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        content="David Turner">
  <title>The design of FreeType 2</title>
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<h1 align=center>
  The design of FreeType&nbsp;2
</h1>

<h3 align=center>
  Copyright&nbsp;1998-2000 David Turner (<a
  href="mailto:david@freetype.org">david@freetype.org</a>)<br>
  Copyright&nbsp;2000 The FreeType Development Team (<a
  href="mailto:devel@freetype.org">devel@freetype.org</a>)
</h3>

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    <h1>
      III. Internal Objects and Classes
    </h1>
  </td></tr>
  </table>

<p>Let us have a look now at the <em>internal</em> objects that
  FreeType&nbsp;2 uses, i.e., those not directly available to client
  applications, and see how they fit into the picture.</p>

<h2>
      1. Memory management
    </h2>

<p>All memory management operations are performed through three specific
    routines of the base layer, namely: <tt>FT_Alloc()</tt>,
    <tt>FT_Realloc()</tt>, and <tt>FT_Free()</tt>.  Each one of these
    functions expects a <tt>FT_Memory</tt> handle as its first
    parameter.</p>

<p>The latter is a pointer to a simple object used to describe the
    current memory pool/manager.  It contains a simple table of
    alloc/realloc/free functions.  A memory manager is created at library
    initialization time by <tt>FT_Init_FreeType()</tt>, calling the function
    <tt>FT_New_Memory()</tt> provided by the <tt>ftsystem</tt>
    component.</p>

<p>By default, this manager uses the ANSI <tt>malloc()</tt>,
    <tt>realloc()</tt>, and <tt>free()</tt> functions.  However, as
    <tt>ftsystem</tt> is a replaceable part of the base layer, a specific
    build of the library could provide a different default memory
    manager.</p>

<p>Even with a default build, client applications are still able to
    provide their own memory manager by not calling
    <tt>FT_Init_FreeType()</tt> but follow these simple steps:</p>

<ol>
      <li>
        <p>Create a new <tt>FT_Memory</tt> object by hand.  The definition
        of <tt>FT_MemoryRec</tt> is located in the public file
        <tt>&lt;freetype/ftsystem.h&gt;</tt>.</p>
      </li>
      <li>
        <p>Call <tt>FT_New_Library()</tt> to create a new library instance
        using your custom memory manager.  This new library doesn't yet
        contain any registered modules.</p>
      </li>
      <li>
        <p>Register the set of default modules by calling the function
        <tt>FT_Add_Default_Modules()</tt> provided by the <tt>ftinit</tt>
        component, or manually register your drivers by repeatedly
        calling <tt>FT_Add_Module()</tt>.</p>
      </li>
    </ol>

<hr>

<h2>
      2. Input streams
    </h2>

<p>Font files are always read through <tt>FT_Stream</tt> objects.  The
    definition of <tt>FT_StreamRec</tt> is located in the public file
    <tt>&lt;freetype/ftsystem.h&gt;</tt>, which allows client developers to
    provide their own implementation of streams if they wish so.</p>

<p>The function <tt>FT_New_Face()</tt> will always automatically create
    a new stream object from the C&nbsp;pathname given as its second
    argument.  This is achieved by calling the function
    <tt>FT_New_Stream()</tt> provided by the <tt>ftsystem</tt> component.
    As the latter is replaceable, the implementation of streams may vary
    greatly between platforms.</p>

<p>As an example, the default implementation of streams is located in
    the file <tt>src/base/ftsystem.c</tt> and uses the ANSI
    <tt>fopen()</tt>, <tt>fseek()</tt>, and <tt>fread()</tt> calls.
    However, the Unix build of FreeType&nbsp;2 provides an alternative
    implementation that uses memory-mapped files, when available on the host
    platform, resulting in a significant access speed-up.</p>

<p>FreeType distinguishes between memory-based and disk-based streams.
    In the first case, all data is directly accessed in memory (e.g.
    ROM-based, write-only static data and memory-mapped files), while in the
    second, portions of the font files are read in chunks called
    <em>frames</em>, and temporarily buffered similarly through typical
    seek/read operations.</p>

<p>The FreeType stream sub-system also implements extremely efficient
    algorithms to very quickly load structures from font files while
    ensuring complete safety in the case of a "broken file".</p>

<p>The function <tt>FT_New_Memory_Face()</tt> can be used to directly
    create/open a <tt>FT_Face</tt> object from data that is readily
    available in memory (including ROM-based fonts).</p>

<p>Finally, in the case where a custom input stream is needed, client
    applications can use the function <tt>FT_Open_Face()</tt>, which can
    accept custom input streams.  This may be useful in the case of
    compressed or remote font files, or even embedded font files that need
    to be extracted from certain documents.</p>

<p>Note that each face owns a single stream, which is also destroyed by
    <tt>FT_Done_Face()</tt>.  Generally speaking, it is certainly
    <em>not</em> a good idea to keep numerous <tt>FT_Face</tt> objects
    opened.</p>

<hr>

<h2>
      3. Modules
    </h2>

<p>A FreeType&nbsp;2 module is itself a piece of code.  However, the
    library creates a single <tt>FT_Module</tt> object for each module that
    is registered when <tt>FT_Add_Module()</tt> is called.</p>

<p>The definition of <tt>FT_ModuleRec</tt> is not publicly available to
    client applications.  However, each <em>module type</em> is described by
    a simple public structure named <tt>FT_Module_Class</tt>, defined in
    <tt>&lt;freetype/ftmodule.h&gt;</tt>, and is described later in this
    document:</p>

<p>You need a pointer to an <tt>FT_Module_Class</tt> structure when
    calling <tt>FT_Add_Module()</tt>, whose declaration is:</p>

<font color="blue"><pre>
    FT_Error  FT_Add_Module( FT_Library              library,
                             const FT_Module_Class*  clazz );</pre>
    </font>

<p>Calling this function will do the following:</p>

<ul>
      <li>
        <p>It will check whether the library already holds a module object
        corresponding to the same module name as the one found in
        <tt>FT_Module_Class</tt>.</p>
      </li>
      <li>
        <p>If this is the case, it will compare the module version number to
        see whether it is possible to <em>upgrade</em> the module to a new
        version.  If the module class's version number is smaller than the
        already installed one, the function returns immediately.  Similarly,
        it checks that the version of FreeType&nbsp;2 that is running is
        correct compared to the one required by the module.</p>
      </li>
      <li>
        <p>It creates a new <tt>FT_Module</tt> object, using data and flags
        of the module class to determine its byte size and how to properly
        initialize it.</p>
      </li>
      <li>
        <p>If a module initializer is present in the module class, it will
        be called to complete the module object's initialization.</p>
      </li>
      <li>
        <p>The new module is added to the library's list of "registered"
        modules.  In case of an upgrade, the previous module object is
        simply destroyed.</p>
      </li>
    </ul>

<p>Note that this function doesn't return an <tt>FT_Module</tt> handle,
    given that module objects are completely internal to the library (and
    client applications shouldn't normally mess with them&nbsp;:-)</p>

<p>Finally, it is important to understand that FreeType&nbsp;2
    recognizes and manages several kinds of modules.  These will be
    explained in more details later in this document, but we will list for
    now the following types:</p>

<ul>
      <li>
        <p><em>Renderer</em> modules are used to convert native glyph images
        to bitmaps/pixmaps.  FreeType&nbsp;2 comes with two renderer modules
        by default: one to generate monochrome bitmaps, the other to
        generate high-quality anti-aliased pixmaps.</p>
      </li>
      <li>
        <p><em>Font driver</em> modules are used to support one or more font
        formats.  Typically, each font driver provides a specific
        implementation/derivative of <tt>FT_Face</tt>, <tt>FT_Size</tt>,
        <tt>FT_GlyphSlot</tt>, as well as <tt>FT_CharMap</tt>.</p>
      </li>
      <li>
        <p><em>Helper</em> modules are shared by several font drivers.  For
        example, the <tt>sfnt</tt> module parses and manages tables found in
        SFNT-based font formats; it is then used by both the TrueType and
        OpenType font drivers.</p>
      </li>
      <li>
        <p>Finally, the <em>auto-hinter</em> module has a specific place in
        the library's design, as its role is to process vectorial glyph
        outlines, independently of their native font format, to produce
        optimal results at small pixel sizes.</p>
      </li>
    </ul>

<p>Note that every <tt>FT_Face</tt> object is <em>owned</em> by the
    corresponding font driver, depending on the original font file's format.
    This means that all face objects are destroyed when a module is
    removed/unregistered from a library instance (typically by calling the
    <tt>FT_Remove_Module()</tt> function).</p>

<p><em>Because of this, you should always take care that no
    <tt>FT_Face</tt> object is opened when you upgrade or remove a module
    from a library, as this could cause unexpected object deletion!</em></p>

<hr>

<h2>
      4. Libraries
    </h2>

<p>We now come back to our well-known <tt>FT_Library</tt> object.  From
    what have been said before, we already know that a library instance owns
    at least the following:</p>

<ul>
      <li>
        <p>A memory manager object (<tt>FT_Memory</tt>), used for all
        allocation/releases within the instance.</p>
      </li>
      <li>
        <p>A list of <tt>FT_Module</tt> objects, corresponding to the
        "installed" or "registered" modules of the instance.  This list can
        be changed at any time through <tt>FT_Add_Module()</tt> and
        <tt>FT_Remove_Module()</tt>.</p>
      </li>
      <li>
        <p>Remember that face objects are owned by font drivers that are
        themselves modules owned by the library.</p>
      </li>
    </ul>

<p>There is however another object owned by the library instance that
    hasn't been described yet: the <em>raster pool</em>.</p>

<p>The <em>raster pool</em> is simply a block of memory of fixed size
    that is used internally as a "scratch area" for various memory-hungry
    transient operations, avoiding memory allocation.  For example, it is
    used by each renderer when converting a vectorial glyph outline into a
    bitmap (actually, that's where its name comes from&nbsp;:-).</p>

<p>The size of the raster pool is fixed at initialisation time (it
    defaults to 16kByte) and cannot be changed at run-time (though we could
    fix this if there is a real need for that).</p>

<p>When a transient operation needs more memory than the pool's size, it
    can decide to either allocate a heap block as an exceptional condition,
    or sub-divide recursively the task to perform in order to never exceed
    the pool's threshold.</p>

<p>This extremely memory-conservative behaviour is certainly one of the
    keys to FreeType's performance in certain areas (most importantly in
    glyph rendering/scanline-conversion).</p>

<hr>

<h2>
      5. Summary
    </h2>

<p>Finally, the following picture illustrates what has been said in this
    section, as well as the previous, by presenting the complete object
    graph of FreeType&nbsp;2's base design:</p>

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