[luatex] How to access user-defined kerning

[Henri Menke]

Dear list,

I'm trying to determine the kerning between two letters (here A and V).  For the
sake of this example I'm inserting negative kerning of -400/1000 em.  However,
when I examine the kern in the kerning callback this massive enhancement of the
negative kern doesn't show up but I only see the kerning from the font itself.

How can I access the user-defined kerns?  MWE is below.

Cheers, Henri

---

\input luaotfload.sty

\directlua{
local function show_kerns(head)
   for item in node.traverse(head) do
      if item.id == node.id("glyph") and item.char == 65 then
         % Get current char and font
         local nut = node.direct.todirect(item)
         local char = node.direct.getchar(nut)
         local font = node.direct.getfont(nut)
         print(font)
         % Get next char and font
         local next = node.next(item)
         local nextnut = node.direct.todirect(next)
         local nextchar = node.direct.getchar(nextnut)
         local nextfont = node.direct.getfont(nextnut)
         % Determine the kern
         local data = fonts.hashes.identifiers[font]
         local kern = fonts.handlers.otf.getkern(data,char,nextchar)
         print(kern)
      end
   end
end
%
luatexbase.add_to_callback("kerning",
                           function(head)
                              show_kerns(head)
                              node.kerning(head)
                           end,
                           "show_kerns")
}

\directlua{
fonts.handlers.otf.addfeature{
   name = "ktest",
   type = "kern",
   data = { ["A"] = { ["V"] =  -400 } } % make them overlap for the example
}
}

\font\test="Latin Modern Roman:+ktest"\test
AV

\bye