Fontfeaturefiles

(This page was written and tested under ConTeXt ver: 2010.08.20 00:00 MKIV)

Recently there was the question how to modify a font at runtime. For OpenType fonts Khaled recalled an article he wrote in fontspec manual (section 11) about font feature files: you just load the feature file with featurefile=foo in your font features definition.

This is an example on how missing ligatures can be used:

\definefontfeature[thetimes][default][mode=node,featurefile=times_fl.fea]
\starttypescript [serif] [TimesNR] [name]
    \setups[font:fallback:serif]
    \definefontsynonym [Serif]          [file:Times New Roman.ttf]            [features=thetimes]
    \definefontsynonym [SerifBold]      [file:Times New Roman Bold.ttf]       [features=thetimes]
    \definefontsynonym [SerifItalic]    [file:Times New Roman Italic.ttf]     [features=thetimes]
    \definefontsynonym [SerifBoldItalic][file:Times New Roman Bold Italic.ttf][features=thetimes]
\stoptypescript

\starttypescript[TimesNR]
    \definetypeface [TimesNR] [rm] [serif] [TimesNR]   [default]
\stoptypescript

\starttypescript[TheTimes]
    \definetypeface [TheTimes] [rm] [serif] [TimesNR]   [default]
\stoptypescript

\mainlanguage[de]
\usetypescript[TheTimes]
\setupbodyfont[TheTimes, 10pt]
 
\starttext
 
Hoffnung finden: fliegen
 
\stoptext

The feature file that is used "times_fl.fea" looks like this:

lookup fligatures {
  lookupflag 0;
    sub \f \i by \fi;
    sub \f \l by \fl;
} fligatures;

feature liga {
  script DFLT;
    language dflt;
      lookup fligatures;
  script latn;
      language dflt;
      lookup fligatures;
} liga;

This is another example showing how improved kerning can be added to the font via feature files:

\definefontfeature[thetimes][default][mode=node,featurefile=times_kern.fea]
\starttypescript [serif] [TimesNR] [name]
  \setups[font:fallback:serif]
    \definefontsynonym [Serif]          [file:Times New Roman.ttf]            [features=thetimes]
    \definefontsynonym [SerifBold]      [file:Times New Roman Bold.ttf]       [features=thetimes]
    \definefontsynonym [SerifItalic]    [file:Times New Roman Italic.ttf]     [features=thetimes]
    \definefontsynonym [SerifBoldItalic][file:Times New Roman Bold Italic.ttf][features=thetimes]
\stoptypescript
 
\starttypescript[TimesNR]
    \definetypeface [TimesNR] [rm] [serif] [TimesNR]   [default]
\stoptypescript
 
\starttypescript[TheTimes]
    \definetypeface [TheTimes] [rm] [serif] [TimesNR]   [default]
\stoptypescript
 
\mainlanguage[de]
\usetypescript[TheTimes]
\setupbodyfont[TheTimes, 10pt]
 
\starttext
 
„W
 
\stoptext

The feature file that is used here "times_kern.fea" looks like this:

lookup MyKerns {
 lookupflag 0;
    pos \quotedblbase \W -150;
} MyKerns;

feature kern {
  script DFLT;
    language dflt ;
    lookup MyKerns;
  script latn;
    language dflt ;
    lookup MyKerns;
} kern;

These font features are loaded in the font cache.

Thus, for experimenting and eg. changing the value "-150" one must clear the cache between two runs: mtxrun --script cache --erase

For example when running context minimal in Mac you just enter in Terminal:

source /Users/YourName/context/tex/setuptex /Users/YourName/context/tex
mtxrun --script cache --erase

Update

Today (1. Sept 2010.) Hans announced that with the next release this clearing isn't needed anymore:

"when you change the file or the name (or remove it) the font will be recached".