[luatex] information about ligatures

[Paul Isambert]

Stephan Hennig <mailing_list at arcor.de> a écrit:
> Am 31.12.2013 09:37, schrieb Paul Isambert:
> > Stephan Hennig <mailing_list at arcor.de> a écrit:
> > 
> > Ligatures are char nodes (id 37) with special subtype 2, and they have
> > a “components” field which is a nodelist containing the ligature’s
> > components.
> 
> I have already read about subtype 2 and the components field, but have
> never seen a glyph node of that subtype in pre_linebreak_filter.
> Instead, I can see glyph nodes of subtype 256 corresponding to standard
> Unicode ligatures, e.g., 0xfb02 (fl).  That is, bit 8 is set in subtype,
> which I can't find any documentation about.  For that reason, I have
> never checked the 'components' field, but it is indeed there.  Thanks!

[...]

> In fact, all top-level glyph nodes seem to be of subtype 256 in
> pre_linebreak_filter.  What does that mean?  (You can find the full node
> list corresponding to TeX input 'flavour specific office trick' at the
> end of this mail.  With a proper font, the ck ligature is also present
> there.)
> 
> Can somebody please provide TeX input that results in a glyph node with
> bit 1 of subtype set?

My two cents is that you’re using luaotfload and that luaotfload sets
“subtype” to 256 for some reason; you’ll see ligatures with proper
subtype 2 if you let TeX do its job with fonts (so use TFM, obviously).
E.g. this in plain TeX:

    \setbox0=\hbox{fi}
    \directlua{%
      texio.write_nl("This ligature has subtype " .. tex.box[0].head.subtype)
    }
    \bye

It should print “This ligature has subtype 2”.

Why luaotfload sets “subtype” to 256 – if that is indeed the case –, I
really don’t know.

Best,
Paul