[DaveBlake]

There is one thing I cannot seem to figure out. It happens when an albums contains the work of two composers.

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How would this difference be caused? I have been comparing the tags, but don't see the logic.

It is about how they are tagged and if the composer is also an album artist, and be aware that Musicbrainz is not entirely consistent how they deal with multiple album composers either. The logic is my initial attempt, so I am happy to discuss and adjust if real world testing reveals flaws. At the moment I am the only dev with a serious interest in classical music, so many things are only going to be revealed by user testing. Also what Kodi does has to work with all kinds of music, even pop songs have composers etc.

It is all down to the artist, album artist and composer tags, and the logic best explained with your example. Assuming you have taken the tags exactly from Musicbrainz.

Ravel: Gaspard de la Nuit / Prokofiev: Piano Sonata no. 6
Tracks 1 to 3 have
ARTIST: Maurice Ravel
ALBUM ARTIST: Ravel / Sergei Sergeyevich Prokofiev / Ivo Pogorelich
COMPOSER: Maurice Ravel

Tracks 4 to 7
ARTIST: Sergei Sergeyevich Prokofiev
ALBUM ARTIST: Ravel / Sergei Sergeyevich Prokofiev / Ivo Pogorelich
COMPOSER: Sergei Sergeyevich Prokofiev

Ravel and Prokofiev are both (along with Pogorelich) album artists for all tracks. They will appear individually in the artists list even when albumartistsonly flag is enabled, when you drill down from one of them you will see all the songs on the album. A composers node will list Ravel and Prokofiev, but what should happen drilling down from there? Currently you see the songs that they composed AND the songs where they are album artist.

Maybe worth noting that since Pogorelich is tagged as Performer (Piano) he will show on a piano node or playlist (if you are interested in pianists and create one).

In comparison:
Wendy Carlos - Switched-on Box Set
All tracks have
ARTIST: Wendy Carlos
ALBUM ARTIST: Wendy Carlos
and then tracks have different COMPOSER tag (or none) but Wendy Carlos is never credited as the composer.

Wendy Carlos appears in the standard artists node while Bach, Scarletti etc. will not unless you have them as artists on other albums. Bach, Scarletti etc. are in the composers node, drill down you see the songs that they composed AND the songs where they are album artist. So on this album, only see the composed songs.

Why do that, and it was a choice on my part? Well I like to play albums, so having selected an album by composer (my usual approach) I want to play all of it not just the tracks by that composer. If I decided to listen to some Prokofiev piano, I would happily listen to the entire recording and have the Ravel too. To support the concept of an "album composer", even though in data it is only the songs that have a composer as data, an album is related to a composer if they are also in the album artist tag of all tracks.

Ideally this would be optional, and the user could choose to see just composed songs not whole album, or switch between them, but not got that far yet!

Hope that at least makes the logic clearer.