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In MkIV there are two kinds of font feature directly visible to the user: ''meta-featurefeatureset'' and ''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]'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".

== Meta-features Featuresets ==

A meta-feature featureset is a composed of a set of open-type features. Meta-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.

<{{cmd>|definefontfeature</cmd>}}

Now when we want to change the default, we use <{{cmd>\|definefontfeature</cmd> }} to specify local meta-featuresfeaturesets. The command <{{cmd>|feature</cmd> }} and its siblings is then used to control the action of those meta-featuresfeaturesets. Here are the commands:

\revivefeature [f:mine] \feature [old][f:mine] \feature[>][f:mine]

Let us now go through a complete sample file with examples. Let us first define our global default meta-feature featureset and activate it with a typescript:

Now when we want to change the default, we use <{{cmd>\|definefontfeature</cmd> }} to specify meta-featuresfeaturesets. The command <{{cmd>|feature</cmd> }} and its siblings is then used to control the action of those meta-featuresfeaturesets.

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 meta-featuresfeaturesets. TeX-Gyre Schola has a decent set of open-type features. Using a few of them, let us set up a few meta-featuresfeaturesets:

Above, all of our meta-features featuresets are mapped to open-type features that plug into the stack.

Of course, a meta-feature featureset set can include more than one open-type feature. In advanced applications this will be the norm:

NOTE: In this case we have to specify 'lnum=no' because it is already defined in our default feature set in the preamble etc. Depending on the The order of lookups in the font one has to watch out for thisitself could conceivably have an impact as well.

Above, each meta-feature featureset is mapped to an open-type feature set that plugs into or unplugs from the stack. But we can mix things up:

The above two illustrate the point that meta-features featuresets hide from the user which actual open-type features are being plugged into or unplugged from the stack.

Now let's put some of the above meta-features featuresets in action. Note the use of synonyms in both the activated code and in the comments:

\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]

\definedefinefontfeature[f:smallcaps] [smcp=yes]\SAMPLE{123 Idrīs ?` !` VA fi}definefontfeature[f:thinimacron] [ss04=yes]\definefontfeature[f:upsidedown] [dlig=yes]

\featuredefinefontfeature[+f:ligatures] [f:smallcapsliga=yes] \SAMPLE \blank% \feature[more]definefontfeature[f:smallcapsnoligatures] \SAMPLE \blank\addfeature [f:wideimacronliga=no] \SAMPLE \blank

\featuredefinefontfeature[-f:oldstyleupsidedown] [f:upsidedownonum=yes,dlig=yes] \SAMPLE \blank% \feature[less]definefontfeature[f:upsidedownnewstylerightsideup] \SAMPLE \blank% \subtractfeature [f:upsidedownonum=no,dlig=no] \SAMPLE \blank

% Replace all features\definefontfeature[f:oldstylewmacron] [ss04=no, including the default set defined in the preamble. Note that kerning is now gone!onum=yes]\definefontfeature[f:newstylesmallcaps] [smcp=yes,onum=no]

\featuredefineparagraphs[=X][f:oldstyleupsidedown] \SAMPLE \blank% \feature[new][f:oldstyleupsidedown] \SAMPLE \blank% \replacefeature[n=2, rule=on, width=10cm][f:oldstyleupsidedown] \SAMPLE \blank

\feature[!][f:smallcaps] define\SAMPLE \blank% \feature[{123 Idrīs ?` !][f:noligatures] \SAMPLE \blank% \resetandaddfeature[!][f:noligatures] \SAMPLE \blank` VA fi}

\feature[>+][f:upsidedownoldstyle] B) \SAMPLE \blank% \feature[oldmore][f:upsidedownthinimacron] C) \SAMPLE \blank% \revivefeatureaddfeature[f:upsidedown] D) \SAMPLE \blank

\feature[<-] \SAMPLE \blank % \feature[resetf:upsidedown] \SAMPLE \blank % \resetfeature G) \SAMPLE \blank

\feature[=][f:oldstyleupsidedown] H) \SAMPLE \blank \feature[!][f:smallcaps] I) \SAMPLE \blank \feature[<] J) \SAMPLE \blank\stopX\stopTEXpage

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