[XeTeX] XeTeX, fontspec, OTF, and fontdimens

[mskala at ansuz.sooke.bc.ca]

On Sun, 20 Feb 2011, Khaled Hosny wrote:
> I lost track of what the original issue is, so I tried testing your
> example with a little modification (removing default font feature and
> using different font), and all monspaced fonts I tried are giving me,
> guess what, monospaced results. Now I'm more confused.

In September of last year I had the issue that my inter-sentence spaces
were wrong, with a commercial monospace font used at only one size.  I
asked on this list and was told about the WordSpace option to fontspec,
which also led me to discover the PunctuationSpace option.  That thread
starts here:
   http://tug.org/mailman/htdig/xetex/2010-September/018463.html

After that I made a general practice of using WordSpace and
PunctuationSpace when using monospace fonts.  The new problem arose when,
in a different project, I wanted to also scale the fonts with the LaTeX
size commands.  So I wrote something like this:

\fontspec[WordSpace={1,0,0},PunctuationSpace=3]{FontName}

small text

\Large
large text

and I found that the space in "large text" was too small - it was the
same size as the space in "small text," not having scaled when the font
scaled.

Everything subsequent to that flows from attempting to get that scenario
to work without breaking other things.  The workaround of not using
WordSpace allows the space to scale properly, but then it becomes
stretchable.  Things that could remove the stretchability seem like they
must be repeated for every size change.  And so on.

And it's less than perfect that I have to do any special configuration at
all.  Non-stretchable word space equal to the fixed width of the font, and
extra punctuation space of either zero or also equal to the fixed width of
the font, are the only things that make sense as defaults for monospace
fonts.  (Yes, it's easy to imagine someone could want something else, like
double-justification of monospace, but that's not a sensible *default*.)
If XeTeX can get monospace right by default for older fonts like CMTT but
not for OpenType fonts, then XeTeX isn't as compatible with OpenType as I
wish it could be.

My immediate problem is solved.  I can live with the workaround of using
the everysel package to add code at every font selection to reset the
stretchability and shrinkability to zero.  However, I think fontspec
should be changed to remember the WordSpace settings as multipliers rather
than as lengths, and to recalculate the multiplication at every size
change.  That would allow my original code to work as intended; it would
also be consistent with the current documentation of fontspec, which says
the WordSpace settings are multipliers applied to the default widths,
implying that the results of the multiplication change when the default
widths change; and it would allow for others with more complicated
requirements than mine to get what they want in a clean way (for instance,
typesetting documents that contain monospace fonts but aren't ENTIRELY
written with monospace fonts).

The same "remember the multiplier instead of the result of multiplication"
should apply to PunctuationSpace as well.

A more speculative wishlist item would be that the XeTeX engine should
recognize when a font is monospace, and set the default fontdimens in that
case to stretchability and shrinkability zero, extra space equal to word
space.  A bitter argument exists about whether a sentence space in a
monospace setting should be equal to one or two word spaces (I prefer
two), but I don't think anybody would seriously claim that it should
default to a non-integer number of word spaces; nonetheless 1.333 is the
current default.  However, recognizing monospace fonts would require the
XeTeX engine to be *able* to recognize monospace fonts, which seems to be
a harder problem than it ought to be; and it could also break existing
documents that rely on the existing, broken, default widths.
-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/