FreeType 2 Modules

Introduction

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:

what is a module, and what kind of modules are recognized by the library?
how are modules registered and managed by the library?
how to write a new module, especially new font drivers?
how to select specific modules for a given build of the library ?
how to compile modules as stand-alone DLLs / shared objects?

Overview

1. Library design:

The design of the library is pretty basic:

client applications typically call the FreeType 2 high-level API, whose functions are implemented in a single component called the Base Layer.
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.
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".

This is illustrated by the following graphics:

Note that, however, FT2 comes with a set of optional components that can be ommited from certain builds, and whose purpose vary between two situations:

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.

As an example, see the the ftglyph component that is used to manipulate glyph images more conveniently than the default API.
some other components complement the base layer, by providing additional routines. Most of them allow client applications to access format-specific data.

For example, the ftmm component provides high-level functions to specify Multiple Master coordinates for MM Type 1 fonts.

This is illustrated by the following graphics:

Module Classes

The library is capable of managing and using several kinds of modules:

renderer modules are used to convert scalable glyph images to bitmaps. FreeType 2 comes by default with two of them:

raster1

supports the conversion of vectorial outlines (described by a FT_Outline object) to monochrome bitmaps.

smooth

supports the conversion of the same outlines to high-quality anti-aliased pixmaps.

The specification and interface of renderers is described in details within this document.

Note that most font formats use FT_Outline 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 :-).

font driver modules are used to support one or more specific font format. By default, FT2 comes with the following modules:

`truetype`	supports TrueType font files
`type1`	supports Postscript Type 1 fonts, both in binary (.pfb) or ASCII (.pfa) formats, including Multiple Master fonts.
`cid`	supports Postscript CID-keyed fonts
`cff`	supports OpenType, CFF as well as CEF fonts (CEF is a derivative of CFF used by Adobe in its SVG viewer).
`winfonts`	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 bezier outlines through the 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 a few examples of helper modules that come with FreeType 2:

`sfnt`	used to support font formats based on the "`SFNT`" storage scheme. This means TrueType & OpenType fonts as well as other variants (like TrueType fonts that only contain embedded bitmaps).
`psnames`	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.

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 it's own hinting engine.

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 initialisation 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's 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 !.

Let's study the definition of FT_Module_Class, and explain it a bit. 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;

here's a description of its fields:

module_flags	this is 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 a `FT_Outline` object ft_module_driver_has_hinter if the module is a font driver that provides its own hinting scheme/algorithm
module_size	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 when the module needs to sub-class the base `FT_ModuleRec` class.
module_name	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 `FT_Library` object. However, `FT_Add_Module` uses the module_version field to detect module upgrades and perform them cleanly, even at run-time.
module_version	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 `FT_Add_Module`.
module_requires	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
module_requires	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. Note that is is optional, and can be set to NULL. Other interfaces can also be accessed through the get_interface field.
module_init	this is a pointer to a function used to initialise 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 initialise the fields of `FT_ModuleRec` subclasses. Most module classes set it to NULL to indicate that no extra initialisation is necessary
module_done	this is a pointer to a function used to finalise 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 finalisation is necessary
get_interface	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.

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 not detail its structure here).

When FT_Add_Module is called, it first allocate a new module instance, using the module_size class field to determine its byte size. The function initializes a the root FT_ModuleRec fields, then calls the class-specific initializer module_init when this field is not set to NULL.

Renderer Modules

As said previously, renderers are used to convert scalable glyph images to bitmaps or pixmaps. Each renderer module is defined through a renderer class, whose definition is found in the file <freetype/ftrender.h>. However, a few concepts need to be explained before having a detailed look at this structure.

1. Glyph formats:

Each glyph image that is loaded by FreeType (either through FT_Load_Glyph or FT_Load_Char), has a given image format, described by the field face->glyph->format. It is a 32-byte integer that can take any value. However, the file <freetype/ftimage.h> defines at least the following values:

`ft_glyph_format_bitmap`	this value is used to indicate that the glyph image is a bitmap or pixmap. Its content can then be accessed directly from `face->glyph->bitmap` after the glyph was loaded.
`ft_glyph_format_outline`	this value is used to indicate that the glyph image is a scalable vectorial outline, that can be described by a `FT_Outline` object. Its content can be accessed directly from `face->glyph->outline` after the glyph was loaded. this is the format that is commonly returned by TrueType, Type1 and OpenType / CFF fonts.
`ft_glyph_format_plotter`	this value is equivalent to `ft_glyph_format_outline` 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).
`ft_glyph_format_composite`	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 `FT_LOAD_NO_RECURSE` flag. The list of subglyphs that make up the composite can be accessed directly as `face->glyph->subglyphs` after the glyph was loaded.

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 face->glyph->other.

2. The `FT_Outline` type:

This structure, which is also defined, and heavily commented, in the file <freetype/ftimage.h>, 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.

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.

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:

raster1, used to convert them to monochrome bitmaps
smooth, used to convert them to high-quality anti-aliased pixmaps

3. Bitmaps and scan-conversion:

Bitmaps and pixmaps are described through a FT_Bitmap structure, which is defined and heavily commented in <freetype/ftimage.h>


  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 Driver Modules

Library Initialisation & Dynamic Builds

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

Such applications must normally call the FT_Init_FreeType function before using the library. This function is in charge of two things:

First, it creates a FT_Library object (by calling the public function FT_New_Library). This new object is "empty" and has no module registered in it.
Then, it registers all "default modules" by repeatedly calling FT_Add_Module.

It is important to notice that the default implementation of FT_Init_FreeType, which is located in the source file "src/base/ftinit.c" always uses a static list of modules that is generated at compile time from the configuration file <freetype/config/ftmodule.h>.

There are cases where this may be inadequate. For example, one might want to compile modules as independent DLLs in a specific location (like "/usr/lib/freetype/module/"), and have the library initialisation function load the modules dynamically by parsing the directory's content

This is possible, and we're going to explain how to do it.

a. Building the library as a DLL (i.e. "shared object" on Unix)

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.

When building dynamic libraries, certain compilers require specific directives to declare exported DLL entry points. For example, the "__cdecl" 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).

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.

To allow such compilations, the FT_EXPORT_DEF() 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 <freetype/freetype.h>:


   FT_EXPORT_DEF(FT_Error)  FT_Init_FreeType( void );

the definition of FT_EXPORT_DEF(x) defaults to "extern x", except when a specific definition is given in the library's system-specific configuration file <freetype/config/ftconfig.h>. This allows project builders to specify the exact compilation directive they need.

Similarly, the FT_EXPORT_FUNC(x) macro is defined and used to define exported functions within the FreeType 2 source code. However, it is only used at compilation time.

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).

b. Building modules as DLLs

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.

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 BASE_DEF(x) macro. The latter is similar to FT_EXPORT_DEF and defaults to "extern x" unless you specify a specific definition in <freetype/config/ftconfig.h>.