[luatex] What are user-defined whatsit nodes?

[Stephan Hennig]

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.

I'd like to generalize the idea a bit.  There are other typographic
problems that can be solved by node list operations.  Think about
long-s/round-s substitution.  I'd welcome luaotfload doing the actual
substitution (the font handling), but would like to give hints to
luaotfload about where to apply the substitution and where not.  What do
you think?

Best regards,
Stephan Hennig