Previous: Artificial Fonts, Up: Fonts and Symbols


5.17.8 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 entities 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 number 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 number 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 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 point 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 point 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 point size.

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

Note that 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 it.

Sometimes, when typesetting letters of different fonts, more or less space at such boundaries are needed. There are two escapes to help with this.

— Escape: \/

Increase the width of the preceding glyph so that the spacing between that glyph and the following glyph is correct if the following glyph is a roman glyph. For example, if an italic f is immediately followed by a roman right parenthesis, then in many fonts the top right portion of the f overlaps the top left of the right parenthesis. Use this escape sequence whenever an italic glyph is immediately followed by a roman glyph without any intervening space. This small amount of space is also called italic correction.

— Escape: \,

Modify the spacing of the following glyph so that the spacing between that glyph and the preceding glyph is correct if the preceding glyph is a roman glyph. Use this escape sequence whenever a roman glyph is immediately followed by an italic glyph without any intervening space. In analogy to above, this space could be called left italic correction, but this term isn't used widely.

— Escape: \&

Insert a zero-width character, which is invisible. Its intended use is to stop interaction of a character with its surrounding.

— Escape: \)

This escape is similar to \& except that it behaves like a character declared with the cflags request to be transparent for the purposes of an end-of-sentence character.

Its main usage is in macro definitions to protect against arguments starting with a control character.

          
          .de xxx
          \)\\$1
          ..
          .de yyy
          \&\\$1
          ..
          This is a test.\c
          .xxx '
          This is a test.
              ⇒This is a test.'  This is a test.
          This is a test.\c
          .yyy '
          This is a test.
              ⇒This is a test.' This is a test.