Mfcttrf

Consider the following example:

documentclassstandalone
usepackageunicode-math
setmathfontTeX Gyre Termes Math
begindocument
(fBigl(Bigr))
enddocument

If you compile it with xelatex, you get

But if you compile it with lualatex, you get a narrower

Definitely, the results differ, so at least one of the engines or unicode-math is wrong about the spacing between f and the left parenthesis. Subjectively, the output of xelatex is more pleasant than that of lualatex, so, I presume, lualatex (or the code inside unicode-math run by lualatex only) is the culprit. But, I'm unaware of the "official" specification of how it should be, so, all bets are off.

1. How large is the distance between f and ( supposed to be for the most pleasant reading?




2. Who is the culprit? (I.e., who deviates from the way it is supposed to be?)




3. Is there any way to repair the culprit or at least to achieve independency of the engine used for compilation more or less automatically?

Weakly related: Change bounding box of math glyphs in LuaTeX .
However, there, Ulrike said in her answer that "you are at the end of the math and luatex doesn't insert the italic correction at the boundary between math and text." Here, on the contrary, we are still inside math. If you insert Uchar"200B or 🦆 right after f, you get more space for both engines, and the discrepancy remains. Moreover, it's far from automatic even if the discrepancy would have gone away.

EDIT: Concerning

This question already has an answer here:
Change bounding box of math glyphs in LuaTeX

It doesn't. The answer from there doesn't fit here. Feel free to test it.

Crosspost: http://latex.org/forum/viewtopic.php?f=46&t=32655&p=109799

What does the relevant specification actually say?

Italics correction can be used in the following situations:

• When a run of slanted characters is followed by a straight character (such as an operator or a delimiter), the italics correction of the last glyph is added to its advance width.

(The OpenType MATH table specification, emphasis mine)

Now the TeX engine has to decide how to translate this to TeX concepts. XeTeX generally classifies mathopen atoms as "delimiters" and therefore "straight characters", but LuaTeX only classifies TeX delimiters, (left, right, etc.) as "delimiters".

I tend to agree with XeTeX here.

To get consistant behaviour, you can add explicitly add the italic correction through /, so in your example:

documentclassstandalone
usepackageunicode-math,ifluatex
setmathfontTeX Gyre Termes Math
ifluatex
 mathitalicsmode=1
fi
begindocument
(f/Bigl(Bigr))
enddocument

This can also be automated using the mlist_to_hlist callback.