Re: [NTG-context] Contextual removal of some ligatures using fonts.handlers.otf.addfeature (LuaTeX 1.13.2)

25 Dec 2021

      Hi Bruce and Hans !

Thank you very much for your answers, that helped me a lot.

In fact, though I would love to switch to ConTeXt one day, I am still using LuaLaTeX — I am a slow 
learner and I know it will take me months to be able to achieve what I need for my job with 
ConTeXt and can do with LuaLaTeX and the Memoir class for the time being (despite sometimes 
being irritated by LaTeX limitations or oddities).

I need the EB Garamond variable font for future jobs, but this font, provided by Google Fonts, is 
such a mess : kerning problems, unusable OpenType features that have to be rewritten from 
scratch, shifts in glyphs table… For sure, the original font by Georg Duffner was much better, but I 
need the variable version. Lots of fixing necessary, but thanks to fonts.handlers.otf.addfeature I 
have almost succeeded to make it a usable Garamond.

Of course, it would be best to help Google improving this font, but I don’t have time yet.

The actual problem was with the italic ligatures "a_s", "e_s", "é_s", "i_s" and "u_s", that should occur 
only at the end of words and not everywhere. Instead of removing the unwanted ligatures, I chose 
another way as soon as I understood how to fully use “current” to describe not only a single glyph, 
but a string if necessary — for instance, current={ { "a", "e", "eacute", "i", "u"}, {"s"} }

Thanks for this wonderful tool !

Thomas Savary
1 le Grand-Plessis
F-85340 L’Île-d’Olonne
Tél. 06 22 82 61 34
https://compo85.fr/
Le samedi 25 décembre 2021, 00:04:51 CET Bruce Horrocks a écrit :
...
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On 24 Dec 2021, at 18:34, Thomas Savary via ntg-context
 wrote:
Hello, dear list !
In order to fix a few bugs in some OpenType features included in EB
Garamond font provided by Google Fonts, I am trying to remove some
ligatures (erroneous in some contexts) with
fonts.handlers.otf.addfeature.
I must be missing something, because I can't get it to work.
Here is a minimal —non working—example :-)
(The attempted removal (“f_i” → “fi” only before “o”) is of course
irrelevant. I chose it just for the sole purpose of this test, to
provide a simple example. What I am trying to achieve is more
complex, with other ligatures.)
fonts.handlers.otf.addfeature{
name = "unfio",
        type = "chainsubstitution",
        lookups = {
{ type = "multiple",
            data = { [ "f_i" ] = { "f", "i" } }
            }
}
        rules = {
{ after = { { "o" } },
            current = { { "f_i" } },
            lookups = { 1 } }
}
}
}
Could someone please explain to me what I did wrong ?
Firstly, you have an extra } in the example above.
Secondly, rules need to come inside a data section. Thus (untested):
fonts.handlers.otf.addfeature{
    name = "unfio",
    type = "chainsubstitution",
    lookups = {
        {
            type = "substitution",
            data = {
                [ "f_i" ] = { "f", "i" }
            }
        }
    },
    data = {
        rules = {
            {
                after = { { "o" } },