OpenType features

An OpenType font can have optional typographic features in its GSUB (glyph substitution) and GPOS (glyph positioning) tables. Each feature has a four-letter tag, such as smcp for small capitals.

Features that are always on

The library applies the features that correct text needs, for every run of text:

Tag	Feature
rvrn	Required variation alternates
ltra, ltrm, rtla, rtlm	Left-to-right and right-to-left alternates and mirrored forms
ccmp	Glyph composition and decomposition
locl	Localized forms
rlig	Required ligatures
mark, mkmk	Mark positioning (accents)
calt	Contextual alternates
clig	Contextual ligatures
liga	Standard ligatures, such as fi and fl
rclt	Required contextual alternates
curs	Cursive positioning
kern	Kerning

In vertical text, vert replaces the horizontal-only features calt, clig, liga, rclt, curs and kern. The complex-script shapers add the features their scripts need, for example init, medi and fina for Arabic.

A feature has an effect only if the font has it. Most fonts have only some of them.

Turn on more features

Add tags to TextOptions.FeatureTags. The FeatureTags enumeration names the registered features, and Tag.Parse makes a tag from its four letters:

using PinataLabs.Typography;
using PinataLabs.Typography.Tables.AdvancedTypographic;

TextOptions options = new(font)
{
    FeatureTags =
    [
        FeatureTags.SmallCapitals,   // smcp
        FeatureTags.OldstyleFigures, // onum
        Tag.Parse("ss01"),           // Stylistic set 1
    ],
};

FeatureTags values convert to Tag without a cast. Use Tag.Parse for tags that the enumeration does not have, such as the stylistic sets ss01 to ss20 and the character variants cv01 to cv99.

The features apply to the whole text. To apply a feature to only part of the text, measure or render that part with its own TextOptions.

Some features that are often useful:

FeatureTags value	Tag	Effect
SmallCapitals	smcp	Lower-case letters as small capitals.
SmallCapitalsFromCapitals	c2sc	Capital letters as small capitals.
OldstyleFigures	onum	Figures that rise and fall like lower-case letters.
LiningFigures	lnum	Figures that are all the height of capitals.
TabularFigures	tnum	Figures that are all the same width, for tables of numbers.
ProportionalFigures	pnum	Figures with their natural widths.
SlashedZero	zero	A zero with a slash, to tell it from the letter O.
Fractions	frac	Fractions such as 1/2 set as one fraction glyph or a built-up fraction.
Ordinals	ordn	Ordinal forms, such as the o in 1º.
DiscretionaryLigatures	dlig	Decorative ligatures, such as ct and st.
HistoricalLigatures	hlig	Historical ligatures.
Swash	swsh	Swash forms.
CaseSensitiveForms	case	Punctuation moved up to fit capital letters.

The full list of registered tags is in the OpenType feature registry.

Fractions

When you turn on frac, the library finds the digits on each side of a slash (/) or a fraction slash (U+2044). It applies the numerator (numr) feature to the digits before the slash and the denominator (dnom) feature to the digits after it. Other digits are not changed.

options.FeatureTags = [FeatureTags.Fractions];
TextMeasurer.MeasureSize("Add 3/4 cup of flour", options);

You cannot turn off a feature that is always on

FeatureTags only adds features. There is no way to turn off a feature from the table at the top of this page, such as liga. Kerning is the exception: use KerningMode.

Kerning

Kerning adjusts the space between pairs of glyphs, for example to move A and V closer together. The library uses the GPOS kern feature. If the font has no GPOS kerning, the library uses the older kern table.

KerningMode	Effect
Standard (default)	Apply kerning from the font.
Auto	Apply kerning when the layout engine chooses. At present this has the same effect as Standard.
None	Do not apply kerning.

options.KerningMode = KerningMode.None;