Just delete the three sqlite database files inside
~/Library/Containers/com.apple.AMPArtworkAgent/Data/Documents/ (so that Music.app will rebuilt the artwork indexes database for you) and you are good to go.
Maybe not, because before that, I also copied everything inside
~/Library/Containers/com.apple.AMPArtworkAgent/Data/Documents/artwork/ from the old machine to the new one. The sqlite database files are presumably responsible for maintaining the index between the artwork image files and the music files. Deleting them is only oneway to trigger the update after the manual import.
By the way, I can confirm if this folder is only used by Music.app or by iTunes.app (before Catalina) as well because my both Macs are running Catalina (I know that’s stupid). However, when I sorted the files by date added, the oldest one is at the time when I upgraded to Catalina so I have my reason to consider it a Music.app-only thing.
Actually, I guess if you don’t have the
~/Music/Music/ folder (which only occurs when you upgraded to Catalina) besides the old
~/Music/iTunes/ folder, the album artworks should be generated automatically at the upgrade because that’s how my old Mac works. If you already have
~/Music/Music/ folder then you must have a previously imported (from iTunes.app to Music.app) library and your artworks should be either on your current or previous computer, thus you should have the artwork files in that folder.
If for some strange reason you don’t have the artwork files in
./artwork/, I would suggest you to try delete the some database files (by the way, use
file command in lieu of the file extension to identify file types) to test if Music.app rebuilds them for you. I didn’t encounter this issue myself so I couldn’t guarantee anything.
Don’t forget to prepare the backups though. I didn’t research on what’s under the hood of the new Music.app, but due to the fact that this library is your only one on macOS Catalina at this time, using
click to switch between libraries won’t work here. Besides, the new presumably proprietary
.musicdb db format is replacing the good ol’
.itl files in iTunes, so pay attention when poking around if you care about integraty of the metadata or the music files itself.
Anyway, that kind of explains why copying the Library (
~/Music/Music/ as well as
~/Music/iTunes) from a Mac to another results in missing album artworks.
The way I find it out is pretty straightforward. I used
fswatch to locate what files changed when I add a song to the new Music.app library, and if you tinker around with the files, it won’t take you too much time to find out how the new Music.app local database might work.
All in all, this might only apply to my case but it should fit into similar situations which involves clean install of macOS Catalina (because I used manual copy instead of Time Machine to transfer the library). I couldn’t find any practical solution on the Internet before I find it out on my own but there are some other person have the same problem. Hope this post can help those who encountered similar issues.