ref: 31fa0bdcd8ad8bf6f6c29f25d13817d7b45f837e
parent: 6043b2404efc77a1c0dc09a193a6526a62e55e28
author: David Turner <[email protected]>
date: Mon Oct 30 13:55:47 EST 2000
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+<head><title>FreeType 2 - Modules</title>
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+<center><table width="500"><tr><td>
+
+<center><h1>FreeType 2 Design - Modules Classes</h1></center>
+
+<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
+<h1>IV. Module Classes</h1>
+</td></tr></table>
+
+<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>
+ <b>renderer</b> 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 or anti-aliased
+ 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>
+
+ <center><table cellpadding=5><tr valign=top><td>
+ <p><b><tt>raster</tt></b></p>
+ </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></p>
+
+ <p><b><tt>smooth</tt></b></p>
+ </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></center>
+
+
+ <li><p>
+ <b>font driver</b> modules are used to support one or more specific
+ font format. By default, FT2 comes with the following font drivers:</p>
+
+ <center><table cellpadding=5><tr valign=top><td>
+ <p><tt><b>truetype</b></tt></p>
+ </td><td>
+ <p>supports TrueType font files</p>
+ </td></tr><tr valign=top><td>
+
+ <p><tt><b>type1</b></tt></p>
+ </td><td>
+ <p>supports Postscript Type 1 fonts, both in binary (.pfb) or ASCII
+ (.pfa) formats, including Multiple Master fonts.</p>
+ </td></tr><tr valign=top><td>
+
+ <p><tt><b>cid</b></tt></p>
+ </td><td>
+ <p>supports Postscript CID-keyed fonts</p>
+ </td></tr><tr valign=top><td>
+
+ <p><tt><b>cff</b></tt></p>
+ </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>
+
+ <p><tt><b>winfonts</b></tt></p>
+ </td><td>
+ <p>supports Windows bitmap fonts (i.e. ".FON" and ".FNT").</p>
+ </td></tr>
+
+ </td></tr></table></center>
+
+ <p>Note that font drivers can support bitmapped or scalable glyph
+ images. A given font driver that supports bezier outlines through
+ the <tt>FT_Outline</tt> can also provide its own hinter, or rely
+ on FreeType's <b>autohinter</b> module.
+ </p></li>
+
+ <li><p>
+ <b>helper</b> 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>
+
+ <table cellpadding=5><tr valign=top><td>
+ <b><tt>sfnt</tt></b>
+ </td><td>
+ used to support font formats based on the "<tt>SFNT</tt>"
+ storage scheme. This means TrueType & OpenType fonts as
+ well as other variants (like TrueType fonts that only
+ contain embedded bitmaps).
+ </td></tr><tr valign=top><td>
+
+ <b><tt>psnames</tt></b>
+ </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>
+
+ <b><tt>psaux</tt></b>
+ </td><td>
+ used to provide various useful functions related to Type 1
+ charstring decoding, as this "feature" is needed by the
+ <b>type1</b>, <b>cid</b> and <b>cff</b> drivers.
+ </td></tr></table></center>
+ </p></li>
+
+
+ <li><p>
+ finally, the <b>autohinter</b> 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 initialisation 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's take a look at this function's declaration:</p>
+
+<pre><font color="blue">
+ extern FT_Error FT_Add_Module( FT_Library library,
+ const FT_Module_Class* clazz );
+</font></pre>
+
+<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>Let's study the definition of <tt>FT_Module_Class</tt>, and explain it
+ a bit. The following code is taken from
+ <tt><freetype/ftmodule.h></tt>:</p>
+
+<pre><font color="blue">
+ 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;
+</font></pre>
+
+<p>here's a description of its fields:</p>
+
+<center><table cellpadding=5><tr valign=top><td>
+<p><b>module_flags</b></p>
+</td><td>
+<p>this is a set of bit flags used to describe the module's
+category. Valid values are:</p>
+ <ul>
+ <li><p>
+ <b>ft_module_font_driver</b> if the module is a font driver
+ </p></li>
+
+ <li><p>
+ <b>ft_module_renderer</b> if the module is a renderer
+ </p></li>
+
+ <li><p>
+ <b>ft_module_hinter</b> if the module is an auto-hinter
+ </p></li>
+
+ <li><p>
+ <b>ft_module_driver_scalable</b> if the module is a font
+ driver supporting scalable glyph formats.
+ </p></li>
+
+ <li><p>
+ <b>ft_module_driver_no_outlines</b> if the module is a
+ font driver supporting scalable glyph formats that <em>cannot</em>
+ be described by a <tt>FT_Outline</tt> object
+ </p></li>
+
+ <li><p>
+ <b>ft_module_driver_has_hinter</b> if the module is a font
+ driver that provides its own hinting scheme/algorithm
+ </p></li>
+ </ul>
+</td></tr><tr valign=top><td>
+
+<p><b>module_size</b></p>
+</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 when the module
+needs to sub-class the base <tt>FT_ModuleRec</tt> class.</p>
+</td></tr><tr valign=top><td>
+
+<p><b>module_name</b></p>
+</td><td>
+<p>this is 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 <b>module_version</b> field to detect module upgrades
+and perform them cleanly, even at run-time.</p>
+</td></tr><tr valign=top><td>
+
+<p><b>module_version</b></p>
+</td><td>
+<p>a 16.16 fixed float number giving the module's major and minor
+ version numbers. It is used to determine wether a module needs
+ to be upgraded when calling <tt>FT_Add_Module</tt>.</p>
+</td></tr><tr valign=top><td>
+
+<p><b>module_requires</b></p>
+</td><td>
+<p>a 16.16 fixed float number giving the version of FreeType 2 that
+ is required to install this module. By default, should be 0x20000
+ for FreeType 2.0</p>
+</td></tr><tr valign=top><td>
+
+<p><b>module_requires</b></p>
+</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,
+where there is one. It's 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 <b>get_interface</b> field.</p>
+</td></tr><tr valign=top><td>
+
+<p><b>module_init</b></p>
+</td><td>
+<p>this is a pointer to a function used to initialise 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
+initialise the fields of <tt>FT_ModuleRec</tt> subclasses.</p>
+
+<p>Most module classes set it to NULL to indicate that no extra
+initialisation is necessary</p>
+</td></tr><tr valign=top><td>
+
+<p><b>module_done</b></p>
+</td><td>
+<p>this is a pointer to a function used to finalise 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
+finalisation is necessary</p>
+</td></tr><tr valign=top><td>
+
+<p><b>get_interface</b></p>
+</td><td>
+<p>this is 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><tr valign=top><td>
+
+</td></tr></table></center>
+
+
+<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's not point to look so far
+ in the library's design.</p>
+
+<p>When <tt>FT_Add_Module</tt> is called, it first allocate a new
+ module instance, using the <tt><b>module_size</b></tt> class
+ field to determine its byte size. The function initializes
+ a the root <tt>FT_ModuleRec</tt> fields, then calls
+ the class-specific initializer <tt><b>module_init</b></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 now detail these classes in the next chapters</p>
+
+</td></tr></table></center>
+</body>
+</html>