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<center><h1>The Design of FreeType 2</h1></center>
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<h1>II. Public Objects and Classes</h1>
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<p>We will now detail 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/classes.</p>
<h2>1. Object Orientation in FreeType 2:</h2>
<p>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 FT2 source code:</p>
<ol>
<li><p>
each object type/class has a corresponding <em>structure type</em> <b>and</b>
a corresponding <em>structure pointer type</em>. the latter is called the
<em>handle type</em> for the type/class.</p>
<p>Consider that we need to manage objects of type "foo" in FT2.
We would define the following structure and handle types as
follow:</p>
<pre><font color="blue">
typedef struct FT_FooRec_* FT_Foo;
typedef struct FT_FooRec_
{
// fields for the "foo" class
...
} FT_FooRec;
</font></pre>
<p>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").
<em>Note that each class type has a corresponding handle type</em>.
</p>
<li><p>
class derivation is achieved internally by wrapping base class
structures into new ones. As an example, let's define a "foobar"
class that is derived from "foo". We would do something like:</p>
<pre><font color="blue">
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;
</font></pre>
<p>As you can see, we ensure that a "foobar" object is also a "foo"
object by placing a <tt>FT_FooRec</tt> at the start of the
<tt>FT_FooBarRec</tt> definition. It is called <b>root</b>
by convention.</p>
<p>Note that a <tt>FT_FooBar</tt> handle also points to a "foo" object
and can be typecasted to <tt>FT_Foo</tt>. Similarly, when the
library handles a <tt>FT_Foo</tt> handle to client applications,
the object can be really implemented as a <tt>FT_FooBar</tt> or any
derived class from "foo".</p>
</p></li>
</ul>
<p>Note that in the following sections of this chapter, we will refer
to "the <tt>FT_Foo</tt> class" to indicate the type of objects
handled through <tt>FT_Foo</tt> pointers, be they implemented as
"foo" or "foobar".</p>
<hr>
<h2>2. The <em><b>FT_Library</b></em> class:</h2>
<p>This type corresponds to a handle to a single instance of the
library. Note that the corresponding structure <tt>FT_LibraryRec</tt>
is not defined in public header files, making client applications
unable to access its internal fields.</p>
<p>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).</p>
<p>Typical client applications should call <tt>FT_Init_FreeType</tt>,
in order to create a new library object, ready to be used for
further action.</p>
<p>Another alternative is to create a fresh new library instance
by calling the function <tt>FT_New_Library</tt>, defined in the
<tt><freetype/ftmodule.h></tt> 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 <tt>FT_Add_Module</tt> manually.</p>
<p>Calling <tt>FT_Init_FreeType</tt> 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 and/or
computed is determined at build time, and depends on the content
of the <b>ftinit</b> component. This process is explained in
details later in this document.</p>
<p>For now, one should consider that library objects are created
with <tt>FT_Init_FreeType</tt>, and destroyed along with all
children with <tt>FT_Done_FreeType</tt>.</p>
<hr>
<h2>3. The <em><b>FT_Face</b></em> class:</h2>
<p>A face object corresponds to a single <em>font face</em>, i.e.
a specific typeface with a specific style. For example, "Arial"
and "Arial Italic" correspond to two distinct faces.</p>
<p>A face object is normally created through <tt>FT_New_Face</tt>.
This function takes the following parameters: a <tt>FT_Library</tt>
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),
as well as the address of a <tt>FT_Face</tt> handle. It returns
an error code:</p>
<pre><font color="blue">
FT_Error FT_New_Face( FT_Library library,
const char* filepathname,
FT_Long face_index,
FT_Face *face );
</font></pre>
<p>in case of success, the function will return 0, and the handle
pointed to by the "face" parameter will be set to a non-NULL value.</p>
<p>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
<tt>FT_FaceRec</tt> definition in the FT2 API Reference.</p>
<hr>
<h2>4. The <em><b>FT_Size</b></em> class:</h2>
<p>Each <tt>FT_Face</tt> object <em>has</em> one or more <tt>FT_Size</tt>
objects. A <em>size object</em> is used to store data specific to a
given character width and height. Each newly created face object
has one size, which is directly accessible as <tt>face->size</tt>.</p>
<p>The content of a size object can be changed by calling either
<tt>FT_Set_Pixel_Sizes</tt> or <tt>FT_Set_Char_Size</tt>.</p>
<p>A new size object can be created with <tt>FT_New_Size</tt>, and
destroyed manually with </tt>FT_Done_Size</tt>. Note that typical
applications don't need to do this normally: they tend to use
the default size object provided with each <tt>FT_Face</tt>.</p>
<p>The public fields of <tt>FT_Size</tt> objects are defined in
a very small structure named <tt>FT_SizeRec</tt>. However, it is
important to understand that some font drivers define their own
derivatives of <tt>FT_Size</tt> to store important internal data
that is re-computed each time the character size changes. Most of
the time, these are size-specific <em>font hints</em>./p>
<p>For example, the TrueType driver stores the scaled CVT table that
results from the execution of the "cvt" program in a <tt>TT_Size</tt>,
while the Type 1 driver stores scaled global metrics (like blue zones)
in a <tt>T1_Size</tt> 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 :-)</p>
<hr>
<h2>5. The <em><b>FT_GlyphSlot</b></em> class:</h2>
<p>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).</p>
<p>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
explicitely provide data to compute a slot's maximum size.</p>
<p>Another reason for glyph slots is that they're also used to hold
format-specific hints for a given glyphs has well as all other
data necessary to correctly load the glyph.</p>
<p>The base <tt>FT_GlyphSlotRec</tt> structure only presents glyph
metrics and images to client applications, while actual implementation
may contain more sophisticated data.</p>
<p>As an example, the TrueType-specific <tt>TT_GlyphSlotRec</tt>
structure contains additional fields to hold glyph-specific bytecode,
transient outlines used during the hinting process, and a few other
things.
the Type1-specific <tt>T1_GlyphSlotRec</tt> structure holds
glyph hints during glyph loading, as well as additional logic used
to properly hint the glyphs when a native T1 hinter is used.</p>
<p>Finally, each face object has a single glyph slot, that is directly
accessible as <tt>face->glyph</tt>.</p>
<hr>
<h2>6. The <em><b>FT_CharMap</b></em> class:</h2>
<p>Finally, the <tt>FT_CharMap</tt> type is used as a handle to
character map objects, or "charmaps" to be brief. A charmap is
simply some sort of table or dictionary which is used to translate
character codes in a given encoding into glyph indices for the
font.</p>
<p>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 ugly "standards".</p>
<p>Each <tt>FT_CharMap</tt> object contains a "platform" and an "encoding"
field used to identify precisely the character repertoire corresponding
to it.</p>
<p>Each font format provides its own derivative of <tt>FT_CharMapRec</tt>
and thus needs to implement these objects.</p>
<hr>
<h2>7. Objects relationships:</h2>
<p>The following diagram summarizes what we just said regarding the
public objects managed by the library, as well as explicitely
describes their relationships:</p>
<p>Note that this picture will be updated at the end of the next
chapter, related to <em>internal objects</em>.</p>
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