[luatex] What are user-defined whatsit nodes?

[Khaled Hosny]

On Mon, Nov 24, 2014 at 08:02:23PM +0100, Stephan Hennig wrote:
> Am 24.11.2014 um 00:38 schrieb Hans Hagen:
> > On 11/23/2014 11:49 PM, Stephan Hennig wrote:
> >>
> >> Is there any interface available that given two glyphs returns
> >> a ligature of those glyphs?
> > 
> > that's not the way open type works ... some fonts can turn ffl into the 
> > ffl ligature, others do it stepwise and yet others implement for 
> > instance ff ligatures by kerning two (different) f's, in fact the whole 
> > concept of ligature in open type is a kind of generic substitution and 
> > does not always relate to ligatures .. the feature can be mixed into 
> > other features as well
> 
> OK.  As I don't want to put focus on the actual low-level ligaturing
> anyway, then, is there any interface to communicate to luaotfload where
> to put ligatures and where not?  The whatsit approach seems to work for
> the time being, but I'd prefer an official communication channel
> nevertheless.

The “official” Unicode was for suppressing a ligature is by inserting a
ZWNJ character, so I guess inserting a whatsit is as close (of course if
one can get a ZWNJ to the output stream so that ligature suppression is
maintained even when text is copied around, that would be even better
IMO).

Regards,
Khaled