fontspec doesn't find Stylistic Variants in Gentium Basic

Question

According to SIL, the font Gentium Basic --- which is a subset of Gentium Plus --- supports "literacy alternates". Consulting the font documentation (for Gentium plus, anyway), the literacy alternates are shown to have a OpenType feature ID of "ss01".

So, why does the following XeTeX code fail in selecting the literacy alternates?

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Gentium Basic}[StylisticSet=01]

\begin{document}
This is a test of literacy álternàtes.
\end{document}

The analogous code works perfectly for Gentium Plus. But with Gentium Basic, I get the following error:

OpenType feature 'Variant=01'(+ss01) not available for 'Gentium Basic'.

I tried also [StylisticSet=1], [StylisticSet=0], [StylisticSet=ss01]. None of them work.

Perhaps the literacy alternates are encoded differently in Gentium Basic than in Gentium Plus? Or perhaps there is another feature of fontspec that I should be using instead? Does anyone know?

Answer 1

As it happens, the stylistic alternative sets are not accessible as a font feature. If the code chosen as the answer to this question is run for Gentium Basic, the following features are discovered:

aalt, ccmp, mark, mkmk.

Running the same code with Gentium Plus results in:

aalt, c2sc, ccmp, kern, mark, mkmk, smcp, liga, ss01, ss04, ss05, ss06, ss07

So, it is clear that stylistic sets are available as font features for Gentium Plus, but not for Gentium Basic.

The "literacy alternates" stylistic set does exist in Gentium Basic, as we can get that feature by including +aalt as an option when loading the font, as @cfr suggested. However, when doing so, all the alternates are selected as well.