luatex fontloader virtual glyph table problem

Question

I'm using Brill-Roman.otffont and with luatex I don't know how to access smallcaps glyphs from virtual fontloader.to_table returned table glyphs. There is no such index and I have no idea how to get that glyphs data. In Brill-Roman.lua there is such glyph (smallcaps D):

  [983164]={
   ["boundingbox"]=804,
   ["index"]=1146,
   ["name"]="D.c2sc",
   ["unicode"]=68,
   ["width"]=559,
  },

NWE

\documentclass{article}
\usepackage{luacode}
\usepackage{fontspec}
\setmainfont[
  Extension=.otf,
  UprightFont=*-Roman,
  UprightFeatures={
    SizeFeatures={
      { Size={9-},
          SmallCapsFeatures={Letters=UppercaseSmallCaps,LetterSpace=5.0},
          WordSpace={1,1.5,0.6}
      }
    }
  }
]{Brill}

\begin{luacode*}

local function get_real_char(font_idx, glyph_idx)
    local real_font = font.getfont(font_idx)
    local myfont = fontloader.open(real_font.filename)
    if myfont then
        metrics = fontloader.to_table(myfont)
        print ("IS_IT?", metrics.glyphs[glyph_idx])
        fontloader.close(myfont)
    end    
end

function print_glyph(h)
    for n in node.traverse(h) do
      if n.id == node.id("glyph") then
        print ("GLYPH:", n.char, n.font)
        get_real_char(n.font, n.char)
      elseif n.id == node.id("hlist") or n.id == node.id("vlist") then
        print_glyph(n.head)
      end
    end
  return h  
end  
luatexbase.add_to_callback("pre_output_filter", print_glyph, "print glyph")            
\end{luacode*}

\begin{document}
\textsc{Data} font
\end{document}

And here is the output:

This is LuaTeX, Version 1.07.0 (TeX Live 2018/W32TeX)  
  (format=lualatex 2018.6.5)  10 JUL 2018 09:43
....
GLYPH:     983164  30
IS_IT?  nil
GLYPH:  983574  30
IS_IT?  nil
GLYPH:  983593  30
IS_IT?  nil
GLYPH:  983574  30
IS_IT?  nil
GLYPH:  102     29
IS_IT?  table: 09814140
GLYPH:  111     29
IS_IT?  table: 0D4A73B8
GLYPH:  110     29
IS_IT?  table: 0E93DCF8
GLYPH:  116     29
IS_IT?  table: 106CDC88

I printed out all the table returned by fontloader and there was nothing with 983164, but there was a lot info with name D.c2sc. It seems it is already remaped to some lookup tables. I don't see any possibilities to attache files here so I can't add that font.
Question how to get n.char data (e.g. "name", "unicode") if it is not in fontloader returned virtual font table.

Answer 1

The table has to be indexed with the character index from the OTF file, not with the unicode codepoint. You can access the codepoint using the LuaTeX font table: Your real_font table contains a character hash indexed by the glyph indices used by LuaTeX and stored in the glyph nodes. In there, every character table has a index field which gives the index in the font file. This index has to be used for accessing the fontloader table. You might want to avoid to_table, the table gets very big.

Most information is also available in the LuaTeX font table directly, so you shouldn't need fontloader to begin with. Like Ulrike Fischer mentioned in a comment, the fontloader library is not normally used for anything, especially not by the luaotfload fontloader.

\documentclass{article}
\usepackage{luacode}
\usepackage{fontspec}
\setmainfont[
  Extension=.ttf,
  UprightFont=*-Roman,
  UprightFeatures={
    SizeFeatures={
      { Size={9-},
          SmallCapsFeatures={Letters=UppercaseSmallCaps,LetterSpace=5.0},
          WordSpace={1,1.5,0.6}
      }
    }
  }
]{Brill}

\begin{luacode*}

local function get_real_char(font_idx, glyph_idx)
    local real_font = font.getfont(font_idx)
    local myfont = fontloader.open(real_font.filename)
    if myfont then
        print ("Name: ", myfont.glyphs[real_font.characters[glyph_idx].index].name)
        fontloader.close(myfont)
    end    
    print ("Unicode without fontloader: ", real_font.characters[glyph_idx].unicode)
end

function print_glyph(h)
    for n in node.traverse(h) do
      if n.id == node.id("glyph") then
        print ("GLYPH:", n.char, n.font)
        get_real_char(n.font, n.char)
      elseif n.id == node.id("hlist") or n.id == node.id("vlist") then
        print_glyph(n.head)
      end
    end
  return h  
end  
luatexbase.add_to_callback("pre_output_filter", print_glyph, "print glyph")            
\end{luacode*}

\begin{document}
\textsc{Data} font
\end{document}