[XeTeX] Fractions

Jonathan Kew jonathan_kew at sil.org
Wed Mar 1 17:31:19 CET 2006

On 1 Mar 2006, at 1:55 pm, Carlo Strub wrote:

> Hi all,
> I have a problem with Fractions and fontspec. Whenever I use the  
> option [Fractions=On] with the Opentype Font Adobe Caslon Pro all  
> numbers and all dots, commata etc. move up. The simplest possible  
> example for this bug is:
> \documentclass[12pt]{article}
> \usepackage{fontspec,xunicode}
> \renewcommand{\encodingdefault}{U}
> \setromanfont[Fractions=On]{Adobe Caslon Pro}
> \begin{document}
> Hello, I was born in 2005 on the 6th of the month.
> \end{document}

It is behaving correctly; this is how the OpenType feature is  
designed. (IMO, it's not as convenient as Apple's approach to this  
type of thing.)

Adobe's "fractions" feature will turn all glyphs that it considers  
potential "numerator" glyphs into small, raised forms, until it  
encounters a slash, and then it'll start turning them into small low  
forms instead. But it doesn't look ahead to see whether a suitable  
slash really is coming, when it first finds a digit.

The intention is that you will only apply such a feature to the  
specific range of text that makes up the fraction; you aren't  
expected to turn it on "globally". This contrasts with Apple's AAT  
features, which are smarter about context.

So here's a sample which shows the OT Fraction feature in use (though  
if you do much of this, you'll want to define some macros to make it  
easier to enter... and perhaps use \newfontinstance to predeclare the  
"fraction" version of the font, rather than re-evaluating the feature  
string every time.



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\setromanfont{Adobe Garamond Pro}


Pi is roughly 3{\addfontfeature{Fractions=On}1/7}, or {\addfontfeature 
{Fractions=On}355/113} to be slightly more accurate.

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