How can I separate an fl ligature in Libertine?

Question

I am having trouble to understand the ligatures behaviour of the Linux Libertine font.

I want to use all ligatures that are possible to have (Ligatures={TeX}) but would like to disable single cases. I would have expected that babel would do that for me. When playing around with this, I found out that I am able to get three different fl ligatures but not a real space between those two letters. What is happening here? Am I doing something wrong? Or shouldn't I separate that two letter completely, even when they belong to different word parts.

See here what happens for the German word "Auflage":

Edit: I insert all mentioned solutions, too. They are differing in the distance between f and l but all are still collisioning. The line break possibilities are differing, too.

Edit 2: I switched from \\ to blank lines between the different cases due to this post. Now it shows the actual hyphenation possibilities for each case in the first picture.

% arara: lualatex
\documentclass[ngerman]{article}

\usepackage[final]{microtype}
\usepackage{fontspec}
\usepackage{luatextra}
\defaultfontfeatures{%
    ,Ligatures={TeX}
    ,Scale=MatchLowercase
}
\setmainfont[Mapping=tex-text]{Linux Libertine O} 
\usepackage{showhyphens}
\usepackage{babel}

\begin{document}
Auflage

Auf{l}age

Auf"-lage

Auf\/lage

Auf"|lage

{\addfontfeatures{Ligatures=NoCommon}Auflage}
\end{document}

Without showhyphens the missing separation is more visible.

Answer 1

Let's walk through all cases:

• Au{f}lage (or Auf{l}age or Auf{}lage})

  (I)

  This does not break up the ligature at all in luatex, and in fact (contrary to widespread belief) has never reliably done so even in pre-luatex engines (eg. pdftex, tex).

• Auf"-lage

  (II)

  This shortcut from German babel inserts a hyphenation point allowing hyphenation in the rest of the word. The side effect is that the ligature is broken up, but there is no kerning between the letters, because even if kerning for this pair were defined in the font it would be inhibited by what has been inserted.

• Auf\/lage

  (result same as II)

  This primitive, meant for italic correction, will insert a kern of 0pt, also resulting in the breaking up of the ligature, but (well...) no kerning. A negative side effect is that there will be no hyphenation in the remainder of the word.

• {\addfontfeatures{Ligatures=NoCommon}Auflage} or (to not disable ligatures in the whole word) even Au{\addfontfeatures{Ligatures=NoCommon}fl}age

  (result same as II)

  Disabling ligatures by means of fontspec, but again no kerning (which tells us that kerning is indeed not defined for this pair in the font). And too much typing anyway.

• Auf"|lage

  (III)

  This shortcut, again from German babel, was actually designed for cases like this. It will break up the ligature, allowing hyphenation in the rest of the word, and insert a kerning of 0.03em, which for this font, however, is simply not enough to completely separate the letters. The kerning amount is hardwired in the language definition file -- you may still change it, though: see below.

So there are three different results: (I) ligature, (II) ligature broken up, but no kerning, and (III) ligature broken up, with a small amount of kerning.

The last case is surely the best approach so far. Now, to change the amount of kerning you could redefine the existing shortcut (after loading babel, surrounded by \makeatletter/\makeatother):

\declare@shorthand{ngerman}{"|}{%
  \textormath{\penalty\@M\discretionary{-}{}{\kern.07em}% <- originally 0.03em
              \allowhyphens}{}}

Or you could even create a new one, to be exclusively used for the fl ligature:

\declare@shorthand{ngerman}{"/}{%
  \textormath{\penalty\@M\discretionary{-}{}{\kern.07em}%
              \allowhyphens}{}}

and then write Auf"/lage.

Another possibility has already been mentioned: the selnolig package. This would surely be the best solution as it doesn't require any effort whatsoever on the part of the user. Unfortunately, however, selnolig will also only break up the ligature and not insert any kerning. It would be a feature request for @Mico to make this customisable, ideally font and glyph-dependent.

---

Finally, not quite related: Did you notice that there is no hyphenation point after Auf- in the cases of intact ligatures? This is not a bug in the German hyphenation patterns, but rather in luaotfload, see here. It can be solved by loading the font with Renderer=Basic. Of course, in this case it won't matter, but it would with a word like auffliegen.

Answer 2

There are a couple of possible answers.

One is to use {} to encase the second letter of the (not to be made into a) ligature pair, in this case: Auf{l}age, Suf{f}rage. OOPS, I JUST NOTICED: this solution doesn't work under LuaLatex. Instead, try the explicit correction for italics, Auf\/lage.

A second possible approach is the selnolig (selective no ligature) package. It's described as providing extensive rules for the selective suppression of so-called common ligatures for English and Ger­man language doc­u­ments. The package can be used in German by invoking: \usepackage[ngerman]{selnolig}.

Answer 3

Activate the option Ligatures = NoCommon. See the fontspec manual under 10.1.

\documentclass{article}
\usepackage{fontspec}
\setmainfont[Ligatures={TeX,NoCommon}]{Linux Libertine O}
\begin{document}
Auflage
\end{document}

To turn off ligatures for "single cases" (if I understand you correctly when you use that phrase), you can add the Ligatures = NoCommon option at will with \addfontfeatures, see the fontspec manual under 7.2.

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\begin{document}
Auflage {\addfontfeatures{Ligatures=NoCommon}Auflage}
\end{document}

If you want to push the two characters in the fl sequence further apart, you can add \kern when you disable the ligature:

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\newcommand{\fl}{f{\kern0.075em}l}
\begin{document}
Auflage Au\fl{}age
\end{document}

I personally wouldn't kern more than 0.075em, because I think the stems and feet of the two characters otherwise are separated too much, which in my view is esthetically inferior. There is, after all, a reason why the fl ligature is commonplace in non-monospaced fonts.

As mico suggests, you can use an f with a shorter arm. That way that f and the l don't collide:

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\newcommand{\fl}{{\char"E0E0}l}
\begin{document}
Auflage Au\fl{}age
\end{document}

Answer 4

The following is really just a lengthy follow-up comment on some of the issues discussed in the earlier answers that address the OP's questions. Because this comment is too long to fit in an ordinary-comment space, I'm posting it as a separate answer.

I believe the OP raises two separate issues:

• How to break up the fl-ligature in a word such as "Auflage" -- while not suppressing all fl-ligatures, let alone all f-ligatures, globally; and
• How to assure that the now-separate f and l glyphs don't "collide" visually.

The other answers have already mentioned various TeX- and babel-based methods for suppressing an fl-ligature on a case-by-case basis. A quick comment on the method that relies on inserting an "italic correction," viz., \/ (an "explicit kern" in TeX jargon) to break up a ligature: In my view, this method may produce a reasonable result (for some, but not necessarily) all font families, if it's used on upright-shape and normal-weight letters. Unfortunately, it tends to produce awful-looking results if the words in question are set in italic and/or bold letters. If you don't use the selnolig package for the purpose of suppressing typographic ligatures, I would strongly recommend either employing the babel shortcut "| method or the insertion of an "implicit kern", such as \kern0pt or \hspace{0pt}.

For what it's worth, the selnolig package -- which, by the way, requires use of the LuaLaTeX format -- employs an entirely different approach to suppressing typographic ligatures: It inserts so-called "whatsits" during the phase of the typesetting process when LuaTeX converts various character pairs and triples (such as ff, fi, fl, ffi, ffl, ft, fft, etc.) into single glyphs. Once the ligatures are suppressed, the "whatsits" disappear and therefore do not affect further phases, such as the hyphenation stage, of the paragraph building process.

Regarding the second issue: I think that adding a non-zero (implicit) kern between f and l for words whose fl ligature should be broken up is not ideal from a typographical point of view. This issue is particularly acute for fonts in which the f glyph has a long "arm" -- as is the case with Linux Libertine. While inserting a kern of sufficient width will indeed succeed in separating the f and l glyphs visually, it comes at the cost of creating a noticeable (and hence distracting) visual hole in words such as "Auflage", "drauflegen", "reiflich", and so on.

In my view, a better solution involves selective replacement of the ordinary "long-arm" f character -- if followed by i, j, l (or any other character that might create an unsightly collision) -- with a "short-arm" f glyph from the same font family. The good news is that quite a few Opentype font families offer such "short-armed" f glyphs. The bad news is that it takes some work to perform this character substitution. In the next version of selnolig, which I hope to release in the next couple of weeks, this method is implemented for the font family EB Garamond, which features short-armed versions of both f and ff. I'm currently investigating possibilities for implementing the long-arm-f to short-arm-f substitution for Linux Libertine as well. Stay tuned...