[bug #61828] make .tkf use same amount of space before and after each affected glyph

[Dave]

URL:
  <https://savannah.gnu.org/bugs/?61828>

                 Summary: make .tkf use same amount of space before and after
each affected glyph
                 Project: GNU troff
            Submitted by: barx
            Submitted on: Sat 15 Jan 2022 01:33:44 AM CST
                Category: Core
                Severity: 3 - Normal
              Item Group: Rendering/Cosmetics
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None

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Details:

= Background =

I ought to have learned by now that not much is gained by putting multiple
problems, even closely related problems, into the same bug report.  If they
can be addressed separately, it makes more sense to file separate reports for
them.

This report concerns itself with the second of the two problems listed in bug
#58562.

Of those two problems, I bet this is the easier one to solve.  Additionally,
because this detail of the behavior (that .tkf adds all extra space to only
one side of each glyph) has never been documented, I believe it's also
permissible to change that behavior without violating users' expectations of
back compatibility.  This change will alter some typeset output, but I wager
almost always for the better, unless some user is abusing .tkf to achieve some
specific effect based on its current (though, I reiterate, undocumented)
behavior.

The behavior is the same in groff 1.22.4 and a recent build
(1.23.0.rc1.1499-34c5f).

= The bug =

Here is the problem description airlifted in from #58562.

This bug only applies to typeset output; .tkf has no effect in terminal
output.

This test case uses somewhat greater track kerning than might be used in a
real-world application, in order to make the problem more visible.  But the
same problem occurs on a smaller scale with smaller adjustments.


.ll 4i
.nh
.tkf B 4 4 32 16
We the people of the United States,
in order to form a more perfect union,
establish justice,
insure domestic tranquility,
provide for the common defense,
promote the general welfare, and
secure the blessing of \fBliberty
to ourselves\fP and our posterity,
do ordain and establish the Constitution of the United States of America.


As output from this shows, .tkf adds all the extra space to the right of the
glyph, producing inconsistent spacing before and after the bolded passage.

It would be better for .tkf to add half the space to the left of each glyph
and half to the right, effectively centering each glyph within its wider
window.




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