How to find out which glyphs are different in an OpenType style set?

Question

With fontspec, and XeTeX or LuaTeX it's possible to access a font's OpenType features, including the Style Sets, e.g. like this:

\addfontfeatures{RawFeature=+ss01;+ss18}

TeX.sx has already taught me how to find out which features are available. Assuming I don't have proper documentation of my font at hand, is there a way to find out which glyphs (in which contexts) will actually come out differently when I use a certain style set (or another OpenType feature)? It also seems to depend on the script and font family (e.g. small caps). I feel like LuaTeX would be able to do this.

A nice font with lots of features to test this is Junicode. For this specific case, however, all the features are neatly described in Specimens and User's Guide.

Answer 1

There’s DTL OTMaster, which can be used for free for inspecting a font (and for a fee, for font editing, too). It takes a little time to find out all the functions in the program, but e.g. the alternate glyphs available in a font can be found out by selecting Tools > 'GPOS'/'GSUB' Table Viewer and then 'GSUB' table from the first dropdown (Layout Table), then selecting different options in the Features menu. Then the Subtable view will show the glyphs affected by a feature, as a mapping table under “Report” and as a set of large-size glyphs under “Image”.