We will now try to explain more precisely the types of modules
that FreeType 2 is capable of managing. Note that each one of them
is described with more details in the following chapters of this
Renderer modules are used to manage scalable glyph images.
This means transforming them, computing their bounding
box, and converting them to either monochrome
or anti-aliased bitmaps.
Note that FreeType 2 is capable of dealing with any
kind of glyph images, as long as a renderer module is provided for it.
The library comes by default with two renderers:
Supports the conversion of vectorial outlines (described by a
FT_Outline object) to monochrome bitmaps.
Supports the conversion of the same outlines to high-quality
anti-aliased pixmaps (using 256 levels of gray). Note
that this renderer also supports direct span generation.
Font driver modules are used to support one or more
specific font format. By default, FreeType 2 comes with the
following font drivers:
supports TrueType font files
supports Postscript Type 1 fonts, both in binary
(.pfb) or ASCII (.pfa) formats, including
Multiple Master fonts.
supports Postscript CID-keyed fonts
supports OpenType, CFF as well as CEF fonts (CEF is a
derivative of CFF used by Adobe in its SVG viewer)
supports Windows bitmap fonts (i.e. .fon and
Note that font drivers can support bitmapped or scalable glyph
images. A given font driver that supports Bézier outlines
through FT_Outline can also provide its own hinter, or rely
on FreeType's autohinter module.
Helper modules are used to hold shared code that is often
used by several font drivers, or even other modules. Here are the
used to support font formats based on the SFNT storage
scheme: TrueType & OpenType fonts as well as other variants (like
TrueType fonts that only contain embedded bitmaps)
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.
used to provide various useful functions related to Type 1
charstring decoding, as this "feature" is needed by the
type1, cid, and cff drivers.
Finally, the autohinter 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 its own hinting engine.
This module's purpose and design is also heavily described on the
FreeType web site.
We will now study how modules are described, then managed by the
1. The FT_Module_Class structure
As described later in this document, library initialization is
performed by calling the FT_Init_FreeType() function. The
latter is in charge of creating a new "empty" FT_Library
object, then register each "default" module by repeatedly calling the
Similarly, client applications can call FT_Add_Module() any
time they wish in order to register a new module in the library. Let us
take a look at this function's declaration:
extern FT_Error FT_Add_Module(
const FT_Module_Class* clazz );
As one can see, this function expects a handle to a library object,
as well as a pointer to a FT_Module_Class 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!
Here the definition of FT_Module_Class, with some
explanation. The following code is taken from
typedef struct FT_Module_Class_
const FT_String* module_name;
const void* module_interface;
A description of its fields:
A set of bit flags used to describe the module's category. Valid
ft_module_font_driver if the module is a font driver
ft_module_renderer if the module is a renderer
ft_module_hinter if the module is an auto-hinter
ft_module_driver_scalable if the module is a font
driver supporting scalable glyph formats
ft_module_driver_no_outlines if the module is a font
driver supporting scalable glyph formats that cannot be
described by an FT_Outline object
ft_module_driver_has_hinter if the module is a font
driver that provides its own hinting scheme/algorithm
An integer that gives the size in bytes of a given
module object. This should never be less than
sizeof(FT_ModuleRec), but can be more if the module needs
to sub-class the base FT_ModuleRec class.
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 FT_Library object. However,
FT_Add_Module() uses the module_version field to
detect module upgrades and perform them cleanly, even at
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 FT_Add_Module().
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
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.
Note that is is optional, and can be set to NULL. Other
interfaces can also be accessed through the get_interface()
A pointer to a function used to initialize the fields of a fresh
new FT_Module object. It is called after the
module's base fields have been set by the library, and is generally
used to initialize the fields of FT_ModuleRec
Most module classes set it to NULL to indicate that no extra
initialization is necessary.
A pointer to a function used to finalize the fields of a given
FT_Module object. Note that it is called before
the library unsets the module's base fields, and is generally used
to finalize the fields of FT_ModuleRec subclasses.
Most module classes set it to NULL to indicate that no extra
finalization is necessary
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.
2. The FT_Module type
The FT_Module type is a handle (i.e. a pointer) to a given
module object/instance, whose base structure is given by the internal
FT_ModuleRec type. We will intentionally not describe
this structure here, as there is no point to look so far into the
When FT_Add_Module is called, it first allocates a new
module instance, using the module_size class field to determine
its byte size. The function initializes the root FT_ModuleRec
field, then calls the class-specific initializer module_init
when this field is not set to NULL.
Note that the library defines several sub-classes of
FT_ModuleRec, which are, as you could have guessed:
FT_Renderer for renderer modules
FT_Driver for font driver modules
FT_AutoHinter for the auto-hinter
Helper modules use the base FT_ModuleRec type. We will
describe these classes in the next chapters.