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 that 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:

• If the symbol has been defined with the char request, use it. This hides a symbol with the same name in the current font.
• Check the current font.
• If the symbol has been defined with the fchar request, use it.
• Check whether the current font has a font-specific list of special fonts; test all fonts in the order of appearance in the last fspecial call if appropriate.
• If the symbol has been defined with the fschar request for the current font, use it.
• Check all fonts in the order of appearance in the last special call.
• If the symbol has been defined with the schar request, use it.
• As a last resort, consult all fonts loaded up to now for special fonts and check them, starting with the lowest font number. Note that this can sometimes lead to surprising results since the fonts line in the DESC file often contains empty positions, which are filled later on. For example, consider the following:

  fonts 3 0 0 FOO
  

  This mounts font foo at font position 3. We assume that FOO is a special font, containing glyph foo, and that no font has been loaded yet. The line

  .fspecial BAR BAZ
  

  makes font BAZ special only if font BAR is active. We further assume that BAZ is really a special font, i.e., the font description file contains the special keyword, and that it also contains glyph foo with a special shape fitting to font BAR. After executing fspecial, font BAR is loaded at font position 1, and BAZ at position 2.

  We now switch to a new font XXX, trying to access glyph foo that is assumed to be missing. There are neither font-specific special fonts for XXX nor any other fonts made special with the special request, so gtroff starts the search for special fonts in the list of already mounted fonts, with increasing font positions. Consequently, it finds BAZ before FOO even for XXX, which is not the intended behaviour.

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 that can’t be derived algorithmically is called groff glyph list (GGL).

• A glyph for Unicode character U+XXXX[X[X]], which is not a composite character is named uXXXX[X[X]]. X must be an uppercase hexadecimal digit. Examples: u1234, u008E, u12DB8. The largest Unicode value is 0x10FFFF. There must be at least four X digits; if necessary, add leading zeroes (after the ‘u’). No zero padding is allowed for character codes greater than 0xFFFF. Surrogates (i.e., Unicode values greater than 0xFFFF represented with character codes from the surrogate area U+D800-U+DFFF) are not allowed too.
• A glyph representing more than a single input character is named

  ‘u’ component1 ‘_’ component2 ‘_’ component3 …
  

  Example: u0045_0302_0301.

  For simplicity, all Unicode characters that are composites must be decomposed maximally (this is normalization form D in the Unicode standard); for example, u00CA_0301 is not a valid glyph name since U+00CA (LATIN CAPITAL LETTER E WITH CIRCUMFLEX) can be further decomposed into U+0045 (LATIN CAPITAL LETTER E) and U+0302 (COMBINING CIRCUMFLEX ACCENT). u0045_0302_0301 is thus the glyph name for U+1EBE, LATIN CAPITAL LETTER E WITH CIRCUMFLEX AND ACUTE.

• groff maintains a table to decompose all algorithmically derived glyph names that are composites itself. For example, u0100 (LATIN LETTER A WITH MACRON) is automatically decomposed into u0041_0304. Additionally, a glyph name of the GGL is preferred to an algorithmically derived glyph name; groff also automatically does the mapping. Example: The glyph u0045_0302 is mapped to ^E.
• glyph names of the GGL can’t be used in composite glyph names; for example, ^E_u0301 is invalid.

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.¹⁷

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:

• Any component that is found in the GGL is converted to the uXXXX form.
• Any component uXXXX that is found in the list of decomposable glyphs is decomposed.
• The resulting elements are then concatenated with ‘_’ in between, dropping the leading ‘u’ in all elements but the first.

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.¹⁸ 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.¹⁹ 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]’, ‘\[rq]’, and ‘\[cq]’ 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 that 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).²⁰ 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.