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'''This page is at the very initial stages of development: Please do not change it yet unless you know the topic well!'''
[[LuaTeX]] and [[MkIV]] allow advanced usage and manipulation of [[Open Type]] <i>font open-type features</i>. Some features, such as <i>oldstyleonum</i> (oldstyle) and <i>smallcapssmcp</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) .
= Features in general =
In MkIV there are two kinds of font feature directly visible to the user: ''meta-feature'' and ''open-type feature''.
== Open-type feature ==
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]'. 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".
Meta-features are specified using <cmd>\definefontfeature</cmd>. In the preamble to our document, an environment file, or even in a typescript file; we specify the default set of <i>open-type</i> features to be used. For example, using TeX-Gyre Schola we can specify the following default open-type features:
<texcode>
\definefontfeature
[schola-preset]
[mode=node,script=latn,language=dflt,kern=yes,liga=yes,lnum=yes]
</texcode>
<cmd>definefontfeature</cmd>
