[luatex] linebreak_filter example implementation

[Paul Isambert]

Élie Roux <elie.roux at telecom-bretagne.eu> a écrit:
> > Well, modifying linebreak_filter would be a good idea indeed :)
> >
> > Otherwise, you could put the clef in a \cleaders:
> >
> >      \newbox\clefbox
> >      \setbox\clefbox=\hbox{<clef>}
> >      \def\clef{\cleaders\copy\clefbox\hskip\wd\clefbox}
> >
> >      note ... note ... \clef ... note ... note
> >
> > then it will automatically disappear at the beginning of a line. Of
> > course the above code is cumbersome, all the more as<clef>  won’t
> > interact with surrounding glyphs if that must happen (e.g. spacing).
> >
> > So you could use the symbol as usual, and then in pre_linebreak_filter
> > (i.e. after kerning, ligaturing, etc., have occurred, i.e. after the
> > glyph has been processed) turn the sublist made of that glyph and
> > associated material into an hlist, and create a leader node pointing
> > to it (really, a glue with a “leader” field pointing to the created
> > box).
> >
> > I guess it’ll work, and it’s much simpler than tinkering with the
> > algorithm (all the more as the Lua version is much slower than the TeX
> > version, if I remember correctly a recent paper by Hans).
> 
> Hmmm, this opens some interesting perspectives! I have to say I would 
> *never* have thought to something like this... I've never used cleaders 
> but I'll learn...
> 
> If between two book readings you ever have a little time to help... ;)

See attached file, which still requires some work:

- Basic optimization (local’s, etc.); the leaders nodes could be
created beforehand (one for each key) and reused with “node.copy()”.
- Populate the “clefs” table.
- Define the “getnodes()” function properly (I don’t know what nodes
are associated with a clef, so I couldn’t do that for you).
- Replace the “callback.register()” call with whatever you use.

As a quick test, you can:

- Add “[65] = true” to the “clefs”.
- Run it on some text with interspersed A’s and a small \hsize.
- You’ll see the A’s disappear at linebreaks. Wow.

> Thank you very much,

Welcome. After all, you’re the one who converted me to LuaTeX :)

Best,
Paul
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