Support for non-standard `GP1`/`GRP1` frames.

[Cantello]

I am trying to copy one tag from field "TXXX:grouping" to field "GRP1". Should be simple in theory but I somehow cannot grasp the concept. I start with id3 -2 -wTXXX:grouping "%{GRP1}" mp3file.mp3 (similar to the below example) but this does not write the GRP1 field to the TXXX field. If I just add text instead of the %{GRP1}, it works.

Any ideas? It almost seems as if GRP1 is not in the list of compatible frames (as per compatability section), so maybe I just use another TXXX frame to copy to the desired frame...

[Cantello]

I now simply used a workaround where I first write the tag to TXXX:series and then to TXXX:grouping. Works well so far.

[squell]

Yes, the GRP1 frame, is not defined in ID3v2.3 and so not supported. I see that this apparently is an instance where iTunes just defines a custom tag: id3/ID3v2.4#18 without regard for what some person on the internet wrote in the ID3v2 spec 20 years ago. Fair enough. :-)

I recall back in the day iTunes also stubbornly being the only program that would write ID3v2.2 tags.

Also see: https://discussions.apple.com/thread/7900430?sortBy=rank

I'll re-open this issue as a "add reading support for GRP1" (I probably don't want to encourage people to write GRP1/GP1 frames). Although it would be nice to have an example of a ID3v2 tag that has one so I can see what the structure is (otherwise I would have to guess). Maybe you can send one?