forked from minhngoc25a/freetype2
Revised.
This commit is contained in:
Werner Lemberg
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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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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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="Author" content="blob">
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<meta name="GENERATOR" content="Mozilla/4.5 [fr] (Win98; I) [Netscape]">
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<title>FreeType Glyph Conventions</title>
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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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</head>
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<body>
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<body text="#000000"
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bgcolor="#FFFFFF"
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@ -14,156 +15,181 @@
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vlink="#51188E"
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alink="#FF0000">
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<center><h1>
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FreeType Glyph Conventions
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</h1></center>
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<h1 align=center>
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FreeType Glyph Conventions
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</h1>
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<center><h2>
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version 2.1
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</h2></center>
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<center><h3>
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Copyright 1998-2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br>
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Copyright 2000 The FreeType Development Team (<a href="devel@freetype.org">devel@freetype.org</a>)
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</h3></center>
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<center><table width=650><tr><td>
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<center><table width="100%" border=0 cellpadding=5><tr bgcolor="#CCFFCC" valign=center>
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<td align=center width="30%">
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</td>
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<td align=center width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center width="30%">
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<a href="glyphs-2.html">Next</a>
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</td>
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</tr></table></center>
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<table width="100%" cellpadding=5><tr bgcolor="#CCCCFF" valign=center><td>
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<h2>
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I. Basic typographic concepts
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<h2 align=center>
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Version 2.1
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</h2>
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</td></tr></table>
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<a name="section-1"><h3>
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1. Font files, format and information
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</h3><blockquote>
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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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<p>
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A font is a collection of various character images that can
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be used to display or print text. The images in a single font share some
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common properties, including look, style, serifs, etc.. Typographically
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speaking, one has to distinguish between a <b>font family</b> and its
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multiple <b>font faces</b>, which usually differ in style though come
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from the same template.</p>
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<center>
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<table width="65%">
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<tr><td>
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For example, "<i>Palatino Regular</i>" and "<i>Palatino Italic</i>" are
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two distinct <i>faces</i> from the same famous <i>family</i>, called
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"<i>Palatino</i>" itself.</p>
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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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<p>The single term font is nearly always used in ambiguous ways to refer
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to either a given family or given face, depending on the context. For example,
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most users of word-processors use "font" to describe a font family (e.g.
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Courier, Palatino, etc..); however most of these families are implemented
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through several data files depending on the file format : for TrueType,
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this is usually one per face (i.e. ARIAL.TFF for "Arial Regular", ARIALI.TTF
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for "Arial Italic", etc..). The file is also called a "font" but really
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contains a font face.
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</p>
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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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<p>A <i>digital font</i> is thus a data file that may contain <i>one or
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more font faces</i>. For each of these, it contains character images,
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character metrics, as well as other kind of information important to the
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layout of text and the processing of specific character encodings. In some
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awkward formats, like Adobe Type1, a single font face is described through
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several files (i.e. one contains the character images, another one the
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character metrics). We will ignore this implementation issue in most of
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this document and consider digital fonts as single files, though FreeType
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2.0 is able to support multiple-files fonts correctly.
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</p>
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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>As a convenience, a font file containing more than one face is called
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a font collection. This case is rather rare but can be seen in many asian
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fonts, which contain images for two or more scripts for a given language.
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</p>
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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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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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</blockquote><h3><a name="section-2">
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2. Character images and mappings :
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</h3><blockquote>
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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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and glyphs can be very complex, but won't be discussed in this document.
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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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<p>The character images are called <b>glyphs</b>. A single character
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can have several distinct images, i.e. several glyphs, depending on script,
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usage or context. Several characters can also take a single glyph (good
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examples are roman ligatures like "oe" and "fi" which can be represented
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by a single glyph). The relationships between characters
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and glyphs can be a very complex one but won't be detailed in this document.
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Moreover, some formats use more or less awkward schemes to store and access
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the glyphs. For the sake of clarity, we'll only retain the following notions
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when working with FreeType :</p>
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<p>Each glyph image is associated to various metrics which are used to
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describe how must be placed and managed when rendering text. Though
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they are described in more details in section III, they relate to
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glyph placement, cursor advances as well as text layout. They are
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extremely important to compute the flow of text when rendering a string
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of text.</p>
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<ul>
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<p><li>
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A font file contains a set of glyphs, each one can be stored as a bitmap,
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a vector representation or any other scheme (e.g. most scalable formats
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use a combination of math representation and control data/programs). These
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glyphs can be stored in any order in the font file, and is typically accessed
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through a simple glyph index.
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</li></p>
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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><li>
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The font file contains one (or more) table, called a character map (or
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charmap in short), which is used to convert character codes for a given
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encoding (e.g. ASCII, Unicode, DBCS, Big5, etc..) into glyph indexes
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relative to the font file. A single font face may contain several charmaps.
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For example, most TrueType fonts contain an Apple-specific charmap as well
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as a Unicode charmap, which makes them usable on both Mac and Windows
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platforms.
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</li></p>
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</ul>
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<p>Though these metrics also exist for non-scalable formats, they only
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apply for a set of given character dimensions and resolutions, and
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they are usually expressed in pixels then.</p>
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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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</blockquote><h3><a name="section-3">
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3. Character and font metrics :
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</h3><blockquote>
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<p>Each glyph image is associated to various metrics which are used to
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describe the way it must be placed and managed when rendering text. Though
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they are described in more details in section III, they relate to glyph
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placement, cursor advances as well as text layouts. They are extremely
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important to compute the flow of text when rendering string of text.
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</p>
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<p>Each scalable format also contains some global metrics, expressed in
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notional units, used to describe some properties of all glyphs in a same
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face. For example : the maximum glyph bounding box, the ascender, descender
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and text height for the font.
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</p>
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<p>Though these metrics also exist for non-scalable formats, they only
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apply for a set of given character dimensions and resolutions, and they're
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usually expressed in pixels then.</p>
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<center><table width="100%" border=0 cellpadding=5><tr bgcolor="#CCFFCC" valign=center>
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<td align=center width="30%">
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||||
</td>
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<td align=center width="30%">
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<a href="index.html">Contents</a>
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</td>
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<td align=center width="30%">
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||||
<a href="glyphs-2.html">Next</a>
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</td>
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</tr></table></center>
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</td></tr></table></center>
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</td></tr>
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</table>
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</center>
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||||
</body>
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||||
</html>
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|
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