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