200 lines
6.6 KiB
HTML
200 lines
6.6 KiB
HTML
<!doctype html public "-//w3c//dtd html 4.0 transitional//en"
|
|
"http://www.w3.org/TR/REC-html40/loose.dtd">
|
|
<html>
|
|
<head>
|
|
<meta http-equiv="Content-Type"
|
|
content="text/html; charset=iso-8859-1">
|
|
<meta name="Author"
|
|
content="David Turner">
|
|
<title>FreeType Glyph Conventions</title>
|
|
</head>
|
|
|
|
<body text="#000000"
|
|
bgcolor="#FFFFFF"
|
|
link="#0000EF"
|
|
vlink="#51188E"
|
|
alink="#FF0000">
|
|
|
|
<h1 align=center>
|
|
FreeType Glyph Conventions
|
|
</h1>
|
|
|
|
<h2 align=center>
|
|
Version 2.1
|
|
</h2>
|
|
|
|
<h3 align=center>
|
|
Copyright 1998-2000 David Turner (<a
|
|
href="mailto:david@freetype.org">david@freetype.org</a>)<br>
|
|
Copyright 2000 The FreeType Development Team (<a
|
|
href="mailto:devel@freetype.org">devel@freetype.org</a>)
|
|
</h3>
|
|
|
|
<center>
|
|
<table width="65%">
|
|
<tr><td>
|
|
|
|
<center>
|
|
<table width="100%"
|
|
border=0
|
|
cellpadding=5>
|
|
<tr bgcolor="#CCFFCC"
|
|
valign=center>
|
|
<td align=center
|
|
width="30%">
|
|
|
|
</td>
|
|
<td align=center
|
|
width="30%">
|
|
<a href="index.html">Contents</a>
|
|
</td>
|
|
<td align=center
|
|
width="30%">
|
|
<a href="glyphs-2.html">Next</a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</center>
|
|
|
|
<p><hr></p>
|
|
|
|
<table width="100%">
|
|
<tr bgcolor="#CCCCFF"
|
|
valign=center><td>
|
|
<h2>
|
|
I. Basic typographic concepts
|
|
</h2>
|
|
</td></tr>
|
|
</table>
|
|
|
|
<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 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 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 it must be placed and managed when rendering text. These
|
|
are described in more details in section 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.</p>
|
|
|
|
|
|
<p><hr></p>
|
|
|
|
<center>
|
|
<table width="100%"
|
|
border=0
|
|
cellpadding=5>
|
|
<tr bgcolor="#CCFFCC" valign=center>
|
|
<td align=center
|
|
width="30%">
|
|
|
|
</td>
|
|
<td align=center
|
|
width="30%">
|
|
<a href="index.html">Contents</a>
|
|
</td>
|
|
<td align=center
|
|
width="30%">
|
|
<a href="glyphs-2.html">Next</a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</center>
|
|
|
|
</td></tr>
|
|
</table>
|
|
</center>
|
|
|
|
</body>
|
|
</html>
|