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5.19.8 Ligatures and Kerning

Ligatures are groups of characters that are run together, i.e, producing a single glyph. For example, the letters ‘f’ and ‘i’ can form a ligature ‘fi’ as in the word ‘file’. This produces a cleaner look (albeit subtle) to the printed output. Usually, ligatures are not available in fonts for TTY output devices.

Most PostScript fonts support the fi and fl ligatures. The C/A/T typesetter that was the target of AT&T troff also supported ‘ff’, ‘ffi’, and ‘ffl’ ligatures. Advanced typesetters or ‘expert’ fonts may include ligatures for ‘ft’ and ‘ct’, although GNU troff does not support these (yet).

Only the current font is checked for ligatures and kerns; neither special fonts nor special charcters defined with the char request (and its siblings) are taken into account.

Request: .lg [flag]
Register: \n[.lg]

Switch the ligature mechanism on or off; if the parameter is non-zero or missing, ligatures are enabled, otherwise disabled. Default is on. The current ligature mode can be found in the read-only register .lg (set to 1 or 2 if ligatures are enabled, 0 otherwise).

Setting the ligature mode to 2 enables the two-character ligatures (fi, fl, and ff) and disables the three-character ligatures (ffi and ffl).

Pairwise kerning is another subtle typesetting mechanism that modifies the distance between a glyph pair to improve readability. In most cases (but not always) the distance is decreased. Typewriter-like fonts and fonts for terminals where all glyphs have the same width don’t use kerning.

Request: .kern [flag]
Register: \n[.kern]

Switch kerning on or off. If the parameter is non-zero or missing, enable pairwise kerning, otherwise disable it. The read-only register .kern is set to 1 if pairwise kerning is enabled, 0 otherwise.

If the font description file contains pairwise kerning information, glyphs from that font are kerned. Kerning between two glyphs can be inhibited by placing \& between them: ‘V\&A’.

See Font Description File Format.

Track kerning expands or reduces the space between glyphs. This can be handy, for example, if you need to squeeze a long word onto a single line or spread some text to fill a narrow column. It must be used with great care since it is usually considered bad typography if the reader notices the effect.

Request: .tkf f s1 n1 s2 n2

Enable track kerning for font f. If the current font is f the width of every glyph is increased by an amount between n1 and n2 (n1, n2 can be negative); if the current type size is less than or equal to s1 the width is increased by n1; if it is greater than or equal to s2 the width is increased by n2; if the type size is greater than or equal to s1 and less than or equal to s2 the increase in width is a linear function of the type size.

The default scaling unit is ‘z’ for s1 and s2, ‘p’ for n1 and n2.

The track kerning amount is added even to the rightmost glyph in a line; for large values it is thus recommended to increase the line length by the same amount to compensate.

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