[Dev-luatex] letterspacing and (no) ligatures

Robert w.m.l at gmx.net
Sun Sep 18 04:26:36 CEST 2011

On 07.09.11 08:15, Paul Isambert wrote:
> Le 07/09/2011 05:03, Robert a écrit :
>> On 06.09.11 10:31, Hans Hagen wrote:
>>> I cannot test it here but you should use mode=base which will create a
>>> static font that is handled by tex itself as the tex machinery will copy
>>> the font data when you do that kind of letterspacing. The engine is
>>> ignorant of anything related to the font at the lua end so one might
>>> loose some very specific features provided outside the engine but that's
>>> no big deal as letterspacing spoils the game anyway. Smallcaps and
>>> oldstyles normally are no problem.
>> Thanks, that works. Would you mind elaborating on the difference
>> between base and node mode a bit, or could you point me to the
>> relevant documentation? I'm unable to find anything more specific than
>> that base mode does not support Opentype "fully" (luaotfload). But
>> what exactly is not supported, what would you lose by switching from
>> node to base mode?
> If I'm not mistaken, "base mode" implements OT features like traditional
> TeX fonts, and thus only those features supported by TeX can be used;
> for instance, simple ligatures or kerning is easily done this way,
> because those already exist for TFM fonts.
> But contextual features, for instance, or even simple ligatures with
> more than two input glyphs, aren't supported this way, because TeX knows
> nothing about them; there is nothing devoted to them in the way fonts
> are organised. So you use "node mode", which boils down to inspecting
> lists of nodes before the paragraph is constructed (in one of the
> callbacks before linebreak_filter) and changing what must be changed; of
> course the traditional features can be implemented this way too.

Thanks for the information. I will tell users then to use base mode, for 
the time being.

Probably not surprising, but just for the record (and I guess this won't 
change, either): The same loss of font features as with \letterspacefont 
also happens with \pdfcopyfonts.


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