Photos needs to be not running. Bit 1 of your answer?

I expect it will only scan the current system photo library. Also I expect that the faces that it remembers are stored in that library and won’t transfer to the other libraries - although you could be pleasantly surprised!

I use Powerphotos to manage several photo libraries and move photos and albums around, although it won’t do the face scans that you are after.
https://fatcatsoftware.com/powerphotos/