Unicode math: italic correction at the end of the math list

10 Feb 2025

      Hello

I suppose that the luatex binary is fixed for TeXlive 2025, so I can 
open an additional problem and I won't disturb you while finalizing the 
binaries:).

Use this test file:

-------------------

\input luaotfload.sty
\font\f=[latinmodern-math]:mode=base;script=math;
\textfont0=\f  \Umathcode `f= 0 0 "1D453 % italic f from Unicode math font

$f!$, $f$!, $f\/$!

\showboxbreadth=100 \batchmode \showlists
\bye

---------------------

When luatex processes it, we get italic correction only in the first case:

\mathon
.\f 𝑓
.\kern0.9 (italic)
.\f !
.\mathoff
.\tenrm ,
.\glue(\spaceskip) 3.33333 plus 2.08331 minus 0.88889
.\mathon
.\f 𝑓
.\mathoff
.\tenrm !
.\tenrm ,
.\glue(\spaceskip) 3.33333 plus 2.08331 minus 0.88889
.\mathon
.\f 𝑓
.\kern0.0 (font)
.\mathoff
.\tenrm !

Yes, fontforge shows that latinmodern-math italic f has italic 
correction 90, i.e. 0.9pt for the font at 10pt. And the italic 
correction is inserted in the first case according to rule 17 of 
appendix G of TeXbook, where we only need to add: "if the symbol is not 
a text symbol or if \fontdimen2 is zero **or if script=math fontfeature 
is used** then insert italic correction". It sounds OK, it is natural 
extension of the original Knuth's idea: his math scripts were in fonts 
cmmi10 and cmsy10 where \fontdimen2=0pt.

But:

A) why the italic correction isn't inserted in case 2? The rule 17 in 
appendix G of TeXbook says that "if the single-char atom isn't a text 
symbol (i.e. it isn't followed by a next specific atom) then the italic 
correction is inserted". The condition is obviously true for the last 
single-char atom of the math list. But luatex with Unicode math font 
doesn't respect this rule. Why?

B) why there is \kern0.0 in the third case where we want to insert 
explicit italic correction (which is 0.9pt as fontforge says)?

When we comment out the third line in our example (\textfont0=...), i.e. 
the classical plain TeX setting with 7bit tfm fonts is used, then luatex 
inserts italic correction in all three cases. It means that explicit \/ 
works and rule 17 from appendix G is respected.

OK, maybe it is a decision of somebody who implemented Unicode math 
because missing italic corrections at the end of math list looks better, 
for example at the end of a sentence (ef-dot): $f$. I.e. the rule 17 
from TeXbook will be not respected. But if our sentence ends by $f$! or 
by $f$? then the result looks very bad and user is unable to do local 
corrections by explicit \/.

The problem is not due to simplification of the example here. If you try 
lualatex with unicode-math macros or OpTeX, you get the same results.

Note that xetex puts correctly italic corrections in all three cases 
even though a Unicode math font is loaded.

Can someone explain me this confusing behavior?

Thanks

Petr Olsak

Petr Olsak

Petr Olsak

luigi scarso

Petr Olsak

luigi scarso

Ulrike Fischer

luigi scarso

David Carlisle

Petr Olsak

luigi scarso

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