[XeTeX] letter spacing

Jonathan Kew jonathan_kew at sil.org
Sun Jun 4 23:32:42 CEST 2006

On 4 Jun 2006, at 9:42 pm, public at heslin.eclipse.co.uk wrote:

[useful discussion snipped]

> So, Jonathan, it would be great if Xetex would provide this too.

I have a couple of ideas on how we might approach this, and perhaps  
some folks will have comments on which you'd prefer to see  
implemented. (I'm not promising anything, just thinking aloud!)

(1) Letterspacing as a TeX font property, settable from within the  
document. There are a couple of ways this could be specified:

(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

(1.b) specify a \fontdimen that xetex would use to apply such an  
adjustment, so you could say something like
	\font\mycaps="MyFont:+smcp" at 24pt

(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!

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  

(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.)



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