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<h1><a href="../../../index.html">FreeType</a>
Design / II</h1>
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<div id="public-objects">
<h2>II. Public Objects and Classes</h2>
<p>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.</p>
<h3 id="section-1">1. Object Orientation in
FreeType 2</h3>
<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 FreeType 2 source code.</p>
<ol>
<li>
<p>Almost all object types or classes have 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
or class.</p>
<p>Consider that we need to manage objects of type
‘foo’ in FreeType 2. We would define
the following structure and handle types as
follows.</p>
<pre>
typedef struct FT_FooRec_* FT_Foo;
typedef struct FT_FooRec_
{
/* fields for the 'foo' class */
...
} FT_FooRec;</pre>
<p>As a convention, handle types use simple but
meaningful identifiers beginning with <tt>FT_</tt>, as
in <tt>FT_Foo</tt>, while structures use the same name
with a <tt>Rec</tt> suffix appended to it
(‘Rec’ is short for
‘record’).</p>
</li>
<li>
<p>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.</p>
<pre>
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;</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 an <tt>FT_FooBar</tt> handle also points to
a ‘foo’ object and can be typecast
to <tt>FT_Foo</tt>. Similarly, when the library
returns an <tt>FT_Foo</tt> handle to client
applications, the object can be really implemented as
<tt>FT_FooBar</tt> or any derived class from
‘foo’.</p>
</li>
</ol>
<p>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>
<h3 id="section-2">2. The <tt>FT_Library</tt> class</h3>
<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 <em>parent</em> 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 <a href="../reference/ft2-base_interface.html#ft_init_freetype"><code>FT_Init_FreeType</code></a>
in order to create a new library object, ready to be used
for further actions.</p>
<p>Another alternative is to create a fresh new library
instance by calling the function
<a href="../reference/ft2-module_management.html#ft_new_library"><code>FT_New_Library</code></a>,
defined in the
<tt>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
<a href="../reference/ft2-module_management.html#ft_add_module"><code>FT_Add_Module</code></a>
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 or computed is determined at
build time, and depends on the content of
the <tt>ftinit</tt> 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 <a href="../reference/ft2-base_interface.html#ft_done_freetype"><code>FT_Done_FreeType</code></a>.</p>
<h3 id="section-3">3. The <tt>FT_Face</tt> class</h3>
<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 <a href="../reference/ft2-base_interface.html#ft_new_face"><code>FT_New_Face</code></a>.
This function takes the following parameters:
an <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), and the address
of an <tt>FT_Face</tt> handle. It returns an error
code.</p>
<pre>
FT_Error FT_New_Face( FT_Library library,
const char* filepathname,
FT_Long face_index,
FT_Face* face );</pre>
<p>In case of success, the function
returns <tt>FT_Err_Ok</tt> (which is value 0), and
the handle pointed to by the <tt>face</tt> parameter is
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 <a href="../reference/ft2-base_interface.html#ft_facerec"><code>FT_FaceRec</code></a>
definition in the FreeType 2 API Reference.</p>
<h3 id="section-4">4. The <tt>FT_Size</tt> class</h3>
<p>Each <tt>FT_Face</tt> object has one or more
<tt>FT_Size</tt> objects. A <em>size object</em> stores
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 contents of a size object can be changed by calling
<a href="../reference/ft2-base_interface.html#ft_request_size"><code>FT_Request_Size</code></a>, <a href="../reference/ft2-base_interface.html#ft_set_pixel_sizes"><code>FT_Set_Pixel_Sizes</code></a>,
or <a href="../reference/ft2-base_interface.html#ft_set_char_size"><code>FT_Set_Char_Size</code></a>.</p>
<p>A new size object can be created
with <a href="../reference/ft2-sizes_management.html#ft_new_size"><code>FT_New_Size</code></a>,
and destroyed manually
with <a href="../reference/ft2-sizes_management.html#ft_done_size"><code>FT_Done_Size</code></a>.
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 <tt>FT_Face</tt>.</p>
<p>The public fields of <tt>FT_Size</tt> objects are defined
in a very small structure
named <a href="../reference/ft2-base_interface.html#ft_sizerec"><code>FT_SizeRec</code></a>.
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
(Control Value Table) data that results from the execution
of the ‘prep’ program in a <tt>TT_Size</tt>
structure, 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>
<h3 id="section-5">5. The <tt>FT_GlyphSlot</tt> class</h3>
<p>The purpose of a <em>glyph slot</em> 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 explicitly provide
data to compute a slot's maximum size.</p>
<p>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.</p>
<p>The
base <a href="../reference/ft2-base_interface.html#ft_glyphslotrec"><code>FT_GlyphSlotRec</code></a>
structure only presents glyph metrics and images to client
applications, while the 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
Type 1-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 Type 1 hinter is used.</p>
<p>Each face object has a single glyph slot that is directly
accessible as <tt>face->glyph</tt>.</p>
<h3 id="section-6">6. The <tt>FT_CharMap</tt> class</h3>
<p>The <tt>FT_CharMap</tt> type is a handle to character map
objects, or <em>charmaps</em>. A charmap is simply some
sort of table or dictionary 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
encodings.</p>
<p>Each <tt>FT_CharMap</tt> object contains a
‘platform’ and an ‘encoding’ field
to precisely identify the character repertoire
corresponding to it.</p>
<p>Each font format provides its own derivative of
<a href="../reference/ft2-base_interface.html#ft_charmaprec"><code>FT_CharMapRec</code></a>
and thus needs to implement these objects.</p>
<h3 id="section-7">7. Objects Relationship</h3>
<p>The following diagram summarizes what we have just said
regarding the public objects managed by the library; it
also describes their relationship.</p>
<center>
<img src="simple-model.png"
width="453"
height="378"
alt="Simple library model">
</center>
<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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<div class="updated">
<p>Last update: 13-Feb-2018</p>
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