6.2.2 Font Description File Format

On typesetting output devices, each font is typically available at multiple sizes. While paper measurements in the device description file are in absolute units, measurements applicable to fonts must be proportional to the type size. groff achieves this using the precedent set by AT&T device-independent troff: one font size is chosen as a norm, and all others are scaled linearly relative to that basis. The “unit width” is the number of basic units per point when the font is rendered at this nominal size.

For instance, groff’s lbp device uses a unitwidth of 800. Its Times roman font ‘TR’ has a spacewidth of 833; this is also the width of its comma, period, centered period, and mathematical asterisk, while its ‘M’ is 2,963 basic units. Thus, an ‘M’ on the lbp device is 2,963 basic units wide at a notional type size of 800 points.¹²⁶

A font description file has two sections. The first is a sequence of directives, and is parsed similarly to the DESC file described above. Except for the directive names that begin the second section, their ordering is immaterial. Later directives of the same name override earlier ones, spaces and tabs are handled in the same way, and the same comment syntax is supported. Empty lines are ignored throughout.

name f ¶

The name of the font is f. ‘DESC’ is an invalid font name. Simple integers are valid, but their use is discouraged.¹²⁷

spacewidth n ¶

The width of an unadjusted inter-word space is n basic units.

The directives above must appear in the first section; those below are optional.

slant n ¶

The font’s glyphs have a slant of n degrees; a positive n slants in the direction of text flow.

ligatures lig1 … lign [0] ¶

Glyphs lig1, …, lign are ligatures; possible ligatures are ‘ff’, ‘fi’, ‘fl’, ‘ffi’ and ‘ffl’. For compatibility with other troff implementations, the list of ligatures may be terminated with a 0. The list of ligatures must not extend over more than one line.

special ¶

The font is special: when a glyph is requested that is not present in the current font, it is sought in any mounted fonts that bear this property.

Other directives in this section are ignored by GNU troff, but may be used by postprocessors to obtain further information about the font.

The second section contains one or two subsections. These can appear in either order; the first one encountered commences the second section. Each starts with a directive on a line by itself. A charset subsection is mandatory unless the associated DESC file contains the unicode directive. Another subsection, kernpairs, is optional.

The directive charset starts the character set subsection.¹²⁸ It precedes a series of glyph descriptions, one per line. Each such glyph description comprises a set of fields separated by spaces or tabs and organized as follows.

name metrics type code [entity-name] [-- comment]

name identifies the glyph: if name is a printable character c, it corresponds to the troff ordinary character c. If name is a multi-character sequence not beginning with \, it corresponds to the GNU troff special character escape sequence ‘\[name]’. A name consisting of three minus signs, ‘---’, is special and indicates that the glyph is unnamed: such glyphs can be accessed only by the \N escape sequence in troff. A special character named ‘---’ can still be defined using char and similar requests. The name ‘\-’ defines the minus sign glyph. Finally, name can be the unbreakable one-sixth and one-twelfth space escape sequences, \| and \^ (“thin” and “hair” spaces, respectively), in which case only the width metric described below is interpreted; a font can thus customize the widths of these spaces.

The form of the metrics field is as follows.

width[,[height[,[depth[,[italic-correction
  [,[left-italic-correction[,[subscript-correction]]]]]]]]]]

There must not be any spaces, tabs, or newlines between these subfields (which have been split here into two lines only for better legibility). The subfields are in basic units expressed as decimal integers. Unspecified subfields default to 0. Since there is no associated binary format, these values are not required to fit into the C language data type ‘char’ as they are in AT&T device-independent troff.

The width subfield gives the width of the glyph. The height subfield gives the height of the glyph (upward is positive); if a glyph does not extend above the baseline, it should be given a zero height, rather than a negative height. The depth subfield gives the depth of the glyph, that is, the distance below the baseline to which the glyph extends (downward is positive); if a glyph does not extend below the baseline, it should be given a zero depth, rather than a negative depth. Italic corrections are relevant to glyphs in italic or oblique styles. The italic-correction is the amount of space that should be added after an oblique glyph to be followed immediately by an upright glyph. The left-italic-correction is the amount of space that should be added before an oblique glyph to be preceded immediately by an upright glyph. The subscript-correction is the amount of space that should be added after an oblique glyph to be followed by a subscript; it should be less than the italic correction.

For fonts used with typesetting devices, the type field gives a featural description of the glyph: it is a bit mask recording whether the glyph is an ascender, descender, both, or neither. When a \w escape sequence is interpolated, these values are bitwise or-ed together for each glyph and stored in the nr register. In font descriptions for terminal devices, all glyphs might have a type of zero, regardless of their appearance.

0

means the glyph lies entirely between the baseline and a horizontal line at the “x-height” of the font; typical examples are ‘a’, ‘c’, and ‘x’;

1

means the glyph descends below the baseline, like ‘p’;

2

means the glyph ascends above the font’s x-height, like ‘A’ or ‘b’; and

3

means the glyph is both an ascender and a descender—this is true of parentheses in some fonts.

The code field gives a numeric identifier that the postprocessor uses to render the glyph. The glyph can be specified to troff using this code by means of the \N escape sequence. code can be any integer.¹²⁹

The entity-name field defines an identifier for the glyph that the postprocessor uses to print the GNU troff glyph name. This field is optional; it was introduced so that the grohtml output driver could encode its character set. For example, the glyph ‘\[Po]’ is represented by ‘£’ in HTML 4.0. For efficiency, these data are now compiled directly into grohtml. grops uses the field to build sub-encoding arrays for PostScript fonts containing more than 256 glyphs. Anything on the line after the entity-name field or ‘--’ is ignored.

A line in the charset section can also have the form

name "

identifying name as another name for the glyph mentioned in the preceding line. Such aliases can be chained.

The directive kernpairs starts a list of kerning adjustments to be made to adjacent glyph pairs from this font. It contains a sequence of lines formatted as follows.

g1 g2 n

The foregoing means that when glyph g1 is typeset immediately before g2, the space between them should be increased by n. Most kerning pairs should have a negative value for n.