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  <title>FreeType Glyph Conventions</title>
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  FreeType Glyph Conventions
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<h2 align=center>
  Version&nbsp;2.1
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<h3 align=center>
  Copyright&nbsp;1998-2000 David Turner (<a
  href="mailto:david@freetype.org">david@freetype.org</a>)<br>
  Copyright&nbsp;2000 The FreeType Development Team (<a
  href="mailto:devel@freetype.org">devel@freetype.org</a>)
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    <h2>
      I. Basic typographic concepts
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<a name="section-1">
    <h3>
      1. Font files, format and information
    </h3>

<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 <em>font family</em> and its
    multiple <em>font faces</em>, which usually differ in style though come
    from the same template.</p>

For example, "Palatino Regular" and "Palatino Italic" are two distinct
    <em>faces</em> from the same famous <em>family</em>, called "Palatino"
    itself.</p>

<p>The single term <em>font</em> 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.
    <tt>arial.ttf</tt> for "Arial Regular", <tt>ariali.ttf</tt> for "Arial
    Italic", etc.).  The file is also called a "font" but really contains a
    font face.</p>

<p>A <em>digital font</em> is thus a data file that may contain <em>one
    or more font faces</em>.  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's Type&nbsp;1, 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 parts of this document and consider
    digital fonts as single files, though FreeType&nbsp;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 <em>font collection</em>.  This case is rather rare but can be seen in
    many Asian fonts, which contain images for two or more representation
    forms of a given scripts (usually for horizontal and vertical
    layout.</p>

<a name="section-2">
    <h3>
      2. Character images and mappings
    </h3>

<p>The character images are called <em>glyphs</em>.  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 "fi" and "fl" which can be
    represented by a single glyph).  The relationships between characters
    and glyphs can be very complex, but won't be discussed in this document.
    Moreover, some formats use more or less awkward schemes to store and
    access glyphs.  For the sake of clarity, we only retain the following
    notions when working with FreeType:</p>

<ul>
      <li>
        <p>A font file contains a set of glyphs; each one can be stored as a
        bitmap, a vector representation or any other scheme (most scalable
        formats use a combination of mathematical 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.</p>
      </li>
      <li>
        <p>The font file contains one or more tables, called a <em>character
        map</em> (or charmap in short), which is used to convert character
        codes for a given encoding (e.g. ASCII, Unicode, DBCS, Big5, etc..)
        into glyph indices 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.</p>
      </li>
    </ul>

<a name="section-3">
    <h3>
      3. Character and font metrics
    </h3>

<p>Each glyph image is associated with various metrics which are used to
    describe how must be placed and managed when rendering text.  These
    are described in more details in section&nbsp;III, they relate to
    glyph placement, cursor advances as well as text layout.  They are
    extremely important to compute the flow of text when rendering a string
    of text.</p>

<p>Each scalable format also contains some global metrics, expressed in
    notional units, to describe some properties of all glyphs in the same
    face.  Examples for global metrics are 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
    are usually expressed in pixels then.</p>

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