[luatex] Kerning: when does it not work?

[Heilmann, Till A.]

I am trying to understand--as a mere user, I must confess--exactly when and why kerning (or, to be more precise, my own adjustments to the kerning of a given OpenType font made in an OTF feature file) does _not_ work.

In the last couple of weeks, I‘ve received excellent advice from the community and was able to solve a few of my problems. But some problems persist and I keep stumbling over new ones. For example, customized kerning does not work at some positions in URLs typeset with the url package and is also suppressed at some positions of citations and bibliographic entries generated by biblatex (see minimal example below).

>From Khaled Hosny, Ulrike Fischer, Philipp Lehmann and others I learn that a fundamental issue (if not the _one_ cause for all the seemingly diverse problems I encounter) is the existence of penalized breakpoints: Even if no line breaking occurs the kerning is suppressed at that point. Philipp told me that this was a long-standing limitation of TeX that apparently has not been lifted in LuaTeX.

My--fairly general--question is whether I am indeed dealing here with various implications of a single cause within the code of (Lua)TeX and, if yes, if there is any chance of this being addressed in the foreseeable future.

I don't want to sound ungrateful considering the developer's fantastic commitment and the immense work the whole community is putting into (Lua)TeX and its diverse packages. It is simply driving me crazy that I have such a powerful and sophisticated tool for digital typesetting at hand and still I cannot get something as (typographically) fundamental as customized kerning to work. Unfortunately, one cannot always choose the fonts used in a document and many (if not most) professional fonts contain some really poor kern pairs that have to be corrected.

Thanks and best regards
--Till

\begin{filecontents*}{test.bib}
@article{stiegler.b:2009,
  Author = {Bernard Stiegler},
  Journal = {Theory, Culture~\& Society},
  Number = {2–3},
  Pages = {33-45},
  Title = {Teleologics of the Snail},
  Volume = {26},
  Year = {2009}}
\end{filecontents*}
\begin{filecontents*}{mybonum.fea}
languagesystem DFLT dflt ;
languagesystem latn dflt ;
feature kern {
  pos six slash 100 ;
  pos slash two 100 ;
  pos colon slash 100 ;
  pos slash slash -150 ;
} kern ;
\end{filecontents*}
\documentclass{article}
\usepackage{url}
\urlstyle{same}
\usepackage{fontspec}
\usepackage[ngerman]{babel}
\setmainfont[FeatureFile=mybonum.fea]{TeX Gyre Bonum}
\usepackage[style=authortitle-dw,backend=biber]{biblatex}
\bibliography{test}
\makeatletter
\renewcommand*{\jourvolnumsep}{/}
\makeatother
\begin{document}
\fullcite{stiegler.b:2009} 6/2\par
http:// \url{http://}
\end{document}