LuaLaTeX/dvipng: conversion fails due to missing font

Question

I am trying to generate an image from a formula using LuaLaTeX and dvipng. Some of the formulas contain non-ascii characters like umlauts. When executing dvipng, I get errors like this:

This is dvipng 1.14 Copyright 2002-2010 Jan-Ake Larsson
kpathsea:make_tex: Invalid fontname `file:lmroman12-regular:script=latn;+trep;+tlig;', contains ':'
[1dvipng warning: font file:lmroman12-regular:script=latn;+trep;+tlig; at 400 dpi not found, characters will be left blank dvipng warning: unable to draw glyph 72, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 76, skipping dvipng warning: unable to draw glyph 76, skipping dvipng warning: unable to draw glyph 76, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 33, skipping kpathsea:make_tex: Invalid fontname `file:lmroman12-regular:script=latn;+trep;+tlig;', contains ':'
dvipng warning: font file:lmroman12-regular:script=latn;+trep;+tlig; at 400 dpi not found, characters will be left blank dvipng warning: unable to draw glyph 72, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 65, skipping dvipng warning: unable to draw glyph 76, skipping dvipng warning: unable to draw glyph 76, skipping dvipng warning: unable to draw glyph 76, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 79, skipping dvipng warning: unable to draw glyph 33, skipping ]

It complains about a missing font, but it definitely exists under ./share/texmf/fonts/opentype/public/lm/lmroman12-regular.otf.

The error occurs with the following MWE, compiled using lualatex --output-format=dvi test.tex:

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec} % if commented out, it works
\usepackage[active,textmath,displaymath]{preview}
\begin{document}
$\text{ä} < \text{ö}; ø\neq æ$
\end{document}

I'm using texlive-full (2016) on Debian testing. I would appreciate any pointers or explanations, I am stuck at this point, while researching the web for answers.

Answer 1

As Heiko and David pointed out, using dvipng with LuaLaTeX is impossible. For my application, I have to use dvipng, because as far as I know, it is the only utility, which also outputs the baseline to position the image within a HTML document. I ended up writing a few "conversion" functions, which just replace all non-ascii characters with their corresponding LaTeX sequence. There's a pretty extensive unicode-to-latex table at https://raw.githubusercontent.com/w3c/xml-entities/gh-pages/unicode.xml. Thanks to David Carlisle for maintaining it!