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How do I get "Rare/Discretionary" ligatures with the Latin Modern Roman Font in LuaLateX?

So far I have loaded the fontspec package and have tried several different approaches the most promising looking being: (from here)

\documentclass[10pt]{article}
\usepackage{fontspec}
\setmainfont[Ligatures=Rare,SmallCapsFont={Latin Modern Roman Caps}]{Latin Modern Roman}
\begin{document}
Unfortunately though, the Q of Queen still has a short tail, film however has the correct ligature.
\end{document}

I fear the answer may be: Use XeTeX. However I am using LuaLateX for drawing trees and therefore could not move to XeTeX without significant work. (As far as I am aware.)

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    The Latin Modern fonts don't provide that feature.
    – egreg
    Commented Feb 19, 2016 at 21:49
  • @egreg That explains the problem! For future reference where can I check this sort of thing for myself?
    – o.comp
    Commented Feb 19, 2016 at 22:01
  • There's no general method, I'm afraid. The Latin Modern fonts do have the dlig feature, but I don't really know which ones it enables.
    – egreg
    Commented Feb 19, 2016 at 22:14
  • 1
    Looking with FontForge, I see that the dlig table just implements the standard TeX ligatures. As far as I know Libertinus Serif has the long tailed Q (enabled by default).
    – egreg
    Commented Feb 19, 2016 at 22:24
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    otfinfo -f <font file> will tell you which features a font file supports. However, it won't, on its own, tell you what exactly each feature does for that font, as @egreg points out.
    – cfr
    Commented Feb 20, 2016 at 0:11

1 Answer 1

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Ligatures are font dependent; it's the font designer/developer who decides which one to enable in the various tables.

For instance, Latin Modern Roman has a dlig feature, which is enabled in fontspec with Ligatures=Rare or Ligatures=Discretionary, but the relative subtable just shows that essentially the standard TeX ligatures are enabled (which is already done by the default Ligatures=TeX option.

In general it should be a duty of the font distributor to provide instructions about what features the font is endowed with. Unfortunately this is not done very frequently. Using FontForge is most of the times the simplest way to get the relevant information.

The picture shows a screen shot of how I looked at the subtable responsible for the dlig feature.

enter image description here

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