Concept for providing cover art for single tracks

[Tobias Leupold]

Dear Elisa devs,

currently, there seems to be no way to provide CD cover images for single 
tracks. In FileScanner::searchForCoverFile, there is a per-folder search for 
all kinds of images that could be the cover art -- for that album. But a 
collection of single tracks normally don't live in a per-album folder 
structure, so this won't work well for single tracks (at least for my 
collection structure).

Lately, I worried about how one could solve this.

Back in Amarok days, one could add cover images for all tracks. Amarok 
calculated some MD5 ID from the lower-case file name or such IIRC and created 
an Amarok-specific cover archive to be used whilst playing songs. This worked 
fine, but the problem was that a lot of work had to be done to always see 
covers, but the result was application-specific, with no easy way to reuse it 
(e.g. in some script). Also, those IDs were filename-agnostic IIRC.

I think there would be a quite straightforward solution for providing cover 
art for all songs. What do you think of the following approach?

 - We define a folder containing all cover art. This would be typically some
   folder inside the collection root (of course configurable by the user).

 - We read, if present, the MusicBrainz album ID of the respective song

 - If there's a MusicBrainz album ID, we check if the song's folder contains
   some cover art as it's done now. And if not, we check if the cover art
   folder contains a file named [id].jpg/png

I use such an approach to generate normalized songs to be used in my car's 
radio: I decode the song I want to add, calculate it's EBU R 128 loudness 
using ffmpeg, apply it and re-encode it to a format my car will play, with the 
normalized loudness applied (as of course stuff like ReplayGain or EBU R 128 
tags aren't considered). And: I embed the cover art for that album, which I 
find using it's MusicBrainz album ID.

This is quite easy: I do this with a small Python script using Mutagen, which 
exposes the MusicBrainz album ID as "TXXX:MusicBrainz Album Id" or 
"musicbrainz_albumid" (depending on the file type) if one uses the (most 
convenient) mutagen.File extractor. And thus, I know which file to embed.

This way, multiple songs can link to one cover image, no matter if it's a 
whole album or a single song. Also, the file name can be easily found, and it 
is static, even if the album name would be changed (corrected). Also, being an 
UUID, we won't have to worry about collisions. And the file's path doesn't 
matter. And this is reusable, not application-specific.

Well, I thought this would almost be trivial to implement, so I tried to do it 
and file a MR. But sadly, there are some problems for me :-(

I must admit that I don't find my way too well in Elisa's sources. But as far 
as I could grasp it, KFileMetaData is used to extract the metadata that is put 
in the SQLite database. Now, KFileMetaData::Property doesn't list MusicBrainz 
stuff, so I think a custom extractor exposing it would be necessary to be 
implemented. The rest would really be easy I think. Just invoke 
FileScanner::searchForCoverFile with not only the file name, but also the 
MusicBrainz album ID and do one additional lookup.

Anyway ... before I become too obsessed with this idea, I wanted to ask what 
you think about this approach, and if this would be something beneficial for 
Elisa.

Thanks for all feedback!

Cheers, Tobias