459 lines
14 KiB
HTML
459 lines
14 KiB
HTML
<!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"
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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>The design of FreeType 2</title>
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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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<h1 align=center>
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The design of FreeType 2
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</h1>
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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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<a href="design-4.html">Previous</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="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="design-6.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><hr></p>
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<table width="100%">
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<tr bgcolor="#ccccee"><td>
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<h1>
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IV. Module Classes
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</h1>
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</td></tr>
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</table>
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<p>We will now try to explain more precisely the <em>types</em> of modules
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that FreeType 2 is capable of managing. Note that each one of them
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is decribed with more details in the following chapters of this
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document.</p>
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<ul>
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<li>
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<p><em>Renderer</em> modules are used to manage scalable glyph images.
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This means <em>transforming</em> them, computing their <em>bounding
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box</em>, and <em>converting</em> them to either <em>monochrome</em>
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or <em>anti-aliased</em> bitmaps</em>.</p>
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<p>Note that FreeType 2 is capable of dealing with <em>any</em>
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kind of glyph images, as long as a renderer module is provided for it.
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The library comes by default with two renderers:</p>
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<p><table cellpadding=8>
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<tr valign=top>
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<td>
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<tt>raster</tt>
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</td>
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<td>
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<p>Supports the conversion of vectorial outlines (described by a
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<tt>FT_Outline</tt> object) to <em>monochrome</em> bitmaps.
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>smooth</tt>
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</td>
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<td>
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<p>Supports the conversion of the same outlines to high-quality
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<em>anti-aliased</em> pixmaps (using 256 levels of gray). Note
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that this renderer also supports direct span generation.</p>
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</td>
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</tr>
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</table></p>
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<li>
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<p><em>Font driver</em> modules are used to support one or more
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specific font format. By default, FreeType 2 comes with the
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following font drivers:</p>
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<p><table cellpadding=8>
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<tr valign=top>
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<td>
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<tt>truetype</tt>
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</td>
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<td>
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<p>supports TrueType font files</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>type1</tt>
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</td>
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<td>
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<p>supports Postscript Type 1 fonts, both in binary
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(<tt>.pfb</tt>) or ASCII (<tt>.pfa</tt>) formats, including
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Multiple Master fonts.</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>cid</tt>
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</td>
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<td>
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<p>supports Postscript CID-keyed fonts</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>cff</tt>
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</td>
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<td>
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<p>supports OpenType, CFF as well as CEF fonts (CEF is a
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derivative of CFF used by Adobe in its SVG viewer)</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>winfonts</tt>
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</td>
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<td>
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<p>supports Windows bitmap fonts (i.e. <tt>.fon</tt> and
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<tt>.fnt</tt>)</p>
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</td>
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</tr>
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</table></p>
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<p>Note that font drivers can support bitmapped or scalable glyph
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images. A given font driver that supports Bézier outlines
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through <tt>FT_Outline</tt> can also provide its own hinter, or rely
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on FreeType's <tt>autohinter</tt> module.</p>
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</li>
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<li>
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<p><em>Helper</em> modules are used to hold shared code that is often
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used by several font drivers, or even other modules. Here are the
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default helpers:</p>
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<p><table cellpadding=8>
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<tr valign=top>
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<td>
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<tt>sfnt</tt>
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</td>
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<td>
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used to support font formats based on the <tt>SFNT</tt> storage
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scheme: TrueType & OpenType fonts as well as other variants (like
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TrueType fonts that only contain embedded bitmaps)
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>psnames</tt>
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</td>
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<td>
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used to provide various useful functions related to glyph names
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ordering and Postscript encodings/charsets. For example, this
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module is capable of automatically synthetizing a Unicode charmap
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from a Type 1 glyph name dictionary.
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>psaux</tt>
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</td>
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<td>
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used to provide various useful functions related to Type 1
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charstring decoding, as this "feature" is needed by the
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<tt>type1</tt>, <tt>cid</tt>, and <tt>cff</tt> drivers.
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</td>
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</tr>
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</table></p>
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</li>
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<li>
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<p>Finally, the <em>autohinter</em> module has a specific role in
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FreeType 2, as it can be used automatically during glyph loading
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to process individual glyph outlines when a font driver doesn't
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provide it's own hinting engine.</p>
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<p>This module's purpose and design is also heavily described on the
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FreeType web site.</p>
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</li>
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</ul>
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<p>We will now study how modules are described, then managed by the
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library.</p>
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<h3>
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1. The <tt>FT_Module_Class</tt> structure
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</h3>
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<p>As described later in this document, library initialization is
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performed by calling the <tt>FT_Init_FreeType()</tt> function. The
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latter is in charge of creating a new "empty" <tt>FT_Library</tt>
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object, then register each "default" module by repeatedly calling the
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<tt>FT_Add_Module()</tt> function.</p>
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<p>Similarly, client applications can call <tt>FT_Add_Module()</tt> any
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time they wish in order to register a new module in the library. Let us
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take a look at this function's declaration:</p>
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<font color="blue"><pre>
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extern FT_Error FT_Add_Module(
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FT_Library library,
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const FT_Module_Class* clazz );</pre>
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</font>
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<p>As one can see, this function expects a handle to a library object,
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as well as a pointer to a <tt>FT_Module_Class</tt> structure. It
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returns an error code. In case of success, a new module object is
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created and added to the library. Note by the way that the module isn't
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returned directly by the call!</p>
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<p>Here the definition of <tt>FT_Module_Class</tt>, with some
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explanation. The following code is taken from
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<tt><freetype/ftmodule.h></tt>:</p>
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<font color="blue"><pre>
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typedef struct FT_Module_Class_
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{
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FT_ULong module_flags;
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FT_Int module_size;
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const FT_String* module_name;
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FT_Fixed module_version;
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FT_Fixed module_requires;
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const void* module_interface;
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FT_Module_Constructor module_init;
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FT_Module_Destructor module_done;
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FT_Module_Requester get_interface;
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} FT_Module_Class;</pre>
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</font>
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<p>A description of its fields:</p>
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<p><table cellpadding=8>
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<tr valign=top>
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<td>
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<tt>module_flags</tt>
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</td>
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<td>
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<p>A set of bit flags used to describe the module's category. Valid
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values are:</p>
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<ul>
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<li>
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<tt>ft_module_font_driver</tt> if the module is a font driver
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</li>
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<li>
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<tt>ft_module_renderer</tt> if the module is a renderer
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</li>
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<li>
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<tt>ft_module_hinter</tt> if the module is an auto-hinter
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</li>
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<li>
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<tt>ft_module_driver_scalable</tt> if the module is a font
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driver supporting scalable glyph formats
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</li>
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<li>
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<tt>ft_module_driver_no_outlines</tt> if the module is a font
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driver supporting scalable glyph formats that <em>cannot</em> be
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described by an <tt>FT_Outline</tt> object
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</li>
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<li>
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<tt>ft_module_driver_has_hinter</tt> if the module is a font
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driver that provides its own hinting scheme/algorithm
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</li>
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</ul>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_size</tt>
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</td>
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<td>
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<p>An integer that gives the size in <em>bytes</em> of a given
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module object. This should <em>never</em> be less than
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<tt>sizeof(FT_ModuleRec)</tt>, but can be more if the module needs
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to sub-class the base <tt>FT_ModuleRec</tt> class.</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_name</tt>
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</td>
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<td>
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<p>The module's internal name, coded as a simple ASCII
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C string. There can't be two modules with the same name
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registered in a given <tt>FT_Library</tt> object. However,
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<tt>FT_Add_Module()</tt> uses the <tt>module_version</tt> field to
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detect module upgrades and perform them cleanly, even at
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run-time.</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_version</tt>
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</td>
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<td>
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<p>A 16.16 fixed float number giving the module's major and minor
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version numbers. It is used to determine whether a module needs to
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be upgraded when calling <tt>FT_Add_Module()</tt>.</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_requires</tt>
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</td>
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<td>
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<p>A 16.16 fixed float number giving the version of FreeType 2
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that is required to install this module. The default value is
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0x20000 for FreeType version 2.0</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_requires</tt>
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</td>
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<td>
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<p>Most modules support one or more "interfaces", i.e. tables of
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function pointers. This field is used to point to the module's main
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interface, if there is one. It is a short-cut that prevents users
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of the module to call "get_interface()" each time they need to
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access one of the object's common entry points.</p>
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<p>Note that is is optional, and can be set to NULL. Other
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interfaces can also be accessed through the <tt>get_interface()</tt>
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field.</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_init</tt>
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</td>
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<td>
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<p>A pointer to a function used to initialize the fields of a fresh
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new <tt>FT_Module</tt> object. It is called <em>after</em> the
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module's base fields have been set by the library, and is generally
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used to initialize the fields of <tt>FT_ModuleRec</tt>
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subclasses.</p>
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<p>Most module classes set it to NULL to indicate that no extra
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initialization is necessary.</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>module_done</tt>
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</td>
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<td>
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<p>A pointer to a function used to finalize the fields of a given
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<tt>FT_Module</tt> object. Note that it is called <em>before</em>
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the library unsets the module's base fields, and is generally used
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to finalize the fields of <tt>FT_ModuleRec</tt> subclasses.</p>
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<p>Most module classes set it to NULL to indicate that no extra
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finalization is necessary</p>
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</td>
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</tr>
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<tr valign=top>
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<td>
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<tt>get_interface</tt>
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</td>
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<td>
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<p>A pointer to a function used to request the address of a given
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module interface. Set it to NULL if you don't need to support
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additional interfaces but the main one.</p>
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</td>
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</tr>
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</table></p>
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<h3>
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2. The <tt>FT_Module</tt> type
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</h3>
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<p>The <tt>FT_Module</tt> type is a handle (i.e. a pointer) to a given
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module object/instance, whose base structure is given by the internal
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<tt>FT_ModuleRec</tt> type. We will intentionally <em>not</em> describe
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this structure here, as there is no point to look so far into the
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library's design.</p>
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<p>When <tt>FT_Add_Module</tt> is called, it first allocates a new
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module instance, using the <tt>module_size</tt> class field to determine
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its byte size. The function initializes the root <tt>FT_ModuleRec</tt>
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field, then calls the class-specific initializer <tt>module_init</tt>
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when this field is not set to NULL.</p>
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<p>Note that the library defines several sub-classes of
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<tt>FT_ModuleRec</tt>, which are, as you could have guessed:</p>
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<ul>
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<li><p><tt>FT_Renderer</tt> for renderer modules</p>
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<li><p><tt>FT_Driver</tt> for font driver modules</p>
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<li><p><tt>FT_AutoHinter</tt> for the auto-hinter</p>
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</ul>
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<p>Helper modules use the base <tt>FT_ModuleRec</tt> type. We will
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describe these classes in the next chapters.</p>
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<p><hr></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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<a href="design-4.html">Previous</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="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="design-6.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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</body>
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</html>
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