FreeType Design / I

I. Components and APIs

It is better to describe FreeType 2 as a collection of components. Each one of them is a more or less abstract part of the library that is in charge of one specific task. We will now explain the connections and relationships between them.

A first brief description of this system of components could be as follows.

• 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 module components to perform the work. In most cases, the client application doesn't need to know which 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 time, and they form what is called the low-level base API.

This is illustrated by the following graphics (note that component entry points are represented as colored triangles).

A few additional things must be added to complete this picture.

• Some parts of the base layer can be replaced for specific builds of the library, and can thus be considered as components themselves. This is the case for the ftsystem component, which is in charge of implementing memory management and input stream access, as well as ftinit, which is in charge of library initialization (i.e., implementing the FT_Init_FreeType function).

• FreeType 2 comes also with a set of optional components, which can be used either as a convenience layer for client applications (e.g., the ftglyph component, used to provide a simple API to manage glyph images independently of their internal representation), or to access format-specific features (e.g., the ftmm component used to access and manage Multiple Masters and OpenType variations fonts).

• A module is capable of calling functions provided by another module. This is very useful to share code and tables between several font driver modules (for example, the truetype and cff modules both use the routines provided by the sfnt module).

• Finally, FreeType provides services, which are a more light-weight way to access certain features across multiple modules, or to access some functionality provided by a single module.

  Services are internal to FreeType; similar to modules, it is necessary to ‘load’ a service, which can fail if the service's module is not available.

  An example for a service provided by a single module is ‘winfonts’ (see file svwinfnt.h), which allows access to the header of Windows bitmap fonts. An example for a service provided by multiple modules is ‘multi-masters’ (see file svmm.h) to manage the abovementioned ftmm component across the truetype, type1, and cff modules.

The following graphics shows the additional components (without services).

Please take note of the following important points.

• An optional component can use either the high-level or base API. This is the case of ftglyph in the above picture.

• Some optional components can use module-specific interfaces or services ignored by the base layer. In the above example, ftmm directly accesses the Type 1 and TrueType modules to set and query data.

• A replaceable component can provide a function of the high-level API. For example, ftinit provides FT_Init_FreeType to client applications.