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<head><title>FreeType 2 - Modules</title>
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<center><h1>FreeType 2 Design - Modules Classes</h1></center>
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<h1>IV. Module Classes</h1>
<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
<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
<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>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>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>
<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>supports TrueType font files</p>
</td></tr><tr valign=top><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>supports Postscript CID-keyed fonts</p>
</td></tr><tr valign=top><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>supports Windows bitmap fonts (i.e. ".FON" and ".FNT").</p>
<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.
<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>
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>
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>
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.
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>
<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 );
<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
<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;
<p>here's a description of its fields:</p>
<center><table cellpadding=5><tr valign=top><td>
<p>this is a set of bit flags used to describe the module's
category. Valid values are:</p>
<b>ft_module_font_driver</b> if the module is a font driver
<b>ft_module_renderer</b> if the module is a renderer
<b>ft_module_hinter</b> if the module is an auto-hinter
<b>ft_module_driver_scalable</b> if the module is a font
driver supporting scalable glyph formats.
<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
<b>ft_module_driver_has_hinter</b> if the module is a font
driver that provides its own hinting scheme/algorithm
</td></tr><tr valign=top><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>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>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>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>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>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>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>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>
<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>
<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>
<p>Helper modules use the base <tt>FT_ModuleRec</tt> type.
We will now detail these classes in the next chapters</p>
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