The design of FreeType 2

Copyright 1998-2000 David Turner (
Copyright 2000, 2007 The FreeType Development Team (

Previous Contents Next

IV. Module Classes

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 document.

  • 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 .fnt)

    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 default helpers:

    sfnt 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)
    psnames 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.
    psaux 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 library.

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 FT_Add_Module() function.

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(
                       FT_Library              library,
                       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 <freetype/ftmodule.h>:

    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;

A description of its fields:


A set of bit flags used to describe the module's category. Valid values are:

  • 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 run-time.


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() field.


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 subclasses.

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 library's design.

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.

Previous Contents Next