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<p>
Previous: <a href="DESC-File-Format.html" accesskey="p" rel="prev"><samp class="file">DESC</samp> File Format</a>, Up: <a href="Device-and-Font-Description-Files.html" accesskey="u" rel="up">Device and Font Description Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Request-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<h4 class="subsection" id="Font-Description-File-Format-1">6.2.2 Font Description File Format</h4>
<a class="index-entry-id" id="index-font-file_002c-format"></a>
<a class="index-entry-id" id="index-font-description-file_002c-format"></a>
<a class="index-entry-id" id="index-format-of-font-files"></a>
<a class="index-entry-id" id="index-format-of-font-description-files"></a>
<p>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. <code class="code">groff</code> achieves this using the
precedent set by <abbr class="acronym">AT&T</abbr> device-independent <code class="code">troff</code>: 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.
</p>
<p>For instance, <code class="code">groff</code>’s <code class="code">lbp</code> device uses a <code class="code">unitwidth</code>
of 800. Its Times roman font ‘<samp class="samp">TR</samp>’ has a <code class="code">spacewidth</code>
of 833; this is also the width of its comma, period, centered
period, and mathematical asterisk, while its ‘<samp class="samp">M</samp>’ is 2,963 basic
units. Thus, an ‘<samp class="samp">M</samp>’ on the <code class="code">lbp</code> device is 2,963 basic units
wide at a notional type size of 800 points.<a class="footnote" id="DOCF126" href="groff.html_fot.html#FOOT126"><sup>126</sup></a>
</p>
<p>A font description file has two sections. The first is a sequence of
directives, and is parsed similarly to the <samp class="file">DESC</samp> 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,
<a class="index-entry-id" id="index-comments-in-font-description-files"></a>
<a class="index-entry-id" id="index-font-description-files_002c-comments"></a>
<a class="index-entry-id" id="index-_0023-1"></a>
and the same comment syntax is supported. Empty lines are ignored
throughout.
</p>
<dl class="table">
<dt id='index-name'><span><code class="code">name <var class="var">f</var></code><a class="copiable-link" href='#index-name'> ¶</a></span></dt>
<dd><p>The name of the font is <var class="var">f</var>. ‘<samp class="samp">DESC</samp>’ is an invalid font
name. Simple integers are valid, but their use is
discouraged.<a class="footnote" id="DOCF127" href="groff.html_fot.html#FOOT127"><sup>127</sup></a>
</p>
</dd>
<dt id='index-spacewidth'><span><code class="code">spacewidth <var class="var">n</var></code><a class="copiable-link" href='#index-spacewidth'> ¶</a></span></dt>
<dd><p>The width of an unadjusted inter-word space is <var class="var">n</var> basic units.
</p></dd>
</dl>
<p>The directives above must appear in the first section; those below are
optional.
</p>
<dl class="table">
<dt id='index-slant'><span><code class="code">slant <var class="var">n</var></code><a class="copiable-link" href='#index-slant'> ¶</a></span></dt>
<dd><p>The font’s glyphs have a slant of <var class="var">n</var> degrees; a positive
<var class="var">n</var> slants in the direction of text flow.
</p>
</dd>
<dt id='index-ligatures'><span><code class="code">ligatures <var class="var">lig1</var> <span class="r">…</span> <var class="var">lign</var> <span class="r">[</span>0<span class="r">]</span></code><a class="copiable-link" href='#index-ligatures'> ¶</a></span></dt>
<dd><p>Glyphs <var class="var">lig1</var>, …, <var class="var">lign</var> are ligatures; possible ligatures
are ‘<samp class="samp">ff</samp>’, ‘<samp class="samp">fi</samp>’, ‘<samp class="samp">fl</samp>’, ‘<samp class="samp">ffi</samp>’ and ‘<samp class="samp">ffl</samp>’. For
compatibility with other <code class="code">troff</code> implementations, the list of
ligatures may be terminated with a <code class="code">0</code>. The list of ligatures
must not extend over more than one line.
</p>
</dd>
<dt id='index-special-fonts-2'><span><code class="code">special</code><a class="copiable-link" href='#index-special-fonts-2'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-special-1"></a>
<p>The font is <em class="dfn">special</em>: when a glyph is requested that is not present
in the current font, it is sought in any mounted fonts that bear this
property.
</p></dd>
</dl>
<p>Other directives in this section are ignored by GNU <code class="code">troff</code>, but
may be used by postprocessors to obtain further information about the
font.
</p>
<p>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 <code class="code">charset</code>
subsection is mandatory unless the associated <samp class="file">DESC</samp> file contains
the <code class="code">unicode</code> directive. Another subsection, <code class="code">kernpairs</code>,
is optional.
</p>
<a class="index-entry-id" id="index-charset-1"></a>
<p>The directive <code class="code">charset</code> starts the character set
subsection.<a class="footnote" id="DOCF128" href="groff.html_fot.html#FOOT128"><sup>128</sup></a> 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.
</p>
<blockquote class="quotation">
<p><var class="var">name</var> <var class="var">metrics</var> <var class="var">type</var> <var class="var">code</var> [<var class="var">entity-name</var>]
[<code class="code">--</code> <var class="var">comment</var>]
</p></blockquote>
<a class="index-entry-id" id="index-8_002dbit-input"></a>
<a class="index-entry-id" id="index-input_002c-8_002dbit"></a>
<a class="index-entry-id" id="index-accessing-unnamed-glyphs-with-_005cN"></a>
<a class="index-entry-id" id="index-unnamed-glyphs_002c-accessing-with-_005cN"></a>
<a class="index-entry-id" id="index-characters_002c-unnamed_002c-accessing-with-_005cN"></a>
<a class="index-entry-id" id="index-glyphs_002c-unnamed_002c-accessing-with-_005cN"></a>
<a class="index-entry-id" id="index-_002d_002d_002d"></a>
<p><var class="var">name</var> identifies the glyph:
if <var class="var">name</var> is a printable character <var class="var">c</var>, it corresponds to
the <code class="code">troff</code> ordinary character <var class="var">c</var>. If <var class="var">name</var> is a
multi-character sequence not beginning with <code class="code">\</code>, it corresponds to
the GNU <code class="code">troff</code> special character escape sequence
‘<samp class="samp">\[<var class="var">name</var>]</samp>’. A name consisting of three minus signs,
‘<samp class="samp">---</samp>’, is special and indicates that the glyph is unnamed: such
glyphs can be accessed only by the <code class="code">\N</code> escape sequence in
<code class="code">troff</code>. A special character named ‘<samp class="samp">---</samp>’ can still be defined
using <code class="code">char</code> and similar requests. The <var class="var">name</var> ‘<samp class="samp">\-</samp>’
defines the minus sign glyph. Finally, <var class="var">name</var> can be the
unbreakable one-sixth and one-twelfth space escape sequences, <code class="code">\|</code>
and <code class="code">\^</code> (“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.
</p>
<p>The form of the <var class="var">metrics</var> field is as follows.
</p>
<div class="display">
<div class="group"><pre class="display-preformatted"><var class="var">width</var>[<code class="code">,</code>[<var class="var">height</var>[<code class="code">,</code>[<var class="var">depth</var>[<code class="code">,</code>[<var class="var">italic-correction</var>
[<code class="code">,</code>[<var class="var">left-italic-correction</var>[<code class="code">,</code>[<var class="var">subscript-correction</var>]]]]]]]]]]
</pre></div></div>
<p>There must not be any spaces, tabs, or newlines between these
<em class="dfn">subfields</em> (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 <code class="code">0</code>.
Since there is no associated binary format, these values are not
required to fit into the C language data type ‘<samp class="samp">char</samp>’ as they are in
<abbr class="acronym">AT&T</abbr> device-independent <code class="code">troff</code>.
</p>
<p>The <var class="var">width</var> subfield gives the width of the glyph. The <var class="var">height</var>
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 <var class="var">depth</var> 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 <var class="var">italic-correction</var> is the amount of space that should
be added after an oblique glyph to be followed immediately by an upright
glyph. The <var class="var">left-italic-correction</var> is the amount of space that
should be added before an oblique glyph to be preceded immediately by an
upright glyph. The <var class="var">subscript-correction</var> 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.
</p>
<p>For fonts used with typesetting devices, the <var class="var">type</var> 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 <code class="code">\w</code>
escape sequence is interpolated, these values are bitwise or-ed
together for each glyph and stored in the <code class="code">nr</code> register. In font
descriptions for terminal devices, all glyphs might have a type of zero,
regardless of their appearance.
</p>
<dl class="table">
<dt><code class="code">0</code></dt>
<dd><p>means the glyph lies entirely between the baseline and a horizontal line
at the “x-height” of the font; typical examples are ‘<samp class="samp">a</samp>’,
‘<samp class="samp">c</samp>’, and ‘<samp class="samp">x</samp>’;
</p>
</dd>
<dt><code class="code">1</code></dt>
<dd><p>means the glyph descends below the baseline, like ‘<samp class="samp">p</samp>’;
</p>
</dd>
<dt><code class="code">2</code></dt>
<dd><p>means the glyph ascends above the font’s x-height, like ‘<samp class="samp">A</samp>’ or
‘<samp class="samp">b</samp>’; and
</p>
</dd>
<dt><code class="code">3</code></dt>
<dd><p>means the glyph is both an ascender and a descender—this is true of
parentheses in some fonts.
</p></dd>
</dl>
<p>The <var class="var">code</var> field gives a numeric identifier that the postprocessor
uses to render the glyph. The glyph can be specified to <code class="code">troff</code>
using this code by means of the <code class="code">\N</code> escape sequence. <var class="var">code</var>
can be any integer.<a class="footnote" id="DOCF129" href="groff.html_fot.html#FOOT129"><sup>129</sup></a>
</p>
<p>The <var class="var">entity-name</var> field defines an identifier for the glyph that the
postprocessor uses to print the GNU <code class="code">troff</code> glyph <var class="var">name</var>. This
field is optional; it was introduced so that the <code class="code">grohtml</code> output
driver could encode its character set. For example, the glyph
‘<samp class="samp">\[Po]</samp>’ is represented by ‘<samp class="samp">&pound;</samp>’ in <abbr class="acronym">HTML</abbr> 4.0.
For efficiency, these data are now compiled directly into
<code class="code">grohtml</code>. <code class="code">grops</code> uses the field to build sub-encoding
arrays for PostScript fonts containing more than 256 glyphs. Anything
on the line after the <var class="var">entity-name</var> field or ‘<samp class="samp">--</samp>’ is ignored.
</p>
<p>A line in the <code class="code">charset</code> section can also have the form
</p>
<div class="example">
<div class="group"><pre class="example-preformatted"><var class="var">name</var> "
</pre></div></div>
<p>identifying <var class="var">name</var> as another name for the glyph mentioned in the
preceding line. Such aliases can be chained.
</p>
<a class="index-entry-id" id="index-kernpairs"></a>
<p>The directive <code class="code">kernpairs</code> 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.
</p>
<div class="example">
<div class="group"><pre class="example-preformatted"><var class="var">g1</var> <var class="var">g2</var> <var class="var">n</var>
</pre></div></div>
<p>The foregoing means that when glyph <var class="var">g1</var> is typeset immediately
before <var class="var">g2</var>, the space between them should be increased
by <var class="var">n</var>. Most kerning pairs should have a negative value
for <var class="var">n</var>.
</p>
</div>
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