Is there a way to get glyph/text outlines in MetaPost in LuaLaTeX?

Question

I need to get outline paths for certain text in MetaPost in LuaLaTeX to use them for some further manipulations, similar to what you can do in ConTeXt:

\starttext
    \startMPcode
        picture p;
        p := outlinetext("TEST") scaled 10;
        for i within p:
            j := j + 1;
            drawarrow pathpart i;
        endfor;
    \stopMPcode
\stoptext

outlinetext from ConTeXt doesn't seem to work under LuaLaTeX, and examples like this I couldn't get to work either. Is there a way to achieve this?

Answer 1

My first approach to solve this would be simply to turn on the metafun format provided by luamplib, like this:

\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibsetformat{metafun} 
\begin{mplibcode}
beginfig(1);
     picture p;
        p := outlinetext("BEST") scaled 10;
        for i within p:
            j := j + 1;
            drawarrow pathpart i;
        endfor;

endfig;
\end{mplibcode}
\end{document}

The metafun format provides an outlinetext macro, but unfortunately in my copy of TexLive 2023 it does not work. If I compile the above document with lualatex, I get these error messages (and no output):

mplib warning: error in script: [string "mp.mf _outline_text(1,[===[BEST]===],[===[]===..."]:1: attempt to call a nil value (field 'mf_outline_text')
mplib warning: error in script: [string "mp.mf_get_outline_text(1)"]:1: attempt to call a nil value (field 'mf_get_outline_text')

So a second approach would be to define my own outlinetext macro. The Context implementation is rather complex and deals properly with kerning and vertical movement and so on, but it is not too hard to make a version that ignores all these complexities and works ok for simple cases.

\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\begin{mplibcode}
vardef outlinetext(expr s) = 
  save c, g, x; string c; picture g; numeric x;
  image(
    for i = 1 upto length s:
      c := substring (i-1, i) of s;
      g := glyph ASCII c of defaultfont scaled 1/64;
      x := xpart lrcorner currentpicture;
      for item within g:
        draw pathpart item shifted (x, 0);
      endfor
    endfor
  )
enddef;
beginfig(1);
  % defaultfont := "pplri8r";  % <- uncomment to try another font
  ahangle := 30;
  picture p; p = outlinetext("BESTA&?") scaled 10;
  for item within p:
    drawarrow pathpart item;
  endfor;
endfig;
\end{mplibcode}
\end{document}

Compile this with lualatex to get a PDF that looks like this:

Answer 2

It's fairly easy to trick LaTeX into loading the ConTeXt support code, so we can make a pretty close replica of the ConTeXt outlinetext function:

\documentclass{article}
\pagestyle{empty}

\usepackage{luamplib}

{\catcode`\%=12
\directlua{
    do --[[ Define some variables that ConTeXt expects to be defined ]]
        nodes.rulecodes = {}
        nodes.nuts = node.direct
        nodes.nuts.isglyph = node.direct.is_glyph
        nodes.nuts.effectiveglue = node.direct.effective_glue
        function nodes.nuts.getreplace(n)
            local _, _, h, _, _, t = node.direct.getdisc(n,true)
            return h, t
        end

        --[[ Load the ConTeXt Lua module ]]
        require("font-mps")
    end

    --[[ Expose the Lua function to Metapost ]]
    luamplib.everymplib[""] = luamplib.everymplib[""] .. [[
        def textoutline (text t) =
            runscript("textoutline('" & t & "')")
        enddef;
    ]]

    function textoutline(text)
        --[[ Create a box holding the provided text ]]
        local box = tonumber(
            luamplib.maketext(text):match("mplibtexboxid=(%d+)")
        )

        --[[ Convert the box to a MetaPost string ]]
        local mp_str = fonts.metapost.boxtomp(box)

        --[[ Clean up the MetaPost code so that it works with LaTeX ]]
        mp_str = mp_str:gsub("mfun_do_outline_text_flush%([^,]-,[^,]-,([%d.-]+),([%d.-]+),[^)]-%)(%b());", "draw %3 shifted (%1,%2);")
                       :gsub("checkbounds%b();", "")

        mp_str = "image(" .. mp_str .. ")"

        --[[ Insert the MetaPost code into the document ]]
        mp.print(mp_str)
    end
}}

\usepackage{fontspec}
\setmainfont{texgyrechorus-mediumitalic}

\begin{document}
    \begin{mplibcode}
        beginfig(1);
            picture t; t := textoutline("\\TeX \\textsf{\\TeX}") scaled 10;
            for item within t:
                fill pathpart item withcolor .9white;
                draw pathpart item dashed evenly withcolor red;
            endfor
        endfig;
    \end{mplibcode}
\end{document}

The example given in the MetaPost manual also works unmodified, with the condition that you can only use old-school Type 1 fonts:

\documentclass{article}
\pagestyle{empty}

\usepackage{luamplib}

\begin{document}
    \begin{mplibcode}
        beginfig(1);
            picture q; path p;
            interim ahlength := 12bp;
            interim ahangle := 25;

            q := glyph "Dcaron" of "ec-lmr10" scaled .2;

            for item within q:
                p := pathpart item;

                drawarrow p withcolor (.6,.9,.6)
                    withpen pencircle scaled 1.5;

                for j=0 upto length p:
                    pickup pencircle scaled .7;
                    draw (point j of p -- precontrol j of p)
                        dashed evenly withcolor blue;

                    draw (point j of p -- postcontrol j of p)
                        dashed evenly withcolor blue;

                    pickup pencircle scaled 3;
                    draw precontrol j of p withcolor red;
                    draw postcontrol j of p withcolor red;

                    pickup pencircle scaled 2;
                    draw point j of p withcolor black;
                endfor
            endfor
        endfig;
    \end{mplibcode}
\end{document}

Thruston's answer shows how to convert the above glyph-drawing function into the desired string-drawing function.