LuaLaTeX: change font id does not work with a non default font

Question

I try to change the font from the Lua side. I tried to change to small caps (see Obtain small caps font from OpenType font when diffining a `\font`, thanks to egreg's code) but, whereas everything works fine with Latin Modern, if I use fourier-otf (but other OpenType packages give same results), the font is changed (numerically) but there is no small caps in the output.

Any ideas?

\documentclass{article}
\usepackage{luacode}

\usepackage{fourier-otf}


\begin{document}

\sbox0{\scshape \global\expandafter\let\expandafter\firstlinefont\the\font}
\font\firstlinefontB={file:lmromancaps10-regular.otf}


\setbox1\hbox{Test}
\setbox2\hbox{Test}
Test
\luadirect{
    local firstlinefont = font.id("firstlinefontB")
    print(firstlinefont)
    glyphn=node.id('glyph')
    h=tex.getbox(1)
    for n in node.traverse_id(glyphn,h.list) do
        if not (n.font == firstlinefont) then
            n.font = firstlinefont
        end 
    end
    tex.print(h)
}

\luadirect{
    firstlinefont = font.id("firstlinefont")
    h=tex.getbox(2)
    print(firstlinefont)
    for n in node.traverse_id(glyphn,h.list) do
        if not (n.font == firstlinefont) then
            n.font = firstlinefont
        end 
    end
    tex.print(h)
}

\end{document}

Answer 1

Erewhon Regular does not have a separate font file for small caps like Latin Modern does, it uses the smcp open type feature to apply GSUB substitutions to map small latin letters the glyphs in the PUA.

I'm not sure how luaotfload access this mapping, nor how can you apply the mapping, or reshape the nodes in the shaped list, but you can save the mapping your self in a table, then replace n.char accordingly.

\documentclass{article}

\usepackage{fourier-otf}

\directlua{
smcp_map = smcp_map or {}
local smallcaps = token.scan_list()
local i = 97
for g in node.traverse_id(node.id('glyph'), smallcaps.list) do
      smcp_map[i] = g.char
      i = i + 1
end
}\hbox{\scshape abcdefghijklmnopqrstuvwxyz}

\begin{document}
\setbox2\hbox{Test}
\directlua{
    local h = tex.getbox(2)
    for n in node.traverse_id(node.id('glyph'),h.list) do
         if smcp_map[n.char] then
             n.char = smcp_map[n.char]
         end
    end
    node.write(h)
}
\end{document}