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354 lines
12 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-2.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-4.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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II. Public Objects and 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 explain the abstractions provided by FreeType 2 to
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client applications to manage font files and data. As you would normally
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expect, these are implemented through objects/classes.</p>
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<h2>
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1. Object Orientation in FreeType 2
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</h2>
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<p>Though written in ANSI C, the library employs a few techniques,
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inherited from object-oriented programming, to make it easy to extend.
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Hence, the following conventions apply in the FreeType 2 source
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code:</p>
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<ol>
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<li>
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<p>Each object type/class has a corresponding <em>structure
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type</em> <b>and</b> a corresponding <em>structure pointer
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type</em>. The latter is called the <em>handle type</em> for the
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type/class.</p>
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<p>Consider that we need to manage objects of type "foo" in
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FreeType 2. We would define the following structure and handle
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types as follows:</p>
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<font color="blue"><pre>
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typedef struct FT_FooRec_* FT_Foo;
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typedef struct FT_FooRec_
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{
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// fields for the "foo" class
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...
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} FT_FooRec;</pre>
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</font>
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<p>As a convention, handle types use simple but meaningful
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identifiers beginning with <tt>FT_</tt>, as in <tt>FT_Foo</tt>,
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while structures use the same name with a <tt>Rec</tt> suffix
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appended to it ("Rec" is short for "record"). <em>Note that each
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class type has a corresponding handle type</em>.</p>
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</li>
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<li>
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<p>Class derivation is achieved internally by wrapping base class
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structures into new ones. As an example, we define a "foobar" class
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that is derived from "foo". We would do something like:</p>
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<font color="blue"><pre>
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typedef struct FT_FooBarRec_* FT_FooBar;
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typedef struct FT_FooBarRec_
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{
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// the base "foo" class fields
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FT_FooRec root;
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// fields proper to the "foobar" class
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...
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} FT_FooBarRec;</pre>
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</font>
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<p>As you can see, we ensure that a "foobar" object is also a "foo"
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object by placing a <tt>FT_FooRec</tt> at the start of the
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<tt>FT_FooBarRec</tt> definition. It is called <b>root</b> by
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convention.</p>
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<p>Note that a <tt>FT_FooBar</tt> handle also points to a "foo"
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object and can be typecasted to <tt>FT_Foo</tt>. Similarly, when
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the library returns a <tt>FT_Foo</tt> handle to client applications,
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the object can be really implemented as a <tt>FT_FooBar</tt> or any
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derived class from "foo".</p>
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</li>
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</ol>
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<p>In the following sections of this chapter, we will refer to "the
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<tt>FT_Foo</tt> class" to indicate the type of objects handled through
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<tt>FT_Foo</tt> pointers, be they implemented as "foo" or "foobar".</p>
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<hr>
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<h2>
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2. The <tt>FT_Library</tt> class
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</h2>
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<p>This type corresponds to a handle to a single instance of the
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library. Note that the corresponding structure <tt>FT_LibraryRec</tt>
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is not defined in public header files, making client applications unable
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to access its internal fields.</p>
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<p>The library object is the <em>parent</em> of all other objects in
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FreeType 2. You need to create a new library instance before doing
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anything else with the library. Similarly, destroying it will
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automatically destroy all its children (i.e. faces and modules).</p>
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<p>Typical client applications should call <tt>FT_Init_FreeType()</tt>
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in order to create a new library object, ready to be used for further
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actions.</p>
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<p>Another alternative is to create a fresh new library instance by
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calling the function <tt>FT_New_Library()</tt>, defined in the
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<tt><freetype/ftmodule.h></tt> public header file. This function
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will however return an "empty" library instance with no module
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registered in it. You can "install" modules in the instance by calling
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<tt>FT_Add_Module()</tt> manually.</p>
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<p>Calling <tt>FT_Init_FreeType()</tt> is a lot more convenient, because
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this function basically registers a set of default modules into each new
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library instance. The way this list is accessed and/or computed is
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determined at build time, and depends on the content of the
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<tt>ftinit</tt> component. This process is explained in details later
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in this document.</p>
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<p>For now, one should consider that library objects are created with
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<tt>FT_Init_FreeType()</tt>, and destroyed along with all children with
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<tt>FT_Done_FreeType()</tt>.</p>
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<hr>
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<h2>
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3. The <tt>FT_Face</tt> class
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</h2>
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<p>A face object corresponds to a single <em>font face</em>, i.e., a
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specific typeface with a specific style. For example, "Arial" and
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"Arial Italic" correspond to two distinct faces.</p>
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<p>A face object is normally created through <tt>FT_New_Face()</tt>.
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This function takes the following parameters: an <tt>FT_Library</tt>
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handle, a C file pathname used to indicate which font file to open, an
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index used to decide which face to load from the file (a single file may
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contain several faces in certain cases), and the address of a
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<tt>FT_Face</tt> handle. It returns an error code:</p>
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<font color="blue"><pre>
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FT_Error FT_New_Face( FT_Library library,
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const char* filepathname,
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FT_Long face_index,
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FT_Face* face );</pre>
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</font>
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<p>In case of success, the function will return 0, and the handle
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pointed to by the <tt>face</tt> parameter will be set to a non-NULL
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value.</p>
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<p>Note that the face object contains several fields used to describe
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global font data that can be accessed directly by client applications.
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For example, the total number of glyphs in the face, the face's family
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name, style name, the EM size for scalable formats, etc. For more
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details, look at the <tt>FT_FaceRec</tt> definition in the
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FreeType 2 API Reference.</p>
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<hr>
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<h2>
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4. The <tt>FT_Size</tt> class
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</h2>
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<p>Each <tt>FT_Face</tt> object <em>has</em> one or more
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<tt>FT_Size</tt> objects. A <em>size object</em> is used to store data
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specific to a given character width and height. Each newly created face
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object has one size, which is directly accessible as
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<tt>face->size</tt>.</p>
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<p>The contents of a size object can be changed by calling either
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<tt>FT_Set_Pixel_Sizes()</tt> or <tt>FT_Set_Char_Size()</tt>.</p>
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<p>A new size object can be created with <tt>FT_New_Size()</tt>, and
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destroyed manually with </tt>FT_Done_Size()</tt>. Note that typical
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applications don't need to do this normally: they tend to use the
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default size object provided with each <tt>FT_Face</tt>.</p>
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<p>The public fields of <tt>FT_Size</tt> objects are defined in a very
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small structure named <tt>FT_SizeRec</tt>. However, it is important to
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understand that some font drivers define their own derivatives of
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<tt>FT_Size</tt> to store important internal data that is re-computed
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each time the character size changes. Most of the time, these are
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size-specific <em>font hints</em>./p>
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<p>For example, the TrueType driver stores the scaled CVT table that
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results from the execution of the "cvt" program in a <tt>TT_Size</tt>
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structure, while the Type 1 driver stores scaled global metrics
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(like blue zones) in a <tt>T1_Size</tt> object. Don't worry if you
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don't understand the current paragraph; most of this stuff is highly
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font format specific and doesn't need to be explained to client
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developers :-)</p>
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<hr>
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<h2>
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5. The <tt>FT_GlyphSlot</tt> class
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</h2>
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<p>The purpose of a glyph slot is to provide a place where glyph images
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can be loaded one by one easily, independently of the glyph image format
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(bitmap, vector outline, or anything else).</p>
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<p>Ideally, once a glyph slot is created, any glyph image can be loaded
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into it without additional memory allocation. In practice, this is only
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possible with certain formats like TrueType which explicitly provide
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data to compute a slot's maximum size.</p>
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<p>Another reason for glyph slots is that they are also used to hold
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format-specific hints for a given glyphs as well as all other data
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necessary to correctly load the glyph.</p>
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<p>The base <tt>FT_GlyphSlotRec</tt> structure only presents glyph
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metrics and images to client applications, while actual implementation
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may contain more sophisticated data.</p>
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<p>As an example, the TrueType-specific <tt>TT_GlyphSlotRec</tt>
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structure contains additional fields to hold glyph-specific bytecode,
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transient outlines used during the hinting process, and a few other
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things.
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The Type 1-specific <tt>T1_GlyphSlotRec</tt> structure holds glyph
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hints during glyph loading, as well as additional logic used to properly
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hint the glyphs when a native Type 1 hinter is used.</p>
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<p>Finally, each face object has a single glyph slot that is directly
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accessible as <tt>face->glyph</tt>.</p>
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<hr>
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<h2>
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6. The <tt>FT_CharMap</tt> class
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</h2>
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<p>The <tt>FT_CharMap</tt> type is used as a handle to character map
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objects, or <em>charmaps</em>. A charmap is simply some sort of table
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or dictionary which is used to translate character codes in a given
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encoding into glyph indices for the font.</p>
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<p>A single face may contain several charmaps. Each one of them
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corresponds to a given character repertoire, like Unicode, Apple Roman,
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Windows codepages, and other encodings.</p>
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<p>Each <tt>FT_CharMap</tt> object contains a "platform" and an
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"encoding" field used to identify precisely the character repertoire
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corresponding to it.</p>
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<p>Each font format provides its own derivative of
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<tt>FT_CharMapRec</tt> and thus needs to implement these objects.</p>
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<hr>
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<h2>
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7. Objects relationships
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</h2>
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<p>The following diagram summarizes what we have just said regarding the
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public objects managed by the library, as well as explicitely describes
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their relationships</p>
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<center>
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<img src="simple-model.png"
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width=453 height=378
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alt="Simple library model">
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</center>
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<p>Note that this picture will be updated at the end of the next
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chapter, related to <em>internal objects</em>.</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-2.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-4.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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