FreeType Design / II

II. Public Objects and Classes

We will now explain the abstractions provided by FreeType 2 to client applications to manage font files and data. As you would normally expect, these are implemented through objects and classes.

1. Object Orientation in FreeType 2

Though written in ANSI C, the library employs a few techniques, inherited from object-oriented programming, to make it easy to extend. Hence, the following conventions apply in the FreeType 2 source code.

1. Almost all object types or classes have a corresponding structure type and a corresponding structure pointer type. The latter is called the handle type for the type or class.

   Consider that we need to manage objects of type ‘foo’ in FreeType 2. We would define the following structure and handle types as follows.

   typedef struct FT_FooRec_*  FT_Foo;
   
   typedef struct  FT_FooRec_
   {
     /* fields for the 'foo' class */
     ...
   
   } FT_FooRec;

   As a convention, handle types use simple but meaningful identifiers beginning with FT_, as in FT_Foo, while structures use the same name with a Rec suffix appended to it (‘Rec’ is short for ‘record’).

2. Class derivation is achieved internally by wrapping base class structures into new ones. As an example, we define a ‘foobar’ class that is derived from ‘foo’. We would do something like this.

   typedef struct FT_FooBarRec_*  FT_FooBar;
   
   typedef struct  FT_FooBarRec_
   {
     /* the base 'foo' class fields */
     FT_FooRec  root;
   
     /* fields proper to the 'foobar' class */
     ...
   } FT_FooBarRec;

   As you can see, we ensure that a ‘foobar’ object is also a ‘foo’ object by placing a FT_FooRec at the start of the FT_FooBarRec definition. It is called root by convention.

   Note that an FT_FooBar handle also points to a ‘foo’ object and can be typecast to FT_Foo. Similarly, when the library returns an FT_Foo handle to client applications, the object can be really implemented as FT_FooBar or any derived class from ‘foo’.

In the following sections of this chapter, we will refer to ‘the FT_Foo class’ to indicate the type of objects handled through FT_Foo pointers, be they implemented as ‘foo’ or ‘foobar’.

2. The FT_Library class

This type corresponds to a handle to a single instance of the library. Note that the corresponding structure FT_LibraryRec is not defined in public header files, making client applications unable to access its internal fields.

The library object is the parent of all other objects in FreeType 2. You need to create a new library instance before doing anything else with the library. Similarly, destroying it will automatically destroy all its children (i.e., faces and modules).

Typical client applications should call FT_Init_FreeType in order to create a new library object, ready to be used for further actions.

Another alternative is to create a fresh new library instance by calling the function FT_New_Library, defined in the ftmodule.h public header file. This function will however return an ‘empty’ library instance with no module registered in it. You can ‘install’ modules in the instance by calling FT_Add_Module manually.

Calling FT_Init_FreeType is a lot more convenient, because this function basically registers a set of default modules into each new library instance. The way this list is accessed or computed is determined at build time, and depends on the content of the ftinit component. This process is explained in details later in this document.

For now, one should consider that library objects are created with FT_Init_FreeType, and destroyed along with all children with FT_Done_FreeType.

3. The FT_Face class

A face object corresponds to a single font face, i.e., a specific typeface with a specific style. For example, ‘Arial’ and ‘Arial Italic’ correspond to two distinct faces.

A face object is normally created through FT_New_Face. This function takes the following parameters: an FT_Library handle, a C file pathname used to indicate which font file to open, an index used to decide which face to load from the file (a single file may contain several faces in certain cases), and the address of an FT_Face handle. It returns an error code.

FT_Error  FT_New_Face( FT_Library   library,
                       const char*  filepathname,
                       FT_Long      face_index,
                       FT_Face*     face );

In case of success, the function returns FT_Err_Ok (which is value 0), and the handle pointed to by the face parameter is set to a non-NULL value.

Note that the face object contains several fields used to describe global font data that can be accessed directly by client applications, for example, the total number of glyphs in the face, the face's family name, style name, the EM size for scalable formats, etc. For more details, look at the FT_FaceRec definition in the FreeType 2 API Reference.

4. The FT_Size class

Each FT_Face object has one or more FT_Size objects. A size object stores data specific to a given character width and height. Each newly created face object has one size, which is directly accessible as face->size.

The contents of a size object can be changed by calling FT_Request_Size, FT_Set_Pixel_Sizes, or FT_Set_Char_Size.

A new size object can be created with FT_New_Size, and destroyed manually with FT_Done_Size. Note that typical applications don't need to do this normally: usually it is fully sufficient to use the default size object provided with each FT_Face.

The public fields of FT_Size objects are defined in a very small structure named FT_SizeRec. However, it is important to understand that some font drivers define their own derivatives of FT_Size to store important internal data that is re-computed each time the character size changes. Most of the time, these are size-specific font hints.

For example, the TrueType driver stores the scaled CVT (Control Value Table) data that results from the execution of the ‘prep’ program in a TT_Size structure, while the Type 1 driver stores scaled global metrics (like blue zones) in a T1_Size object. Don't worry if you don't understand the current paragraph; most of this stuff is highly font format specific and doesn't need to be explained to client developers :-)

5. The FT_GlyphSlot class

The purpose of a glyph slot is to provide a place where glyph images can be loaded one by one easily, independently of the glyph image format (bitmap, vector outline, or anything else).

Ideally, once a glyph slot is created, any glyph image can be loaded into it without additional memory allocation. In practice, this is only possible with certain formats like TrueType which explicitly provide data to compute a slot's maximum size.

Another reason for glyph slots is that they are also used to hold format-specific hints for a given glyphs as well as all other data necessary to correctly load the glyph.

The base FT_GlyphSlotRec structure only presents glyph metrics and images to client applications, while the actual implementation may contain more sophisticated data.

As an example, the TrueType-specific TT_GlyphSlotRec structure contains additional fields to hold glyph-specific bytecode, transient outlines used during the hinting process, and a few other things. The Type 1-specific T1_GlyphSlotRec structure holds glyph hints during glyph loading, as well as additional logic used to properly hint the glyphs when a native Type 1 hinter is used.

Each face object has a single glyph slot that is directly accessible as face->glyph.

6. The FT_CharMap class

The FT_CharMap type is a handle to character map objects, or charmaps. A charmap is simply some sort of table or dictionary to translate character codes in a given encoding into glyph indices for the font.

A single face may contain several charmaps. Each one of them corresponds to a given character repertoire, like Unicode, Apple Roman, Windows codepages, and other encodings.

Each FT_CharMap object contains a ‘platform’ and an ‘encoding’ field to precisely identify the character repertoire corresponding to it.

Each font format provides its own derivative of FT_CharMapRec and thus needs to implement these objects.

7. Objects Relationship

The following diagram summarizes what we have just said regarding the public objects managed by the library; it also describes their relationship.

Note that this picture will be updated at the end of the next chapter, related to internal objects.