[XeTeX] letter spacing

[public at heslin.eclipse.co.uk]

Jonathan Kew <jonathan_kew at sil.org> writes:

> (1.a) a "pseudo-feature" in the \font definition, something like \font
> \mycaps="MyFont:+smcp,letterspace=0.05" to add 5% of the em-size  
> between letters

That would be most similar to the way fontinst used to work (and the way
otftotfm now works).  It's handy when the extra inter-letter space
scales up in size with the size of the font.

> (2) Letterspacing by inserting glue; a new parameter such as  
> \XeTeXinterletterglue that (if non-zero) would be automatically  
> inserted between characters. This would make it easy to do things  
> like spacing text out to exactly fit a given width (by making the  
> interletter glue stretchable). Obviously, this could be horrendously
> abused, too, but that in itself is not a reason to reject it!

I don't think this would be useful at all for the purposes under
discussion.  The point is not to fill up a given horizontal space, but
to provide a fixed amount of extra space between letters, which text in
caps needs, and which is a function of the point size and the general
look of the (small) caps in the font.

> In all these cases, things get a little tricky when we're dealing  
> with general Unicode text, not just English typography. For example,
> we mustn't allow letterspacing adjustments to affect the relative  
> positioning of base characters and diacritics, or to interfere with  
> script-specific glyph processing that's specified in the OpenType  
> tables.

Letterspacing corresponds to a specific need for typesetting full and
small caps in the Latin alphabet -- but within that domain, it's an
essential requirement.  There may be other scripts where it is needed --
I have seen letterspaced caps in Greek, for example, but there are
others (e.g. Arabic) where I suppose it would make no sense at all.  If
someone turns on this feature in a context where it makes no sense, then
let them worry about the consequences.  Unfortunately, when you empower
people to do the good things they need to do, you also give them the
power to steal sheep ...

> (There's also "(3) Access to AAT tracking tables, when rendering with
> ATSUI on Mac OS X", but so few fonts actually include a 'trak' table,
> as far as I've seen, that I'm not sure this is worth doing.)

I can only guess, but I suppose that this is a matter where font
designers just expect the user application to implement a tracking
feature.

Best wishes,

Peter

-- 
Peter Heslin (http://www.dur.ac.uk/p.j.heslin)