FreeType Design / III

III. Internal Objects and Classes

Let us have a look now at the internal objects that FreeType 2 uses, i.e., those not directly available to client applications, and see how they fit into the picture.

1. Memory Management

Most memory management operations are performed through three specific routines of the base layer: FT_Alloc, FT_Realloc, and FT_Free. Each one of these functions expects a FT_Memory handle as its first parameter. Note, however, that there exist more, similar variants for specific purposes which we skip here for simplicity.

FT_Memory is a pointer to a simple object that describes the current memory pool or manager. It contains a small table of alloc, realloc, and free functions. A memory manager is created at library initialization time by FT_Init_FreeType, calling the (internal) function FT_New_Memory provided by the ftsystem component.

By default, this manager uses the ANSI functions malloc, realloc, and free. However, as ftsystem is a replaceable part of the base layer, a specific build of the library could provide a different default memory manager.

Even with a default build, client applications are still able to provide their own memory manager by not calling FT_Init_FreeType but follow these simple steps.

1. Create a new FT_Memory object by hand. The definition of FT_MemoryRec is located in the public header file ftsystem.h.

2. Call FT_New_Library to create a new library instance using your custom memory manager. This new library doesn't yet contain any registered modules.

3. Register the set of default modules by calling the function FT_Add_Default_Modules provided by the ftinit component, or manually register your drivers by repeatedly calling FT_Add_Module.

2. Input Streams

Font files are always read through FT_Stream objects. The definition of FT_StreamRec is located in the public header file ftsystem.h, which allows client developers to provide their own implementation of streams if they wish so.

The function FT_New_Face always automatically creates a new stream object from the C pathname given as its second argument. This is achieved by calling the (internal) function FT_Stream_Open provided by the ftsystem component. As the latter is replaceable, the implementation of streams may vary greatly between platforms.

As an example, the default implementation of streams is located in the file src/base/ftsystem.c and uses the ANSI functions fopen, fseek, and fread. However, the Unix build of FreeType 2 provides an alternative implementation that uses memory-mapped files, when available on the host platform, resulting in a significant access speed-up.

FreeType distinguishes between memory-based and disk-based streams. In the first case, all data is directly accessed in memory (e.g., ROM-based, write-only static data, and memory-mapped files), while in the second, portions of the font files are read in chunks called frames, and temporarily buffered similarly through typical seek and read operations.

The FreeType stream sub-system also implements extremely efficient algorithms to very quickly load structures from font files while ensuring complete safety in the case of a ‘broken file’.

The function FT_New_Memory_Face can be used to directly create and open an FT_Face object from data that is readily available in memory (including ROM-based fonts).

Finally, in the case where a custom input stream is needed, client applications can use the function FT_Open_Face, which can accept custom input streams. This may be useful in the case of compressed or remote font files, or even embedded font files that need to be extracted from certain documents.

Note that each face owns a single stream, which is also destroyed by FT_Done_Face.

3. Modules

A FreeType 2 module is itself a piece of code. However, the library creates a single FT_Module object for each module that is registered when FT_Add_Module is called.

The definition of FT_ModuleRec is not publicly available to client applications. However, each module type is described by a simple public structure named FT_Module_Class, defined in header file ftmodule.h, and is described later in this document.

You need a pointer to an FT_Module_Class structure when calling FT_Add_Module.

FT_Error
FT_Add_Module( FT_Library              library,
               const FT_Module_Class*  clazz );

This function does the following tasks.

• Check whether the library already holds a module object corresponding to the same module name as the one found in FT_Module_Class.

• If this is the case, compare the module version number to see whether it is possible to upgrade the module to a new version. If the module class's version number is smaller than the already installed one, return immediately. Similarly, check that the version of FreeType 2 that is running is correct compared to the one required by the module.

• Create a new FT_Module object, using data and flags of the module class to determine its byte size and how to properly initialize it.

• If a module initializer is present in the module class, call it to complete the module object's initialization.

• Add the new module to the library's list of ‘registered’ modules. In case of an upgrade, destroy the previous module object.

Note that this function doesn't return an FT_Module handle, given that module objects are completely internal to the library (and client applications shouldn't normally mess with them :-)

Finally, it is important to understand that FreeType 2 recognizes and manages several kinds of modules. These will be explained later on in this document.

• Renderer modules are used to convert native glyph images to bitmaps or pixmaps. FreeType 2 comes with two renderer modules by default: one to generate monochrome bitmaps, the other to generate anti-aliased pixmaps.

• Font driver modules are used to support one or more font formats. Typically, each font driver provides a specific implementation or derivative of FT_Face, FT_Size, FT_GlyphSlot, as well as FT_CharMap.

• Helper modules are shared by several font drivers. For example, the sfnt module parses and manages tables found in SFNT-based font formats; it is then used by both the TrueType font and CFF drivers.

• Finally, the auto-hinter module has a specific place in the library's design, as its role is to process vectorial glyph outlines, independently of their native font format, to produce optimal results at small pixel sizes.

Note that every FT_Face object is owned by the corresponding font driver, depending on the original font file's format. This means that all face objects are destroyed when a module is removed or unregistered from a library instance (typically by calling the FT_Remove_Module function). Because of this, you should always take care that no FT_Face object is opened when you upgrade or remove a module from a library, as this could cause unexpected object deletion!

4. Summary

Finally, the following picture illustrates what has been said in this section, as well as the previous, by presenting the complete object graph of FreeType 2's base design.