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5.17.4 Using Symbols

A glyph is a graphical representation of a character. While a character is an abstract entity containing semantic information, a glyph is something which can be actually seen on screen or paper. It is possible that a character has multiple glyph representation forms (for example, the character `A' can be either written in a roman or an italic font, yielding two different glyphs); sometimes more than one character maps to a single glyph (this is a ligature – the most common is `fi').

A symbol is simply a named glyph. Within gtroff, all glyph names of a particular font are defined in its font file. If the user requests a glyph not available in this font, gtroff looks up an ordered list of special fonts. By default, the PostScript output device supports the two special fonts ‘SS’ (slanted symbols) and ‘S’ (symbols) (the former is looked up before the latter). Other output devices use different names for special fonts. Fonts mounted with the fonts keyword in the DESC file are globally available. To install additional special fonts locally (i.e. for a particular font), use the fspecial request.

Here the exact rules how gtroff searches a given symbol:

See Font Files, and Special Fonts, for more details.

The list of available symbols is device dependent; see the groff_char(7) man page for a complete list of all glyphs. For example, say

     
     man -Tdvi groff_char > groff_char.dvi

for a list using the default DVI fonts (not all versions of the man program support the -T option). If you want to use an additional macro package to change the used fonts, groff must be called directly:

     
     groff -Tdvi -mec -man groff_char.7 > groff_char.dvi

Glyph names not listed in groff_char(7) are derived algorithmically, using a simplified version of the Adobe Glyph List (AGL) algorithm which is described in http://partners.adobe.com/public/developer/opentype/index_glyph.html. The (frozen) set of glyph names which can't be derived algorithmically is called groff glyph list (GGL).

— Escape: \(nm
— Escape: \[name]
— Escape: \[component1 component2 ...]

Insert a symbol name (two-character name nm) or a composite glyph with component glyphs component1, component2, ... There is no special syntax for one-character names – the natural form ‘\n’ would collide with escapes.1

If name is undefined, a warning of type ‘char’ is generated, and the escape is ignored. See Debugging, for information about warnings.

groff resolves \[...] with more than a single component as follows:

No check for the existence of any component (similar to tr request) is done.

Examples:

\[A ho]
A’ maps to u0041, ‘ho’ maps to u02DB, thus the final glyph name would be u0041_02DB. Note this is not the expected result: The ogonek glyph ‘ho’ is a spacing ogonek, but for a proper composite a non-spacing ogonek (U+0328) is necessary. Looking into the file composite.tmac one can find ‘.composite ho u0328 which changes the mapping of ‘ho’ while a composite glyph name is constructed, causing the final glyph name to be u0041_0328.
\[^E u0301]
\[^E aa]
\[E a^ aa]
\[E ^ ']
^E’ maps to u0045_0302, thus the final glyph name is u0045_0302_0301 in all forms (assuming proper calls of the composite request).

It is not possible to define glyphs with names like ‘A ho within a groff font file. This is not really a limitation; instead, you have to define u0041_0328.

— Escape: \C'xxx'

Typeset the glyph named xxx.2 Normally it is more convenient to use \[xxx], but \C has the advantage that it is compatible with newer versions of AT&T troff and is available in compatibility mode.

— Request: .composite from to

Map glyph name from to glyph name to if it is used in \[...] with more than one component. See above for examples.

This mapping is based on glyph names only; no check for the existence of either glyph is done.

A set of default mappings for many accents can be found in the file composite.tmac which is loaded at start-up.

— Escape: \N'n'

Typeset the glyph with code n in the current font (n is not the input character code). The number n can be any non-negative decimal integer. Most devices only have glyphs with codes between 0 and 255; the Unicode output device uses codes in the range 0–65535. If the current font does not contain a glyph with that code, special fonts are not searched. The \N escape sequence can be conveniently used in conjunction with the char request:

          
          .char \[phone] \f[ZD]\N'37'

The code of each glyph is given in the fourth column in the font description file after the charset command. It is possible to include unnamed glyphs in the font description file by using a name of ‘---’; the \N escape sequence is the only way to use these.

No kerning is applied to glyphs accessed with \N.

Some escape sequences directly map onto special glyphs.

— Escape: \'

This is a backslash followed by the apostrophe character, ASCII character 0x27 (EBCDIC character 0x7D). The same as \[aa], the acute accent.

— Escape: \`

This is a backslash followed by ASCII character 0x60 (EBCDIC character 0x79 usually). The same as \[ga], the grave accent.

— Escape: \-

This is the same as \[-], the minus sign in the current font.

— Escape: \_

This is the same as \[ul], the underline character.

— Request: .cflags n c1 c2 ...

Input characters and symbols have certain properties associated with it.3 These properties can be modified with the cflags request. The first argument is the sum of the desired flags and the remaining arguments are the characters or symbols to have those properties. It is possible to omit the spaces between the characters or symbols. Instead of single characters or symbols you can also use character classes (see Character Classes for more details).

1
The character ends sentences (initially characters ‘.?!’ have this property).
2
Lines can be broken before the character (initially no characters have this property). This only works if both the characters before and after have non-zero hyphenation codes (as set with the hcode request). Use value 64 to override this behaviour.
4
Lines can be broken after the character (initially the character ‘-’ and the symbols ‘\[hy]’ and ‘\[em]’ have this property). This only works if both the characters before and after have non-zero hyphenation codes (as set with the hcode request). Use value 64 to override this behaviour.
8
The character overlaps horizontally if used as a horizontal line building element. Initially the symbols ‘\[ul]’, ‘\[rn]’, ‘\[ru]’, ‘\[radicalex]’, and ‘\[sqrtex]’ have this property.
16
The character overlaps vertically if used as vertical line building element. Initially symbol ‘\[br]’ has this property.
32
An end-of-sentence character followed by any number of characters with this property is treated as the end of a sentence if followed by a newline or two spaces; in other words the character is transparent for the purposes of end-of-sentence recognition – this is the same as having a zero space factor in TeX (initially characters ‘"')]*’ and the symbols ‘\[dg]’ and ‘\[rq]’ have this property).
64
Ignore hyphenation code values of the surrounding characters. Use this in combination with values 2 and 4 (initially no characters have this property). For example, if you need an automatic break point after the hyphen in number ranges like `3000-5000', insert
               
               .cflags 68 -

into your document. Note, however, that this can lead to bad layout if done without thinking; in most situations, a better solution instead of changing the cflags value is to insert \: right after the hyphen at the places which really need a break point.

128
Prohibit a line break before the character, but allow a line break after the character. This works only in combination with flags 256 and 512 (see below) and has no effect otherwise.
256
Prohibit a line break after the character, but allow a line break before the character. This works only in combination with flags 128 and 512 (see below) and has no effect otherwise.
512
Allow line break before or after the character. This works only in combination with flags 128 and 256 and has no effect otherwise.

Contrary to flag values 2 and 4, the flags 128, 256, and 512 work pairwise. If, for example, the left character has value 512, and the right character 128, no line break gets inserted. If we use value 6 instead for the left character, a line break after the character can't be suppressed since the right neighbour character doesn't get examined.

— Request: .char g [string]
— Request: .fchar g [string]
— Request: .fschar f g [string]
— Request: .schar g [string]

Define a new glyph g to be string (which can be empty).4 Every time glyph g needs to be printed, string is processed in a temporary environment and the result is wrapped up into a single object. Compatibility mode is turned off and the escape character is set to ‘\’ while string is being processed. Any emboldening, constant spacing or track kerning is applied to this object rather than to individual characters in string.

A glyph defined by these requests can be used just like a normal glyph provided by the output device. In particular, other characters can be translated to it with the tr or trin requests; it can be made the leader character by the lc request; repeated patterns can be drawn with the glyph using the \l and \L escape sequences; words containing the glyph can be hyphenated correctly if the hcode request is used to give the glyph's symbol a hyphenation code.

There is a special anti-recursion feature: Use of g within the glyph's definition is handled like normal characters and symbols not defined with char.

Note that the tr and trin requests take precedence if char accesses the same symbol.

          
          .tr XY
          X
              ⇒ Y
          .char X Z
          X
              ⇒ Y
          .tr XX
          X
              ⇒ Z

The fchar request defines a fallback glyph: gtroff only checks for glyphs defined with fchar if it cannot find the glyph in the current font. gtroff carries out this test before checking special fonts.

fschar defines a fallback glyph for font f: gtroff checks for glyphs defined with fschar after the list of fonts declared as font-specific special fonts with the fspecial request, but before the list of fonts declared as global special fonts with the special request.

Finally, the schar request defines a global fallback glyph: gtroff checks for glyphs defined with schar after the list of fonts declared as global special fonts with the special request, but before the already mounted special fonts.

See Using Symbols, for a detailed description of the glyph searching mechanism in gtroff.

— Request: .rchar c1 c2 ...
— Request: .rfschar f c1 c2 ...

Remove the definitions of glyphs c1, c2... This undoes the effect of a char, fchar, or schar request.

It is possible to omit the whitespace between arguments.

The request rfschar removes glyph definitions defined with fschar for glyph f.

See Special Characters.


Footnotes

[1] Note that a one-character symbol is not the same as an input character, i.e., the character a is not the same as \[a]. By default, groff defines only a single one-character symbol, \[-]; it is usually accessed as \-. On the other hand, gtroff has the special feature that \[charXXX] is the same as the input character with character code XXX. For example, \[char97] is identical to the letter a if ASCII encoding is active.

[2] \C is actually a misnomer since it accesses an output glyph.

[3] Note that the output glyphs themselves don't have such properties. For gtroff, a glyph is a numbered box with a given width, depth, and height, nothing else. All manipulations with the cflags request work on the input level.

[4] char is a misnomer since an output glyph is defined.