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- added first draft of "Modules in FreeType 2" - minor formatting / testing in docmaker.py
2000-10-23 20:32:55 +02:00
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<head><title>FreeType 2 - Modules</title>
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<center><h1>FreeType 2 Modules</h1></center>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Introduction</h1>
</td></tr></table>
<p>
The purpose of this document is to present in great details the way
FreeType 2 uses and manages modules. Among other things, it answers
the following questions:</p>
<ul>
<li><p>
what is a module, and what kind of modules are recognized
by the library?
</p></li>
<li><p>
how are modules registered and managed by the library?
</p></li>
<li><p>
how to write a new module, especially new font drivers?
</p></li>
<li><p>
how to select specific modules for a given build of the
library ?
</p></li>
<li><p>
how to compile modules as stand-alone DLLs / shared objects?
</p></li>
</ul>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Overview</h1>
</td></tr></table>
<h3>1. Library design:</h3>
<p>The design of the library is pretty basic:</p>
<ul>
<li><p>
client applications typically call the FreeType 2 high-level
API, whose functions are implemented in a single component
called the <em>Base Layer</em>.
</p></li>
<li><p>
depending on the context or the task, the base
layer then calls one or more modules to perform the
work. In most cases, the client application doesn't
need to know what module was called.
</p></li>
<li><p>
the base layer also contains a set of routines that are
used for generic things like memory allocation, list
processing, i/o stream parsing, fixed point computation,
etc.. these functions can also be called by a module
at any, and they form what is called the "low-level
base API".
</p></li>
</ul>
<p>This is illustrated by the following graphics:</p>
<center><img src="basic-design.png" width="312" height="312"></center>
<p>Note that, however, FT2 comes with a set of <em>optional</em>
components that can be ommited from certain builds, and whose
purpose vary between two situations:</p>
<ul>
<li><p>
some are located on top of the high-level API and provide
convenience functions that make certain things easier
for typical applications. They typically do not call
modules directly, though they can use the low level
base API for certain tasks.</p>
<p>As an example, see the the <tt>ftglyph</tt> component
that is used to manipulate glyph images more conveniently
than the default API.</p>
</p></li>
<li><p>
some other components complement the base layer, by providing
additional routines. Most of them allow client applications
to access format-specific data.</p>
<p>For example, the <tt>ftmm</tt> component provides high-level
functions to specify Multiple Master coordinates for MM Type 1
fonts.</p>
</p></li>
</ul>
<p>This is illustrated by the following graphics:</p>
<center><img src="detailed-design.png" width="365" height="392"></center>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Module Classes</h1>
</td></tr></table>
<p>
The library is capable of managing and using several kinds of
modules:</p>
<ul>
<li><p>
<b>renderer</b> modules are used to convert scalable glyph images
to bitmaps. FreeType 2 comes by default with two of them:</p>
<center><table cellpadding=5><tr valign=top><td>
<p><b><tt>raster1</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.</p>
</td></tr></table></center>
<p>The specification and interface of renderers is described in
details within this document.</p>
<p>Note that most font formats use <tt>FT_Outline</tt> objects
to describe scalable glyph images. However, FT2 is flexible
and allows specific modules to register and support other
formats. As an example, it's (at least theorically :-) perfectly
possible to write a renderer module that is capable of directly
converting MetaFont glyph definitions to bitmaps or pixmaps !
(of course, this assumes that you also write a MetaFont font
driver to load the definitions :-).
</p></li>
<li><p>
<b>font driver</b> modules are used to support one or more specific
font format. By default, FT2 comes with the following modules:</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 a few examples of helper modules that come with
FreeType 2:</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 function 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></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></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>&lt;freetype/ftmodule.h&gt;</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 not detail its
structure here).</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>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Renderer Modules</h1>
</td></tr></table>
<p>As said previously, <b>renderers</b> are used to convert scalable
glyph images to bitmaps or pixmaps. Each renderer module is defined
through a <b>renderer class</b>, whose definition is found in the
file <tt>&lt;freetype/ftrender.h&gt;</tt>. However, a few concepts
need to be explained before having a detailed look at this structure.
</p>
<h3>1. Glyph formats:</h3>
<p>Each glyph image that is loaded by FreeType (either through
<tt>FT_Load_Glyph</tt> or <tt>FT_Load_Char</tt>), has a given
<em>image format</em>, described by the field
<tt>face-&gt;glyph-&gt;format</tt>. It is a 32-byte integer that
can take any value. However, the file <tt>&lt;freetype/ftimage.h&gt;</tt>
defines at least the following values:</p>
<center><table cellpadding=5>
<tr valign=top><td>
<tt><b>ft_glyph_format_bitmap</b></tt>
</td><td>
this value is used to indicate that the glyph image is a bitmap or pixmap.
Its content can then be accessed directly from
<tt>face-&gt;glyph-&gt;bitmap</tt> after the glyph was loaded.
</td></tr>
<tr valign=top><td>
<tt><b>ft_glyph_format_outline</b></tt>
</td><td>
this value is used to indicate that the glyph image is a scalable vectorial
outline, that can be described by a <tt>FT_Outline</tt> object. Its content
can be accessed directly from
<tt>face-&gt;glyph-&gt;outline</tt> after the glyph was loaded.
this is the format that is commonly returned by TrueType, Type1 and
OpenType / CFF fonts.
</td></tr>
<tr valign=top><td>
<tt><b>ft_glyph_format_plotter</b></tt>
</td><td>
this value is equivalent to <tt><b>ft_glyph_format_outline</b></tt> except
that the outline stored corresponds to path strokes, instead of filled
outlines. It can be returned from certain Type 1 fonts (notably the Hershey
collection of free fonts).
</td></tr>
<tr valign=top><td>
<tt><b>ft_glyph_format_composite</b></tt>
</td><td>
this value is used to indicate that the glyph image is really a "compound"
of several other "sub-glyphs". This value is only returned when a glyph
is loaded with the <tt><b>FT_LOAD_NO_RECURSE</b></tt> flag. The list of
subglyphs that make up the composite can be accessed directly as
<tt>face-&gt;glyph-&gt;subglyphs</tt> after the glyph was loaded.
</td></tr>
</table></center>
<p>Note that this is only a list of pre-defined formats supported by
FreeType. Nothing prevents an application to install a new font
driver that creates other kinds of glyph images. For example, one
could imagine a MetaFont font driver, that would be capable to
parse font definition files and create in-memory "glyph programs",
that could be returned in <tt>face-&gt;glyph-&gt;other</tt>.</p>
<h3>2. The <tt>FT_Outline</tt> type:</h3>
<p>This structure, which is also defined, and heavily commented, in
the file <tt>&lt;freetype/ftimage.h&gt;</tt>, is used to hold
a scalable glyph image that is made of one or more contours, each
contour being described by line segments or bezier arcs (either
quadratic or cubic). The outline itself is stored in a compact
way that makes processing it easy.</p>
<p>Points are placed in a 2D plane that uses the y-upwards convention,
and their coordinates are stored in 1/64th of pixels (also known
as the 26.6 fixed point format). Pixels correspond to single squares
whose borders are on integer coordinates (i.e. mutiples of 64).
In other words, pixel centers are located on half pixel coordinates.</p>
<p>Outlines can be very easily transformed (translated, rotated, etc..)
before being converted to bitmap, which allows for sophisticated
use of text. FreeType 2 comes by default with two "outline renderer"
modules:</p>
<p><ul>
<li><b>raster1</b>, used to convert them to monochrome bitmaps</li>
<li><b>smooth</b>, used to convert them to high-quality anti-aliased
pixmaps</li>
</ul></p>
<h3>3. Bitmaps and scan-conversion:</h3>
<p>Bitmaps and pixmaps are described through a <tt>FT_Bitmap</tt>
structure, which is defined and heavily commented in
<tt>&lt;freetype/ftimage.h&gt;</tt>
<pre><font color="blue">
typedef struct FT_Renderer_Class_
{
FT_Module_Class root;
FT_Glyph_Format glyph_format;
FTRenderer_render render_glyph;
FTRenderer_transform transform_glyph;
FTRenderer_getCBox get_glyph_cbox;
FTRenderer_setMode set_mode;
FT_Raster_Funcs* raster_class;
} FT_Renderer_Class;
</font></pre>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Font Driver Modules</h1>
</td></tr></table>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Library Initialisation & Dynamic Builds</h1>
</td></tr></table>
<p>By default, all components of FreeType 2 are compiled independently,
then grouped into a single static library file that can be installed
or used directly to compile client applications</p>
<p>Such applications must normally call the <tt>FT_Init_FreeType</tt>
function before using the library. This function is in charge of
two things:</p>
<ul>
<li><p>
First, it creates a <tt>FT_Library</tt> object (by calling
the public function <tt>FT_New_Library</tt>). This new
object is "empty" and has no module registered in it.
</p></li>
<li><p>
Then, it registers all "default modules" by repeatedly calling
<tt>FT_Add_Module</tt>.
</p></li>
</ul>
<p>It is important to notice that the default implementation of
<tt>FT_Init_FreeType</tt>, which is located in the source
file <tt>"src/base/ftinit.c"</tt> always uses a <em>static</em>
list of modules that is generated at compile time from the
configuration file <tt>&lt;freetype/config/ftmodule.h&gt;</tt>.
</p>
<p>There are cases where this may be inadequate. For example, one
might want to compile modules as independent DLLs in a specific
location (like <tt>"/usr/lib/freetype/module/"</tt>), and have
the library initialisation function load the modules dynamically
by parsing the directory's content</p>
<p>This is possible, and we're going to explain how to do it.</p>
<h4>a. Building the library as a DLL (i.e. "shared object" on Unix)</h4>
<p>But first of all, let's explain how to build FreeType 2 as a single
DLL or shared object, i.e. one that includes the base layer, all
default modules and optional components into a single file.</p>
<p>When building dynamic libraries, certain compilers require specific
directives to <em>declare</em> exported DLL entry points. For example, the
"<tt>__cdecl</tt>" directive is required by Win32 compilers, as it forces
the use of the "C" parameter passing convention (instead of "smarter"
schemes, which usually use registers and the stack to pass parameters).</p>
<p>To make matter worse, some of these compilers require the directive
before the function's return type, while some others want it between
the return type and the function's identifier.</p>
<p>To allow such compilations, the <tt>FT_EXPORT_DEF()</tt> macro is
used in all public header files in order to declare each high-level
API function of FreeType 2, as in the following example, taken from
<tt>&lt;freetype/freetype.h&gt;</tt>:</p>
<pre><font color="blue">
FT_EXPORT_DEF(FT_Error) FT_Init_FreeType( void );
</font></pre>
<p>the definition of <tt>FT_EXPORT_DEF(x)</tt> defaults to <tt>"extern x"</tt>,
except when a specific definition is given in the library's system-specific
configuration file <tt>&lt;freetype/config/ftconfig.h&gt;</tt>. This
allows project builders to specify the exact compilation directive
they need.</p>
<p>Similarly, the <tt>FT_EXPORT_FUNC(x)</tt> macro is defined and used to
<em>define</em> exported functions within the FreeType 2 source code.
However, it is only used at compilation time.</p>
<p>Note that on Unix, there is no need for specific exportation directives.
However, the code must be compiled in a special way, named Position
Independent Code ("PIC"), which is normally selected through specific
compiler flags (like "-PIC" with gcc).</p>
<h4>b. Building modules as DLLs</h4>
<p>In order to build modules as dynamic libraries, we first need to compile
the base layer (and optional components) as a single DLL. This is very
similar to the case we just described, except that we also need to
export all functions that are part of the "low level base API",
as these will get called by the modules in various cases.</p>
<p>Similarly to the high-level API, all functions of the low-level base
API are declared in the internal header files of FreeType 2 with the
<tt>BASE_DEF(x)</tt> macro. The latter is similar to
<tt>FT_EXPORT_DEF</tt> and defaults to <tt>"extern x"</tt> unless
you specify a specific definition in
<tt>&lt;freetype/config/ftconfig.h&gt;</tt>.</p>
<p>
<hr>
<table width="100%" cellpadding=5><tr bgcolor="#ccccee"><td>
<h1>Conclusion</h1>
</td></tr></table>
</td></tr></table></center>
</body>
</html>