Graphite features (Chinese Pinyin and low diacritics) on Gentium Plus

Question

I'm trying to activate the Graphite features on Gentium Plus (Chinese Pinyin tone marks + low diacritics). This is my minimal example:

\documentclass[utf8,12pt,letterpaper]{article}
\usepackage{fontspec}
\begin{document}
\fontspec[Renderer=Graphite,RawFeature={1057=1},RawFeature={1054=1}]{Gentium Plus}

mā má mǎ mà

\end{document}

However, the features are not activated when I compile with XeLaTeX. On the contrary, I get this warning:

*************************************************
* fontspec warning: "only-luatex-feature"
* 
* Ignored LuaTeX only feature: 'Renderer=Full/Basic'.
*************************************************

Although I didn't chose Full or Basic.

I have taken this into account: Using Graphite fonts on XeLaTeX

Answer 1

After some more investigation, this seems to be a problem with two distinct sources. The first is a bug in fontspec, which causes the Graphite renderer to be inaccessible. This bug has been reported and fixed, and should show up in the next version of fontspec. As a temporary fix, you can add the following code to your document (a full sample document will follow):

\ExplSyntaxOn
\keys_define:nn {fontspec-preparse} {
  Renderer .choice_code:n = {
    \fontspec_update_fontid:n {+rend:\l_keys_choice_tl}
    \int_compare:nTF {\l_keys_choice_int <= 3} {
      \tl_set:Nv \l_fontspec_renderer_tl
        { g_fontspec_renderer_tag_ \l_keys_choice_tl }
    }{
      \fontspec_warning:nx {only-luatex-feature} {Renderer=Full/Basic}
    }
  }
  ,
  Renderer .generate_choices:n = {AAT,ICU,Graphite,Full,Basic}
}
\ExplSyntaxOff

This is only half the problem, however. The most recent version of the Gentium Plus font has changed from numeric font features to alphabetic font features, so what used to be

RawFeature={1054=1}

should now be

RawFeature={lopr=1}

However, XeTeX does not seem to deal correctly with these new font feature designations, and assumes they are numeric values. This can be seen by the output of the aat-info.tex file adapted for Graphite:

Running this on version 1.504 of Gentium Plus yields the old numbering system, (image on the left) whereas running it on the latest version 1.510 of Gentium Plus reveals different numbers, instead of the expected alpha codes (image on the right):

The development version of XeTeX supports the alphanumeric IDs, but in the meantime, it's possible either to pass the "new" numeric values as RawFeatures, or as Khaled Hosny points out in the comments, use the full long names of the features as described in the Gentium Plus documentation. Here is a complete document, showing that the relevant features are now accessible with the different numbers or with the long form names. Be careful to match the names exactly: the ID names are case sensitive, as are the "True" and "False" (which must appear as shown.)

% !TEX TS-program = XeLaTeX

\documentclass[12pt,letterpaper]{article}
\usepackage{fontspec}
\ExplSyntaxOn
\keys_define:nn {fontspec-preparse} {
  Renderer .choice_code:n = {
    \fontspec_update_fontid:n {+rend:\l_keys_choice_tl}
    \int_compare:nTF {\l_keys_choice_int <= 3} {
      \tl_set:Nv \l_fontspec_renderer_tl
        { g_fontspec_renderer_tag_ \l_keys_choice_tl }
    }{
      \fontspec_warning:nx {only-luatex-feature} {Renderer=Full/Basic}
    }
  }
  ,
  Renderer .generate_choices:n = {AAT,ICU,Graphite,Full,Basic}
}
\ExplSyntaxOff
\begin{document}
\newfontfamily\lowfull[Renderer=Graphite,RawFeature={Low-profile diacritics=True}]{Gentium Plus}
\newfontfamily\lownumeric[Renderer=Graphite,RawFeature={1819242610=1}]{Gentium Plus}
\newfontfamily\high[Renderer=Graphite,RawFeature={Low-profile diacritics=False}]{Gentium Plus}


{\lowfull mā má mǎ mà \qquad (Low profile)}

{\lownumeric  mā má mǎ mà \qquad (Low profile)}

{\high  mā má mǎ mà \qquad (Regular profile)}



\end{document}