2000-11-09 07:26:30 +01:00
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en"
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"http://www.w3.org/TR/REC-html40/loose.dtd">
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2000-11-09 01:01:38 +01:00
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<html>
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2000-11-09 07:26:30 +01:00
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<meta http-equiv="Content-Type"
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content="text/html; charset=iso-8859-1">
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<meta name="Author"
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content="David Turner">
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<title>FreeType Glyph Conventions</title>
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2000-11-09 01:01:38 +01:00
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</head>
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<body text="#000000"
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bgcolor="#FFFFFF"
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link="#0000EF"
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vlink="#51188E"
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alink="#FF0000">
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2000-11-09 07:26:30 +01:00
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<h1 align=center>
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FreeType Glyph Conventions
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</h1>
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<h2 align=center>
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Version 2.1
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2000-11-09 01:01:38 +01:00
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</h2>
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2000-11-09 07:26:30 +01:00
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<h3 align=center>
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Copyright 1998-2000 David Turner (<a
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href="mailto:david@freetype.org">david@freetype.org</a>)<br>
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Copyright 2000 The FreeType Development Team (<a
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href="mailto:devel@freetype.org">devel@freetype.org</a>)
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</h3>
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<center>
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<table width="65%">
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<tr><td>
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<center>
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<table width="100%"
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border=0
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cellpadding=5>
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<tr bgcolor="#CCFFCC"
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valign=center>
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<td align=center
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width="30%">
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</td>
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<td align=center
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width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center
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width="30%">
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<a href="glyphs-2.html">Next</a>
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</td>
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</tr>
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</table>
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</center>
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2000-11-09 09:01:18 +01:00
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<p><hr></p>
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2000-11-09 07:26:30 +01:00
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<table width="100%">
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<tr bgcolor="#CCCCFF"
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valign=center><td>
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<h2>
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I. Basic typographic concepts
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</h2>
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</td></tr>
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</table>
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<a name="section-1">
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<h3>
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1. Font files, format and information
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</h3>
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<p>A font is a collection of various character images that can be used
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to display or print text. The images in a single font share some common
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properties, including look, style, serifs, etc. Typographically
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speaking, one has to distinguish between a <em>font family</em> and its
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multiple <em>font faces</em>, which usually differ in style though come
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from the same template.</p>
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For example, "Palatino Regular" and "Palatino Italic" are two distinct
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<em>faces</em> from the same famous <em>family</em>, called "Palatino"
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itself.</p>
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<p>The single term <em>font</em> is nearly always used in ambiguous ways
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to refer to either a given family or given face, depending on the
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context. For example, most users of word-processors use "font" to
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describe a font family (e.g. "Courier", "Palatino", etc.); however most
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of these families are implemented through several data files depending
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2000-11-10 23:39:21 +01:00
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on the file format: For TrueType, this is usually one per face (i.e.
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<tt>arial.ttf</tt> for "Arial Regular", <tt>ariali.ttf</tt> for "Arial
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Italic", etc.). The file is also called a "font" but really contains a
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font face.</p>
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<p>A <em>digital font</em> is thus a data file that may contain <em>one
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or more font faces</em>. For each of these, it contains character
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images, character metrics, as well as other kind of information
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important to the layout of text and the processing of specific character
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encodings. In some awkward formats, like Adobe's Type 1, a single
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font face is described through several files (i.e. one contains the
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character images, another one the character metrics). We will ignore
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this implementation issue in most parts of this document and consider
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digital fonts as single files, though FreeType 2.0 is able to
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support multiple-files fonts correctly.</p>
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<p>As a convenience, a font file containing more than one face is called
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a <em>font collection</em>. This case is rather rare but can be seen in
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many Asian fonts, which contain images for two or more representation
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forms of a given scripts (usually for horizontal and vertical
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layout.</p>
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<a name="section-2">
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<h3>
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2. Character images and mappings
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</h3>
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<p>The character images are called <em>glyphs</em>. A single character
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can have several distinct images, i.e. several glyphs, depending on
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script, usage or context. Several characters can also take a single
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glyph (good examples are Roman ligatures like "fi" and "fl" which can be
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represented by a single glyph). The relationships between characters
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2000-11-10 23:39:21 +01:00
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and glyphs can be very complex, but won't be discussed in this document.
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2000-11-09 07:26:30 +01:00
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Moreover, some formats use more or less awkward schemes to store and
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access glyphs. For the sake of clarity, we only retain the following
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notions when working with FreeType:</p>
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<ul>
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<li>
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<p>A font file contains a set of glyphs; each one can be stored as a
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bitmap, a vector representation or any other scheme (most scalable
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formats use a combination of mathematical representation and control
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data/programs). These glyphs can be stored in any order in the font
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file, and is typically accessed through a simple glyph index.</p>
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</li>
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<li>
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<p>The font file contains one or more tables, called a <em>character
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map</em> (or charmap in short), which is used to convert character
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codes for a given encoding (e.g. ASCII, Unicode, DBCS, Big5, etc..)
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into glyph indices relative to the font file. A single font face
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may contain several charmaps. For example, most TrueType fonts
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contain an Apple-specific charmap as well as a Unicode charmap,
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which makes them usable on both Mac and Windows platforms.</p>
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</li>
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</ul>
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<a name="section-3">
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<h3>
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3. Character and font metrics
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</h3>
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2001-01-19 04:33:30 +01:00
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<p>Each glyph image is associated with various metrics which are used to
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describe how it must be placed and managed when rendering text. These
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are described in more details in section III; they relate to glyph
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placement, cursor advances as well as text layout. They are extremely
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important to compute the flow of text when rendering a string of
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text.</p>
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2000-11-09 07:26:30 +01:00
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<p>Each scalable format also contains some global metrics, expressed in
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notional units, to describe some properties of all glyphs in the same
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face. Examples for global metrics are the maximum glyph bounding box,
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the ascender, descender and text height for the font.</p>
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<p>Though these metrics also exist for non-scalable formats, they only
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2001-01-19 04:33:30 +01:00
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apply for a set of given character dimensions and resolutions, and are
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usually expressed in pixels.</p>
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2000-11-09 07:26:30 +01:00
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2000-11-09 09:01:18 +01:00
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<p><hr></p>
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2000-11-09 07:26:30 +01:00
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<center>
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<table width="100%"
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border=0
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cellpadding=5>
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<tr bgcolor="#CCFFCC" valign=center>
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<td align=center
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width="30%">
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</td>
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<td align=center
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width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center
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width="30%">
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<a href="glyphs-2.html">Next</a>
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</td>
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</tr>
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</table>
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</center>
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</td></tr>
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</table>
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</center>
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2000-11-09 01:01:38 +01:00
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</body>
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</html>
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