[bug #57506] Suspicious "slant" values in devps/TI, devlbp/HI, devlbp/HBI

[Dave]

Follow-up Comment #8, bug #57506 (project groff):

Following up to myself:

[comment #6 comment #6:]
> Huh, so RTFMing suggests this value might be "erroneous" on
> purpose.  afmtodit(1) says:

To be fair, afmtodit(1) is not the most obvious place to look for this nugget.
 A user perusing the font description file and wanting to know what its
contents mean will probably first turn to (currently) section 6.2.2, Font
Description File Format, of the Texinfo manual.  Herein, the entirety of the
"slant" description is: "The font's glyphs have a slant of N degrees; a
positive N slants in the direction of text flow."  The fact that this value
might not accurately reflect the glyphs' slant, due to needing to align accent
marks, should be stated here.

(For context, I first stumbled upon this issue when needing to sic \S on a
glyph that didn't have an italic version, and wanted to use the same slant as
the italic font this glyph was mixing with.  Thus, knowing that this parameter
in the font file might not accurately reflect the font's glyphs is pertinent
to readers of the file, not just to those generating such files.)

I'd submit a patch to address the above Texinfo omission, but I think it's
important to also mention what mechanism uses this value for this purpose, and
this is still murky to me.

> the "is used by groff" line above is awfully vague about what
> accent-placing mechanism uses the value.

Upon further thought, it might not even be strictly accurate: the font
description file is device-specific, so it seems under the purview of the
postprocessor, not of groff, to utilize this information.  (This assumes font
data is appropriately encapsulated and thus not visible to groff itself.  But
in practice I think the barriers are more porous than that.)  Or the "groff"
in this phrase might be merely a shorthand for the pipeline, since groff calls
a postprocessor by default unless the user explicitly requests otherwise.

> The \o escape, for instance, claims to (horizontally) center
> all overstruck glyphs

I should point out that this claim also comes from the Texinfo manual, which
we now know is not comprehensive when it comes to "slant"-related matters.  (I
still have not done any testing.)


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