luaotfload output enhancment for DVI mode

luadvi.lua

local getfont = font.getfont
local setfont = node.direct.setfont
local getdisc = node.direct.getdisc
local traverse_glyph = node.direct.traverse_glyph
local traverse_id = node.direct.traverse_id
local define_font = font.define
local disc_t = node.id'disc'
local mapped_fonts = setmetatable({}, {__index = function(t, fid)
  local font = getfont(fid)
  local mapped = font.backend_font or false
  t[fid] = mapped
  return mapped
end})
local function process(head, font)
  local mapping = mapped_fonts[font]
  if not mapping then return head end
  local mapped_font = mapping.font
  for n, c, f in node.direct.traverse_glyph(head) do if f == font then
    local mapped = mapping[c]
    if mapped then setfont(n, mapped_font, mapped) end
  end end
  for n in node.direct.traverse_id(disc_t, head) do if node.direct.uses_font(n, font) then
    local pre, post, rep = getdisc(n)
    for n, c, f in node.direct.traverse_glyph(pre) do if f == font then
      local mapped = mapping[c]
      if mapped then setfont(n, mapped_font, mapped) end
    end end
    for n, c, f in node.direct.traverse_glyph(post) do if f == font then
      local mapped = mapping[c]
      if mapped then setfont(n, mapped_font, mapped) end
    end end
    for n, c, f in node.direct.traverse_glyph(rep) do if f == font then
      local mapped = mapping[c]
      if mapped then setfont(n, mapped_font, mapped) end
    end end
  end end
end
local function manipulate(tfmdata)
  if 2 ~= (tfmdata.encodingbytes or ((tfmdata.format == 'truetype' or tfmdata.format == 'opentype') and 2 or 1)) then
    return
  end
  local newfont = {}
  for k,v in pairs(tfmdata) do
    newfont[k] = v
  end
  local newchars = {}
  newfont.characters = newchars
  local lookup = {}
  for k,v in pairs(tfmdata.characters) do
    local newchar = {
      width = v.width, -- Only width should really be necessary
      height = v.height,
      depth = v.depth,
    }
    newchars[v.index or k] = newchar
    lookup[k] = v.index or k
  end
  newfont.checksum = string.unpack('>I4', 'LuaF')
  local name = '[' .. newfont.name .. ']:'
  if newfont.subfont and newfont.subfont ~= 1 then
    name = name .. 'index=' .. tostring(math.tointeger(newfont.subfont-1)) .. ';'
  end
  if newfont.extend and newfont.extend ~= 1000 then
    name = name .. 'extend=' .. tostring(math.tointeger(math.round(newfont.extend*65.536))) .. ';'
  end
  if newfont.slant and newfont.slant ~= 0 then
    name = name .. 'slant=' .. tostring(math.tointeger(math.round(newfont.slant*65.536))) .. ';'
  end
  if newfont.mode == 2 and newfont.width and newfont.width ~= 0 then
    name = name .. 'embolden=' .. tostring(math.tointeger(math.round(newfont.width*65.78176))) .. ';'
  end
  newfont.name = name:sub(1,-2)
  tfmdata.backend_font = lookup
  lookup.font = define_font(newfont)
end

fonts.constructors.features.otf.register {
  name = "dvifont",
  default = true,
  manipulators = {
    node = manipulate,
    base = manipulate,
  },
  processors = {
    node = process,
    base = process,
  },
}

test.tex

\RequirePackage{iftex}
\ifLuaTeX
  \directlua{require'luadvi'}
\fi
\documentclass{article}
\pagestyle{empty}

\usepackage{fontawesome5}
\usepackage{fontspec}
\setmainfont{Latin Modern Roman}

\begin{document}
\Huge
\faArrowRight\faArrowLeft
abc
\addfontfeature{FakeBold=1}
def
\addfontfeature{FakeStretch=2}
ghi
\addfontfeature{FakeSlant=1}
jkl
\end{document}

I get a segmentation fault with

dvisvgm --zoom=-1 --exact --font-format=woff2 test.dvi

@agrahn That's because dvisvgm doesn't support this way of passing fonts currently. Martin is working on dvisvgm to add this support.

Thank you all, Martin (@mgieseki) and Marcel!