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FreeType Glyph Conventions
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version 2.1
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Copyright 1998-2000 David Turner (<a href="mailto:[email protected]">[email protected]</a>)<br>
Copyright 2000 The FreeType Development Team (<a href="[email protected]">[email protected]</a>)
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<h2>
I. Basic typographic concepts
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1. Font files, format and information
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<p>
A font is a collection of various character images that can
be used to display or print text. The images in a single font share some
common properties, including look, style, serifs, etc.. Typographically
speaking, one has to distinguish between a <b>font family</b> and its
multiple <b>font faces</b>, which usually differ in style though come
from the same template.</p>
For example, "<i>Palatino Regular</i>" and "<i>Palatino Italic</i>" are
two distinct <i>faces</i> from the same famous <i>family</i>, called
"<i>Palatino</i>" itself.</p>
<p>The single term font is nearly always used in ambiguous ways to refer
to either a given family or given face, depending on the context. For example,
most users of word-processors use "font" to describe a font family (e.g.
Courier, Palatino, etc..); however most of these families are implemented
through several data files depending on the file format : for TrueType,
this is usually one per face (i.e. ARIAL.TFF for "Arial Regular", ARIALI.TTF
for "Arial Italic", etc..). The file is also called a "font" but really
contains a font face.
</p>
<p>A <i>digital font</i> is thus a data file that may contain <i>one or
more font faces</i>. For each of these, it contains character images,
character metrics, as well as other kind of information important to the
layout of text and the processing of specific character encodings. In some
awkward formats, like Adobe Type1, a single font face is described through
several files (i.e. one contains the character images, another one the
character metrics). We will ignore this implementation issue in most of
this document and consider digital fonts as single files, though FreeType
2.0 is able to support multiple-files fonts correctly.
</p>
<p>As a convenience, a font file containing more than one face is called
a font collection. This case is rather rare but can be seen in many asian
fonts, which contain images for two or more scripts for a given language.
</p>
</blockquote><h3><a name="section-2">
2. Character images and mappings :
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<p>The character images are called <b>glyphs</b>. A single character
can have several distinct images, i.e. several glyphs, depending on script,
usage or context. Several characters can also take a single glyph (good
examples are roman ligatures like "oe" and "fi" which can be represented
by a single glyph). The relationships between characters
and glyphs can be a very complex one but won't be detailed in this document.
Moreover, some formats use more or less awkward schemes to store and access
the glyphs. For the sake of clarity, we'll only retain the following notions
when working with FreeType :</p>
<ul>
<p><li>
A font file contains a set of glyphs, each one can be stored as a bitmap,
a vector representation or any other scheme (e.g. most scalable formats
use a combination of math representation and control data/programs). These
glyphs can be stored in any order in the font file, and is typically accessed
through a simple glyph index.
</li></p>
<p><li>
The font file contains one (or more) table, called a character map (or
charmap in short), which is used to convert character codes for a given
encoding (e.g. ASCII, Unicode, DBCS, Big5, etc..) into glyph indexes
relative to the font file. A single font face may contain several charmaps.
For example, most TrueType fonts contain an Apple-specific charmap as well
as a Unicode charmap, which makes them usable on both Mac and Windows
platforms.
</li></p>
</ul>
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3. Character and font metrics :
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<p>Each glyph image is associated to various metrics which are used to
describe the way it must be placed and managed when rendering text. Though
they are described in more details in section III, they relate to glyph
placement, cursor advances as well as text layouts. They are extremely
important to compute the flow of text when rendering string of text.
</p>
<p>Each scalable format also contains some global metrics, expressed in
notional units, used to describe some properties of all glyphs in a same
face. For example : the maximum glyph bounding box, the ascender, descender
and text height for the font.
</p>
<p>Though these metrics also exist for non-scalable formats, they only
apply for a set of given character dimensions and resolutions, and they're
usually expressed in pixels then.</p>
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