Fonts that combine special characters as ligature

Question

In german, there are three special characters ä,ö and ü that are often written as two: ae,oe,ue.

Is there a font, that shows an "ä" whenever the letters "ae" appear?

I thought this would be solveable with a pseudo ligature, that combines both letters.

Answer 1

AFAIK, there isn't.

In addition, this "feature" would cause more trouble than gain, because there are lots of words in German where the ligature is not wanted, e.g., aktuell, Bauer, neue, raue, Osloer, Bebraer, ...

If you really want to create such kind of font, you are free to do so: Copy a given font to a new name, convert the tfm (TeX Font metrics) file with the utility tftopl into a property list, and than add ligatures by editing the property list. The relevant section of the property list reads (here taken from the font ecrm1000 for the letter f):

(LIGTABLE
   (LABEL C f)
   (LIG C i O 34)
   (LIG C f O 33)
   (LIG C l O 35)

so you can easily add ligatures to the renamed font like

   (LABEL C a)
   (LIG C e O 304)

and convert it back to a valid tfm file by using tftopl. (In fact, you will probably find the (LABEL C a) statement initiating the section of kernings after the letter a in the tfm file.

Please note, that renaming the fonts is necessary for two reasons:

1. Avoid confusion between the originals and the changed fonts
2. It is required by the licences of typical fonts coming with a TeX distribution (e.g., the Computer Modern fonts or the ec fonts).

Answer 2

You can replace the "ae", "oe", and "ue" strings globally during input processing, by using LuaLaTeX. However, I wouldn't advise anyone actually do so. The following sample sentence illustrates why it wouldn't be a good idea.

% arara: lualatex

\documentclass{article}
\usepackage{fontspec,luacode,luatexbase} 

\begin{luacode}
function double_vowel_to_umlaut ( x )
      x =  string.gsub ( x, "ae", "ä") 
      x =  string.gsub ( x, "oe", "ö") 
      x =  string.gsub ( x, "ue", "ü") 
      return ( x )
end
\end{luacode}
\AtBeginDocument{\directlua{%
   luatexbase.add_to_callback( "process_input_buffer", 
      double_vowel_to_umlaut, "doublevowel_to_umlaut")}}

\begin{document}
Samuel, Joel, und Rafael trafen sich beim Taekwondo zu neuen Duellen. 
\end{document}