Specifically, the combination of f plus î in the italic ends up with the upper curl of the f smashed into the peak of the circumflex (other fonts have the same problem, but the result is even uglier). I would like to improve the situation by using the GentiumPlus user-selectable feature "Low-profile diacritics" (feature ID code
lopr in the latest release (version 1.510),
1054 in previous releases --per the newest version of gentiumPlus-features.pdf, apparently only available online in the font package download-- with values
0=false, as corrected in the SIL tech notes for this font).
This would produce a squatter circumflex that should miss the collision with f.
Other notes on the SIL page regarding features new with GentiumPlus seems to indicate that this feature is available both as Graphite and OpenType features, and since I would like to be able to do this in both LuaLaTeX and XeLaTeX, I would like to use OpenType, since Graphite is not supported in LaTeX (except possibly via the Fontspec back door of including Graphite instructions in a Font Feature File, as mentioned in a post here, but not in the Fontspec manual, where it is supposed that it will only be used for OpenType features).
Now if I am reading the Fontspec documentation correctly, and if I am correct that this exists as an OpenType feature, I can add this feature with
\addICUfeature (in XeLaTeX) or
\addopentypefeature (in LuaLaTeX). The Fonspec manual (page 43) gives the syntax (using the XeLaTeX version of the command name) as
with an example (using the LuaLaTeX version)
Comparing with the syntax and use of
\addAATfeature on the same page, it seems clear that
<option> are the names that would go on the righthand and lefthand sides of the
= in an argument to e.g.
<feature tag> would, in this case, be
+lopr, assuming a prefixed plus/minus sign works as an on or off switch.
BUT I seem not to have done this correctly. I had guessed that
<option> were simply user-created discriptive names, as with
\newcommand. What I used was
in the preamble, and for an activation command (which I grouped with just the combining circumflex character, immediately following the dotless i) I used
My document compiled successfully without stopping, but the result was visually identical to the combined characters without the command, and with the original i-circumflex character. The log shows a fontspec warning
"icu-feature-not-exist-in-font" with an explanation
OpenType feature 'GentiumPlusFeatures=LowProfile' (+lopr) not available for font 'GentiumPlus-R.ttf' with script 'Latin' and language 'Default'.
Finally, I tried adding
Renderer=ICU to the
\addfontfeatures argument, as suggested in a possibly-relevant post here but the result was unchanged. (I am guessing
Renderer=ICU would still be necessary once corrections to the rest are made, but also don't know that
Renderer=ICU would work in LuaLaTeX).
UPDATE: I have tried substituting the name
Low-profile diacritics in the
<feature> field and
<option>, so that by using these, along with
+lopr (plus trial variants
lopr), I am now using only the exact language of the font documentation, but still no success.
I have gone as far as I can with this on my own, and am hoping someone with real expertise will be able to look it over and fix it in a few seconds. It is a small thing, I know... much like a grape seed in one's appendix.