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<{{cmd>|definefontfeature</cmd>}}
\SAMPLE\blank
% Adding\definefontfeature[f:ligatures] [liga=yes]\definefontfeature[f:noligatures] [liga=no]
% Subtracting\definefontfeature[f:oldstylewmacron] [ss04=no,onum=yes]\definefontfeature[f:newstylesmallcaps] [smcp=yes,onum=no]
% Replace all features, including the default set defined in the preamble. Note that kerning is now gone!\startTEXpage[offset=10pt]
% Let's reset and get the original default set back and add smallcaps:\startX A) \SAMPLE\blank
% Let's revive one of the old featuresets\feature[+][f:smallcaps] E) \SAMPLE \blank\nextX\addfeature[f:wideimacron] F) \SAMPLE \blank
% Let's get the original default set back.\feature[=][f:oldstyleupsidedown] H) \SAMPLE \blank
no edit summary
[[LuaTeX]] and [[MkIV]] allow advanced usage and manipulation of [[Open Type]] <i>open-type features</i>. Some features, such as <i>onum</i> (oldstyle) and <i>smcp</i> (smallcaps) are known to most users of Open Type fonts, and virtually every open type font has default features for kerning (<i>kern</i>) and ligatures (<i>liga</i>). More advanced OT fonts can have lots more, including the Stylistic Set feature <i>ss<nn></i> (where <i>nn</i> stands for any numeral between 01 and 99).
== Open-type features ==
The open-type features are specified in the font and are composed of individual "lookups" which specify each substitution and positioning action to be performed in the processing. These open-type features are plugged in and out of the "stack" of lookups to be processed in the MkIV open type handler. These features are represented by a 4-character name (<i>onum</i>, <i>smcp</i>, etc.) and in MkIV are associated with a keyval that turns it on ("yes") or off ("no"). For example, '[onum=yes]' turns on the lookup substitutions in the font which replace a line numeral with an oldstyle one. Here the key 'yes' means "include open-type feature ''onum'' into the processed stack"; the key 'no' means "do not include open-type feature ''onum'' in the processed stack". === Adding features on the fly === Sometimes you want to provisionally add OpenType features to a font. There are at least three ways to do this.# Use a low-level `font.handlers.otf.addfeature` callback function to define new OTF features. See Section 8.14 of [https://www.pragma-ade.com/general/manuals/fonts-mkiv.pdf fonts-mkiv manual] for details of the syntax, or refer to the examples [http://minimals.metatex.org/current/context/test/doc/context/tests/mkiv/fonts/extensions-001.tex here].# Define the new features in an `lfg` "goodies" file and load it with the `goodies` key. See [http://minimals.metatex.org/current/context/test/doc/context/tests/mkiv/fonts/greek-001.tex this example] with the corresponding [http://minimals.metatex.org/current/context/test/doc/context/tests/mkiv/fonts/oxoniensis.lfg lfg] file.# Use a feature file (in the `fea` format) and load it with the `featurefile` key. See [[Fontfeaturefiles]]. === Debugging features === For an OpenType to work properly, all the relevant glyphs must come from the same font (either the original font or one of the fallbacks). For example, the font [https://github.com/aminabedi68/Mikhak/tree/master/Font/ttf Mikhak] now misses glyphs for en dash and em dash, hence the `tlig` feature does not work to automatically change double or triple dashes to en and em dashes. This would not work even if we added fallbacks for those two characters. However, if we also create a fallback for the dash character, then this will work. <texcode>\starttext \definefontfallback[myfallback][dejavuserif*default][0x02d,0x2013,0x2014][check=yes,force=yes] \definefontsynonym[mytestfont][file:Mikhak-Medium.ttf][fallbacks=myfallback] \definedfont[mytestfont*arabic] foo--ofo---oof [\char"002D] [\char"2013] [\char"2014] \stoptext</texcode> OpenType features can be complicated to understand or debug. One useful command to see how they are working is {{cmd|showotfcomposition}}. <texcode>\starttext% -1 for the second argument means RTL. Leave it out for LTR text.\showotfcomposition{mikhak-medium*arabic}{-1}{سلام علیکم}\stoptext</texcode>
== Featuresets ==
A featureset is a composed of a set of open-type features. Featuresets are specified using <{{cmd>|definefontfeature</cmd>}}. For each given body font in the typeface we specify the ''global'' default set of <i>open-type</i> features to be used for that font. This is usually done in the preamble to our document, an environment file, or a typescript file, or the like.
For example, using TeX-Gyre Schola we can specify the following default, global set open-type features in the preamble:
</texcode>
All other open-type features in the font are not included in the processing.
Now when we want to change the default, we use <{{cmd>|definefontfeature</cmd> }} to specify local featuresets. The command <{{cmd>|feature</cmd> }} and its siblings is then used to control the action of those featuresets. Here are the commands:
<texcode>
In the above note that we start with just a global, default feature set of kerns, ligatures, and line numerals.
Now when we want to change the default, we use <{{cmd>|definefontfeature</cmd> }} to specify featuresets. The command <{{cmd>|feature</cmd> }} and its siblings is then used to control the action of those featuresets.
At this stage which open-type feature is being plugged into or unplugged from the stack is invisible to the user. We just speak in terms of featuresets. TeX-Gyre Schola has a decent set of open-type features. Using a few of them, let us set up a few featuresets:
<texcode>
\startTEXpagedefinefontfeature [offsetschola-preset] [mode=node,script=latn,language=dflt,kern=yes,liga=yes,lnum=yes] \starttypescript [serif] [schola-preset] [name] \definefontsynonym [Serif] [name:texgyreschola-regular.otf][features=schola-preset] \stoptypescript \starttypescript [serif] [schola-preset] [size] \definebodyfont [35.8pt,29.8pt,24.8pt,20.7pt,17.3pt,14.4pt,12pt,11pt,10pt,9pt,8pt] [rm] [tf=Serif sa 1] \stoptypescript \starttypescript [schola-default] \definebodyfontenvironment [schola-default] [default] [interlinespace=2.7ex]\definetypeface [schola-default] [rm] [serif] [schola-preset] [schola-preset] % [default] \stoptypescript \usetypescript[schola-default] \setupbodyfont[schola-default,12pt] \definefontfeature[f:smallcaps] [smcp=yes]\definefontfeature[f:thinimacron] [ss04=yes]\definefontfeature[f:upsidedown] [dlig=yes]
\definedefinefontfeature[f:newstyle] [onum=no]\SAMPLE{123 Idrīs ?` !` VA fi}definefontfeature[f:nocaps] [smcp=no]\definefontfeature[f:wideimacron] [ss04=no]\definefontfeature[f:rightsideup] [dlig=no]
\feature[+]definefontfeature[f:oldstyle] \SAMPLE \blank\feature[more][f:thinimacron] \SAMPLE \blank\addfeature [f:upsidedownonum=yes,lnum=no] \SAMPLE \blank
\featuredefinefontfeature[+f:oldstyleupsidedown] [f:smallcapsonum=yes,dlig=yes] \SAMPLE \blank% \feature[more]definefontfeature[f:smallcapsnewstylerightsideup] \SAMPLE \blank\addfeature [f:wideimacrononum=no,dlig=no] \SAMPLE \blank
\featuredefineparagraphs[-X][f:upsidedownn=2, rule=on, width=10cm] \SAMPLE \blank% \feature[less][f:upsidedown] \SAMPLE \blank% \subtractfeature[f:upsidedown] \SAMPLE \blank
\feature[=][f:oldstyleupsidedown] define\SAMPLE \blank% \feature[new][f:oldstyleupsidedown] \SAMPLE \blank% \replacefeature[=][f:oldstyleupsidedown] \SAMPLE \blank{123 Idrīs ?` !` VA fi}
\feature[!+][f:smallcapsoldstyle] B) \SAMPLE \blank% \feature[!more][f:noligaturesthinimacron] C) \SAMPLE \blank% \resetandaddfeature[!]addfeature[f:noligaturesupsidedown] D) \SAMPLE \blank
\feature[>-][f:upsidedown] \SAMPLE \blank% \feature[old][f:upsidedown] \SAMPLE \blank% \revivefeature[f:upsidedown] G) \SAMPLE \blank
\feature[<!] \SAMPLE \blank % \feature[resetf:smallcaps] \SAMPLE \blank % \resetfeature I) \SAMPLE \blank
\feature[<] J) \SAMPLE \blank\stopX\stopTEXpage
</texcode>
[[Category:Fonts]]
[[Category:LuaTeXTools]]
