How do you select individual end alternate/swash characters with LuaLaTeX and fontspec

directluafontspecgaramondluatexopentype

Basic Problem Statement

I would like to enable some (but not all) stylistic alternate ".end" characters from an OpenType font, on condition that they replace terminal characters in a word.

Unfortunately, using the +salt feature enables them globally throughout text, which I do not want.

So far, I have tried using \directlua to build a chainsubstitution, then add it via RawFeature, but this is producing very odd results.

I expect this is likely an error in usage of \directlua on my part—one for which I would greatly appreciate correction.

Indeed, to me, my approach seems a bit of a hack, particularly when it comes to listing all the conditions to trigger substitution. If there is an altogether different method for making this substitution smartly, I am happy to hear about it.

MWE

This is a simple example of the problem. I am using Adobe Garamond Premier Pro. Regrettably, I am unaware of a free choice at hand which features the same ".end" characters for this situation.

For ease, (I think) I am setting "a.end" to replace "a" whenever "a" occurs just before a period.

With this snippet, the output is as expected only with the normal Roman and boldface fonts. All italic shapes somehow produce the "d.end" character instead.

\documentclass{minimal}
\usepackage{fontspec}

\directlua{
  fonts.handlers.otf.addfeature{
    name = "asub",
    type = "chainsubstitution",
    lookups = {
      {
        type = "substitution",
        data = {
          ["a"] = "a.end",
        },
      },
    },
    data = {
      rules = {
        {
          after  = { { "." } },
          current = { { "a" } },
          lookups = { 1 },
        },
      },
    },
  }
}

\setmainfont[{RawFeature=+asub}]{Garamond Premier Pro}

\begin{document}

a.

{\itshape a. } 

{\bfseries a. }

{\bfseries\itshape a. }

\end{document}

MWE Output

MWE Output

Notes

  • Using +salt does produce the correct "a.end" glyph, no matter what choice of bold and/or italic. On checking the different OTF files with FontForge, I see the "a.end" glyph is not in the same slot for each font, but the name is always the same.
  • This is just one example of such a broken substitution. The "h.end" character behaves similarly.

Best Answer

This was caused by a bug in luaotfload. It will be fixed in version 1.19, but until that is released the development version can be used to avoid the error:

Run:

git clone --branch dev --depth 1 https://github.com/latex3/luaotfload.git
cd luaotfload
l3build install

Then compiling with lualatex-dev will use the development version of luaotfload instead of the last release and the code in the question works.

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