[luatex] First question: Retrieve *family* name of a font / the three main families

[Urs Liska]

Hi all,

this is my first post to this list, and I must say I haven't found a way 
to search the list archives. If there is any I apologize for asking 
without searching the list archives before posting. But search engines 
and the manual haven't provided any help for me, nor did I receive much 
feedback on stackexchange. So I think asking here is "the right thing".

I'm contributing to a LuaLaTeX package 
(https://github.com/jperon/lyluatex) that uses an external program 
(LilyPond) to generate images to be included in the LaTeX document. I 
need to pass the used fonts to LilyPond, and I didn't manage to retrieve 
the necessary information in Lua.

What I need is

a) which families are currently set as roman, sans and mono fonts
b) the "family" name of these.

ad a)
I have managed to retrieve the "current" font, i.e. the one that is 
currently used for typesetting. This is basically done with

     local current_font = fonts.hashes.identifiers[font.current()]

although I don't know if that's the most direct way to get to that 
information.
But this is not the *family* of the font but actually the font face 
currently in use. And I have no idea how to get to the three defined 
font families.

ad b)
According to the documentation (chapter 5 of luatex.pdf) I can access 
the following vlaues from the font object:
     .name
     .fullname
     .psname
     .filename
but none of those provides me reliably with what I need to pass on to 
LilyPond, which seems to be the "family".

For Linux Libertine O it works, but this seems somewhat accidentally. 
This is what the font object gives me:

fullname: Linux Libertine O
psname: LinLibertineO
name: 
[LinLibertine_R.otf]:mode=node;script=latn;language=DFLT;+tlig;+tnum;+lnum;
filename: 
/usr/share/texlive/texmf-dist/fonts/opentype/public/libertine/LinLibertine_R.otf

Other fonts don't work. This is the result for "Adobe Garamond Pro":
fullname: AGaramondPro-Regular
psname: AGaramondPro-Regular
name: AdobeGaramondPro:mode=node;script=latn;language=DFLT;+tlig;
filename: /home/shared/fonts/Adobe Garamond Pro/AGaramondPro-Regular.otf

If I compare this to the output Fontconfig is giving me:

$ fc-list -v | grep -i libertine
     family: "Linux Libertine O"(s)
     fullname: "Linux Libertine O Semibold"(s)
     file: 
"/usr/share/fonts/opentype/linux-libertine/LinLibertine_RZ.otf"(s)
     postscriptname: "LinLibertineOZ"(s)
     ... and the same sets for all font faces

$ fc-list -v | grep -i "adobe.*garamond"
     family: "Adobe Garamond Pro"(s)
     fullname: "AGaramondPro-SemiboldItalic"(s) "Adobe Garamond Pro Sb 
Italic"(s) "Adobe Garamond Pro Semibold Italic"(s)
     file: "/home/uliska/.fonts/Adobe Garamond Pro/AGaramondPro

it seems that the "fullname" property is quite unreliable, and I 
definitely need to get the "family" name.
However, from the "Font structure" section in the LuaTeX manual I don't 
find any way to get that information.

Maybe there's a way to retrieve that information from the actual font 
file? But it would be nice to be able to get it directly from Lua - and 
of course it has to be OS independent, so I guess I can't make use of 
Fontconfig?

Any suggestions are highly welcome.

Thanks in advance and best regards
Urs