[bug #64519] font description file should allow kern pairs with hyphen as first element

[Dave]

URL:
  <https://savannah.gnu.org/bugs/?64519>

                 Summary: font description file should allow kern pairs with
hyphen as first element
                   Group: GNU roff
               Submitter: barx
               Submitted: Sun 06 Aug 2023 10:19:11 PM CDT
                Category: Font - others/general
                Severity: 1 - Wish
              Item Group: Feature change
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None


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Follow-up Comments:


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Date: Sun 06 Aug 2023 10:19:11 PM CDT By: Dave <barx>
Today we examine a three-pronged problem.

=== I. font description file ===

Any pair of glyphs in the "kernpairs" section of the font description file
where the first glyph is a hyphen is ignored.

This is not immediately obvious, because none of the font description files
groff ships for the ps or pdf devices has such a kernpair:

$ egrep '^(-|hy) ' font/devp*/* | wc -l
0

However, my system has a font family that contains such pairs: Linux
Libertine, whose roman font I've locally named LR:

$ egrep '^(-|hy) ' /usr/share/groff/site-font/devps/LR | wc -l
80

So let's look at one such kernpair:

$ egrep '^(-|hy) W ' /usr/share/groff/site-font/devps/LR
- W -48
hy W -48

I chose this one because it has one of the larger adjustments of any of the 40
kernpairs beginning with a hyphen, so any change ought to be visible to the
naked eye, especially at a large enough type size.

So let's craft an example with a large type size and see what we see.

$ cat kern-test
.fam L
.nf
.ps 60
.vs 80
.sp
MAKE-WORK
MAKE-\&WORK

The "- W" (or "hy W") kernpair in the LR font description file ought to
visibly reduce the space between the hyphen and the W on the first line of the
PostScript output.  This should be visible by contrast with the next line,
where the dummy escape prevents kerning in that spot.

But in the generated output, the two lines look distressingly identical.

Just to verify to ourselves that our naked eye is not failing us, we can
generate each line separately and algorithmically compare the results:

$ diff <(fgrep -v E-W kern-test | groff) <(fgrep -v \& kern-test | groff) | wc
-l
0

The hyphen-W kernpair has no effect.

This does help explain the side note in comment 3 of bug #57448.

Groff's own TR font description file contains the opposite kernpair, a
W-hyphen one, and it's trivial to demonstrate via similar means that this does
have a visible effect.  As with all lecturers who hand-wave away "trivial"
demonstrations, I leave this one as an exercise to the reader.

=== II. documentation ===

This lack of efficacy, like the similar one with a space as the first element
of a kernpair (bug #58718), is undocumented in the info manual's three
sentences that explain how items in the "kernpairs" section work.

But one might harbor suspicions that there would be some difficulty defining a
space as the first element of a kernpair, because the "kernpairs" section uses
spaces to delimit the elements.  The hyphen, represented as either "-" or
"hy", should present no such difficulty, so this behavior really defies
expectations.

=== III. afmtodit ===

It further turns out these nonfunctional hyphen-leading kernpair lines were
placed there by groff's own afmtodit:

$ head -1 /usr/share/groff/site-font/devps/LR
# This file has been generated with GNU afmtodit (groff) version 1.22.2

One piece of groff software should not generate lines that are ignored by
another piece.

== The fix ==

But I believe these kernpairs _should_ work, and I've filed this bug
accordingly.  That is, rather than documenting the limitation and fixing
afmtodit to honor it, I think groff should use such kernpairs.

Aesthetic value is subjective, but I suspect most people with a typographical
eye would agree that the PostScript output is more pleasing with the
adjustment that the font description file requests.  You can compare the lines
with and without the adjustment by faking the kerning in our trusty
"kern-test" file from above:

$ sed 's/&/h"-0.048m"/' kern-test | groff > kern-test.ps

In this case the difference _is_ visible to the naked eye--at least, to eyes
as discerning as yours.  The hoi polloi may not see any difference, but you
and I know good typesetting when we see it!  *conspiratorial wink*

This example uses font LR, but if you don't have that, you can demonstrates
the principle just as well using groff's own TR.  In this instance we'll use a
slightly larger adjustment, reflecting the TR font's adjustment in the
W-hyphen case.

$ fgrep 'W - ' /usr/share/groff/current/font/devps/TR
W - -65
$ fgrep -v .fam kern-test | sed 's/&/h"-0.065m"/' | groff > kern-test.ps









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