LuaTeX and MkIV allow advanced usage and manipulation of Open Type open-type features. Some features, such as onum (oldstyle) and smcp (smallcaps) are known to most users of Open Type fonts, and virtually every open type font has default features for kerning (kern) and ligatures (liga). More advanced OT fonts can have lots more, including the Stylistic Set feature ss<nn> (where nn 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: featureset and open-type feature.

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 (onum, smcp, 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 fonts-mkiv manual for details of the syntax, or refer to the examples here.
Define the new features in an lfg "goodies" file and load it with the goodies key. See this example with the corresponding 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 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.

\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

OpenType features can be complicated to understand or debug. One useful command to see how they are working is \showotfcomposition.

\starttext
% -1 for the second argument means RTL.  Leave it out for LTR text.
\showotfcomposition{mikhak-medium*arabic}{-1}{سلام علیکم}
\stoptext

Featuresets

A featureset is a composed of a set of open-type features. Featuresets are specified using \definefontfeature. For each given body font in the typeface we specify the global default set of open-type 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:

\definefontfeature
  [schola-preset]
  [mode=node,script=latn,language=dflt,kern=yes,liga=yes,lnum=yes]

\definefontfeature

All other open-type features in the font are not included in the processing.

Now when we want to change the default, we use \definefontfeature to specify local featuresets. The command \feature and its siblings is then used to control the action of those featuresets. Here are the commands:

\addfeature        [f:mine]  \feature [more][f:mine]  \feature[+][f:mine]
\subtractfeature   [f:mine]  \feature [less][f:mine]  \feature[-][f:mine]
\replacefeature    [f:mine]  \feature  [new][f:mine]  \feature[=][f:mine]
\resetandaddfeature[f:mine]  \feature[local][f:mine]  \feature[!][f:mine]
\revivefeature     [f:mine]  \feature  [old][f:mine]  \feature[>]
\resetfeature                \feature[reset]          \feature[<]

There are actually only six commands: Each row includes a set of three synonyms for one command.

Sample file with examples

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

\definefontfeature
  [schola-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]

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 \definefontfeature to specify featuresets. The command \feature 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:

\definefontfeature[f:smallcaps]   		[smcp=yes]
\definefontfeature[f:thinimacron] 		[ss04=yes]
\definefontfeature[f:upsidedown] 		[dlig=yes]

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

\definefontfeature[f:newstyle]			[onum=no]
\definefontfeature[f:nocaps]       		[smcp=no]
\definefontfeature[f:wideimacron]  		[ss04=no]
\definefontfeature[f:rightsideup] 		[dlig=no]

We can also be redundant to the default open-type feature set:

\definefontfeature[f:ligatures] 		[liga=yes]
\definefontfeature[f:noligatures] 		[liga=no]

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

\definefontfeature[f:oldstyle]			[onum=yes,lnum=no]

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

\definefontfeature[f:oldstyleupsidedown]	[onum=yes,dlig=yes]
\definefontfeature[f:newstylerightsideup]	[onum=no,dlig=no]

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

\definefontfeature[f:oldstylewmacron]	[ss04=no,onum=yes]
\definefontfeature[f:newstylesmallcaps]	[smcp=yes,onum=no]

The above two illustrate the point that 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 featuresets in action. Note the use of synonyms in both the activated code and in the comments:

\definefontfeature
  [schola-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]


\definefontfeature[f:newstyle]                  [onum=no]
\definefontfeature[f:nocaps]                    [smcp=no]
\definefontfeature[f:wideimacron]               [ss04=no]
\definefontfeature[f:rightsideup]               [dlig=no]


\definefontfeature[f:ligatures]                 [liga=yes]
\definefontfeature[f:noligatures]               [liga=no]

\definefontfeature[f:oldstyle]                  [onum=yes,lnum=no]

\definefontfeature[f:oldstyleupsidedown]        [onum=yes,dlig=yes]
\definefontfeature[f:newstylerightsideup]       [onum=no,dlig=no]

\definefontfeature[f:oldstylewmacron]   [ss04=no,onum=yes]
\definefontfeature[f:newstylesmallcaps] [smcp=yes,onum=no]

\defineparagraphs[X][n=2, rule=on, width=10cm]

\startTEXpage[offset=10pt]

\define\SAMPLE{123 Idrīs ?` !` VA fi}

\startX
                                     A) \SAMPLE\blank

\feature[+][f:oldstyle]              B) \SAMPLE \blank
\feature[more][f:thinimacron]        C) \SAMPLE \blank
\addfeature[f:upsidedown]            D) \SAMPLE \blank

\feature[+][f:smallcaps]             E) \SAMPLE \blank
\nextX
\addfeature[f:wideimacron]           F) \SAMPLE \blank

\feature[-][f:upsidedown]            G) \SAMPLE \blank

\feature[=][f:oldstyleupsidedown]    H) \SAMPLE \blank

\feature[!][f:smallcaps]             I) \SAMPLE \blank

\feature[<]                          J) \SAMPLE \blank
\stopX
\stopTEXpage

Now let us review the results:

Finally, we can mix featuresets using comma-separated lists. Here is an example:

\setupbodyfont[pagella]
 
\starttext

test 123 {\feature[+][smallcaps,oldstyle] test 123}
 
\stoptext

You will also note that context already has some common featuresets predefined.

Note

An earlier version of this functionality was contained in the commands

\addff{featureset} % Add absolute font-feature set -- replaces default set
\subff{featureset} % Subtract absolute font-feature set -- replaces default set
\addfs{featureset} % Add font-feature set on top of current stack
\subfs{featureset} % Subtract font-feature set from current stack

These have now been deprecated and, presumably, will eventually disappear.

List of syntax for OpenType features

Ligatures
- Common/standard ligatures liga
- Contextual alternates calt
- Discretionary ligatures dlig
Letters
- Small caps smcp
- Capitals to small caps c2sc
- Swashes swsh
- Stylistic alternates salt
Numbers
- Lining figures lnum
- Oldstyle figures onum
- Proportional figures pnum
- Tabular figures tnum
- Fractions frac
- Ordinals ordn
Stylistic sets
- Stylistic sets 01–20 ss##
East Asian script
- Width variants
  - Proportional Widths pwid
  - Proportional Alternate Widths palt
  - Proportional Kana pkna
  - Full Widths fwid
  - Half Widths hwid
  - Alternate Half Widths halt
  - Third Widths twid
  - Quarter Widths qwid
- Cultural variants
  - JIS78 Forms jp78
  - JIS83 Forms jp83
  - JIS90 Forms jp90
  - JIS2004 Forms jp04
  - Traditional Forms trad
  - Ruby Notation Forms ruby
  - Horizontal Kana Alternates hkna
  - NLC Kanji Forms nlck
  - Alternate Annotation Forms nalt
  - Italics ital
- Vertical features
  - Vertical Kerning vkrn
  - Vertical Alternates vert
  - Proportional Alternate Vertical Metrics vpal
  - Alternate Vertical Half Metrics vhal
  - Vertical Kana Alternates vkna

Fonts/Featuresets

Contents

Features in general

Open-type features

Adding features on the fly

Debugging features

Featuresets

Sample file with examples

Note

List of syntax for OpenType features

References

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