[XeTeX] Fine tune selecting alternate glyphs

David J. Perry hospes.primus at verizon.net
Tue Feb 9 06:24:36 CET 2010

I've been thinking about this issue for a while.  Let me address Szabolcs's 
specific question and then raise a larger issue.

> 1. This method switches the letter shapes for all of δ, ζ, and Ξ.  Is it 
> possible to switch only some of them?
Peter's suggestion to declare a new font (or, perhaps better, a new font 
instance; see the FontSpec manual, p. 5, for the difference) works.  But if 
your whole document (or a significant chunk of it) is in GFS Neohellenic you 
could use addfontfeature, like this:
By putting the whole command in curly brackets you prevent all subsequent 
instances of delta from taking on the alternate form.  You could use the 
LaTeX \newcommand feature to make this even shorter, something like:
Now whenever you type \d1 you will get the alternate delta.  See any LaTeX 
tutorial for help with this.

Peter's suggestion to use the codepoints may or may not work.  Some font 
developers give their alternate glyphs Unicode values in the Private Use 
Area and some do not.  The latter is really the way things should be, but 
because so few applications support stylistic alternates some developers do 
provide PUA values.  I haven't looked at the GFS fonts you mention, but if 
they have PUA values you can use the command \char"xxxx where xxxx is the 
PUA value.

If the fonts in question do not give PUA assignments to their alternate 
glyphs, and if the Alternates=1 doesn't work, you can access glyphs directly 
by their glyph ID, using the command \XeTeXglyphxxxx.  To do this, you must 
know the glyph ID of the alternate you want.  You can find this by opening 
the font in a font editor, if you have one, or with the Windows TrueType 
Viewer by Rogier van
Dalen, http://home.kabelfoon.nl/~slam/fonts/truetypeviewer.html, or the SIL 
ViewGlyph application, http://scripts.sil.org/ViewGlyph_home.  Glyph IDs are 
not something that users normally deal with, so you need specialized 
software to see them.

Now the more general issue:

XeTeX comes with that nifty utility that allows one to identify the OT or 
AAT features that exist in a font.  It would be very helpful if there were a 
similar utility that would identify which characters have stylistic 
alternates.  Running such a utility on a font would produce a list like 
    a - 5 variants, b - 1 variant, e - 4 variants, g - 6 variants, etc.

One could then construct a test document to see which alternates are 
accessed with which numbers.  This is an important issue because some font 
makers don't provide exact information about what characters have alternate 
glyphs, even if they tell you that the font does contain stylistic 
alternates.  (I guess they assume everyone uses an application like InDesign 
or Quark that comes with a palette to show the alternates.)  Without this 
information XeTeX users can't access the variants since they have to be 
called numerically.  Anybody want to write something like this?  (I would do 
it but I don't have the programming skills.).

Best wishes,

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