Capturing out-of-music markup for music braille

[Maurits Lamers]

Hey all,

It has been a while I posted on this mailing-list. As some of you might 
remember, I have been working on a music braille output system for 
Lilypond for quite some time now. The update on that front is that I do 
have a system now that can reliably output (a subset of) music braille 
for Lilypond files version 2.24 and higher.

The concept of this system (named Lybra) is based on the concept 
provided by the event-listener.ly file as included with Lilypond, 
capturing music events, rewrite them to braille and saving the output in 
a separate text file. This allows users to write music the normal way, 
and get the music braille equivalent by including a single lilypond 
file. That inclusion attaches listeners to the music events, which cause 
the braille output file to be created, next to any PDF or midi as 
indicated in the Lilypond source.
While the system is definitely not complete yet, I think that the 
listener interface will be sufficient for all music related information 
I would still need to add.

For a proper braille output though it is important to also be able to 
include the headers, as well as other non-music related markup. One 
specific use case is a song score, with a score block with the music and 
the first verse as lyrics and the remaining verses as blocks of text 
under the song.

My main problem is that I do not know how I can get hold of that 
information. My main entry and exit points are the initialize and 
finalize listeners to the Score context. I have been searching what 
contexts I can get access to a nd whether those contexts can provide 
anything, but if there is anything, I cannot find it.

I did see functions in the Lilypond scheme function list that look 
suitable to provide the headers (which would solve one part of the 
problem), but these functions require parameters to be passed in, and 
there doesn't seem to be documentation on how to get a reference to that 
parameter, (ie the current state at the moment of initialize or finalize).
One example is the headers-property-alist-chain function, another is the 
paper-book-header function. The first requires the headers object, the 
second a paperbook object, but I cannot find how to get a 
pointer/reference to either the current headers or the current paperbook 
object.

So my main questions are:

- is there a way to get a listener or callback of some kind to capture 
out-of-music markup?
- is there a way to get access to the global headers and / or score 
headers (in case of score specific headers)?
- is there documentation on how to get a reference to the required 
parameters for these scheme functions?

Thanks in advance for any suggestions!

cheers

Maurits