I just realized that uuid for thes same photo is different on different computers.

[chengguangnan]

The uique uuid is cloudassetid / fingerprint.

[RhetTbull]

Yes. UUID is unique only to the specific library. If you repair the library or create new library and re-download from iCloud, then UUID will be different. If you move a backup between computers, you can use the osxphotos exportdb --migrate-photos-library command to update the UUIDs in the export database.

osxphotos doesn't use cloudassetid because these exist only for photos in an iCloud database. It also doesn't use fingerprint because Photos computes fingerprints only for photos, not for videos, and because fingerprint is unique only to the actual file not the asset in the database. That is, you could have duplicates in your library that have different metadata, are in different albums, etc. but they have the same fingerprint.

[chengguangnan]

Thank you. Very informative!

[RhetTbull]

By the way, the logic used for the --migrate-photos-library command is this:

If cloud GUID exists, use that because it's guaranteed to be the same asset
If not, and fingerprint exists, use filename:fingerprint
If fingerprint does not exist, use filename:filesize

If photo is in a legacy "shared album", none of these exist for the photo so the "fingerprint" used is "cloud_owner_hashed_id:heigh:width:isphoto:isvideo:date"

This isn't perfect and there could be false matches but it is not likely and it's the best algorithm I've been able to come up with. If someone can devise a more robust matching algorithm I'm happy to accept a PR!

This logic is important and is used in other places in the code -- for example, for the sync command which syncs metadata between two libraries and the import command which checks for duplicates when importing.

I'm not sure why Photos doesn't compute fingerprint for videos -- would be more useful if it did. The fingerprint method (which is a private undocumented method in the Photos framework which I've discovered and figured out how to call) works fine on video files. My guess is that because videos can be very large, checking the fingerprint on import could be slow and thus it's avoided. On modern Macs though the algorithm runs very fast. It computes some sort of hash though I've not figured out which exactly (but also not relevant as osxphotos needs to match Photos).