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<h1><a href="http://freetype.org/index.html">FreeType</a>
Tutorial / I</h1>
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<h2>I. Simple Glyph Loading</h2>
<h3 id="section-1">1. Header Files</h3>
<p>The following are instructions required to compile an
application that uses the FreeType 2 library.</p>
<ol>
<li class="emph">
<p>Locate the FreeType 2 <tt>include</tt>
directory.</p>
<p>You have to add it to your compilation include
path.</p>
<p>In Unix-like environments you can run the
<tt>freetype-config</tt> script with
the <tt>--cflags</tt> option to retrieve the
appropriate compilation flags. This script can also
be used to check the version of the library that is
installed on your system, as well as the required
librarian and linker flags.</p>
</li>
<li class="emph">
<p>Include the file named <tt>ft2build.h</tt>.</p>
<p>It contains various macro declarations that are later
used to <tt>#include</tt> the appropriate public
FreeType 2 header files.</p>
</li>
<li class="emph">
<p>Include the main FreeType 2 API header file.</p>
<p>You should do that using the
macro <tt>FT_FREETYPE_H</tt>, like in the following
example.</p>
<pre>
#include <ft2build.h>
#include FT_FREETYPE_H</pre>
<p><tt>FT_FREETYPE_H</tt> is a special macro defined in
file <tt>ftheader.h</tt>. It contains some
installation-specific macros to name other public
header files of the FreeType 2 API.</p>
<p>You can
read <a href="../reference/ft2-header_file_macros.html">this
section of the FreeType 2 API Reference</a> for a
complete listing of the header macros.</p>
</li>
</ol>
<p>The use of macros in <tt>#include</tt> statements is
ANSI-compliant. It is used for several reasons.</p>
<ul>
<li>It avoids conflicts with (deprecated) FreeType 1.x
public header files.</li>
<li>The macro names are not limited to the DOS 8.3 file
naming limit; names like <tt>FT_MULTIPLE_MASTERS_H</tt>
or <tt>FT_SFNT_NAMES_H</tt> are a lot more readable and
explanatory than the real file names
<tt>ftmm.h</tt> and <tt>ftsnames.h</tt>.</li>
<li>It allows special installation tricks that will not be
discussed here.</li>
</ul>
<h3 id="section-2">2. Library Initialization</h3>
<p>To initialize the FreeType library, create a variable of
type <a href="../reference/ft2-base_interface.html#FT_Library"><tt>FT_Library</tt></a>
named, for example, <tt>library</tt>, and call the
function <a href="../reference/ft2-base_interface.html#FT_Init_FreeType"><tt>FT_Init_FreeType</tt></a>.</p>
<pre>
#include <ft2build.h>
#include FT_FREETYPE_H
FT_Library library;
...
error = FT_Init_FreeType( &library );
if ( error )
{
... an error occurred during library initialization ...
}</pre>
<p>This function is in charge of</p>
<ul>
<li>creating a new instance of the FreeType 2 library
and setting the handle <tt>library</tt> to it, and</li>
<li>loading each module that FreeType knows about in the
library. Among others, your new <tt>library</tt> object
is able to handle TrueType, Type 1, CID-keyed &
OpenType/CFF fonts gracefully.</li>
</ul>
<p>As you can see, the function returns an error code, like
most other functions of the FreeType API. An error code
of 0 (also known
as <tt>FT_Err_Ok</tt>) <em>always</em> means that the
operation was successful; otherwise, the value describes
the error, and <tt>library</tt> is set to NULL.</p>
<h3 id="section-3">3. Loading a Font Face</h3>
<h4>a. From a Font File</h4>
<p>Create a new <tt>face</tt> object by
calling <a href="../reference/ft2-base_interface.html#FT_New_Face"><tt>FT_New_Face</tt></a>.
A <em>face</em> describes a given typeface and style. For
example, ‘Times New Roman Regular’ and
‘Times New Roman Italic’ correspond to two
different faces.</p>
<pre>
FT_Library library; <span class="comment">/* handle to library */</span>
FT_Face face; <span class="comment">/* handle to face object */</span>
error = FT_Init_FreeType( &library );
if ( error ) { ... }
error = FT_New_Face( library,
"/usr/share/fonts/truetype/arial.ttf",
0,
&face );
if ( error == FT_Err_Unknown_File_Format )
{
... the font file could be opened and read, but it appears
... that its font format is unsupported
}
else if ( error )
{
... another error code means that the font file could not
... be opened or read, or that it is broken...
}</pre>
<p>As you can certainly imagine, <tt>FT_New_Face</tt> opens
a font file, then tries to extract one face from it. Its
parameters are as follows.</p>
<dl>
<dt>library</dt>
<dd>A handle to the FreeType library instance where the
face object is created.</dd>
<dt>filepathname</dt>
<dd>The font file pathname (a standard C string).</dd>
<dt>face_index</dt>
<dd>
<p>Certain font formats allow several font faces to be
embedded in a single file.</p>
<p>This index tells which face you want to load. An
error is returned if its value is too large.</p>
<p>Index 0 always works, though.</p>
</dd>
<dt>face</dt>
<dd>
<p>A <em>pointer</em> to the handle that is set to
describe the new face object.</p>
<p>It is set to NULL in case of error.</p>
</dd>
</dl>
<p>To know how many faces a given font file contains, load
its first face (this is, <tt>face_index</tt> should be set
to zero), then check the value
of <tt>face->num_faces</tt>, which indicates how many
faces are embedded in the font file.</p>
<h4>b. From Memory</h4>
<p>In the case where you have already loaded the font file
into memory, you can similarly create a new face object
for it by
calling <a href="../reference/ft2-base_interface.html#FT_New_Memory_Face"><tt>FT_New_Memory_Face</tt></a>.</p>
<pre>
FT_Library library; <span class="comment">/* handle to library */</span>
FT_Face face; <span class="comment">/* handle to face object */</span>
error = FT_Init_FreeType( &library );
if ( error ) { ... }
error = FT_New_Memory_Face( library,
buffer, <span class="comment">/* first byte in memory */</span>
size, <span class="comment">/* size in bytes */</span>
0, <span class="comment">/* face_index */</span>
&face );
if ( error ) { ... }</pre>
<p>As you can see, <tt>FT_New_Memory_Face</tt> takes a
pointer to the font file buffer and its size in bytes
instead of a file pathname. Other than that, it has
exactly the same semantics as
<tt>FT_New_Face</tt>.</p>
<p>Note that you must not deallocate the memory before calling
<a href="../reference/ft2-base_interface.html#FT_Done_Face"><tt>FT_Done_Face</tt></a>.</p>
<h4>c. From Other Sources (Compressed Files, Network,
etc.)</h4>
<p>There are cases where using a file pathname or preloading
the file into memory is not sufficient. With
FreeType 2, it is possible to provide your own
implementation of I/O routines.</p>
<p>This is done through
the <a href="../reference/ft2-base_interface.html#FT_Open_Face"><tt>FT_Open_Face</tt></a>
function, which can be used to open a new font face with a
custom input stream, select a specific driver for opening,
or even pass extra parameters to the font driver when
creating the object. We advise you to look up
the <a href="../reference/ft2-toc.html">FreeType 2
reference manual</a> in order to learn how to use it.</p>
<h3 id="section-4">4. Accessing the Face Data</h3>
<p>A <em>face object</em> models all information that
globally describes the face. Usually, this data can be
accessed directly by dereferencing a handle, like
in <tt>face−>num_glyphs</tt>.</p>
<p>The complete list of available fields is in
the <a href="../reference/ft2-base_interface.html#FT_FaceRec"><tt>FT_FaceRec</tt></a>
structure description. However, we describe here a few of
them in more detail.</p>
<dl>
<dt>num_glyphs</dt>
<dd>This variable gives the number of <em>glyphs</em>
available in the font face. A glyph is a character
image, nothing more – it thus doesn't necessarily
correspond to a <em>character code</em>.</dd>
<dt>face_flags</dt>
<dd>A 32-bit integer containing bit flags that describe
some face properties. For example, the flag
<tt>FT_FACE_FLAG_SCALABLE</tt> indicates that the face's
font format is scalable and that glyph images can be
rendered for all character pixel sizes. For more
information on face flags, please read
the <a href="../reference/ft2-base_interface.html#FT_FACE_FLAG_XXX">FreeType 2
API Reference</a>.</dd>
<dt>units_per_EM</dt>
<dd>This field is only valid for scalable formats (it is
set to 0 otherwise). It indicates the number of
font units covered by the EM.</dd>
<dt>num_fixed_sizes</dt>
<dd>This field gives the number of embedded bitmap strikes
in the current face. A <em>strike</em> is a series of
glyph images for a given character pixel size. For
example, a font face could include strikes for pixel
sizes 10, 12, and 14. Note that even scalable font
formats can have embedded bitmap strikes!</dd>
<dt>available_sizes</dt>
<dd>
<p>A pointer to an array
of <a href="../reference/ft2-base_interface.html#FT_Bitmap_Size"><tt>FT_Bitmap_Size</tt></a>
elements. Each <tt>FT_Bitmap_Size</tt> indicates the
horizontal and vertical <em>character pixel sizes</em>
for each of the strikes that are present in the
face.</p>
<p class="warning">Note that, generally speaking, these
are <em>not</em> the <em>cell size</em> of the bitmap
strikes.</p>
</dd>
</dl>
<h3 id="section-5">5. Setting the Current Pixel Size</h3>
<p>FreeType 2 uses <em>size objects</em> to model all
information related to a given character size for a given
face. For example, a size object holds the value of
certain metrics like the ascender or text height,
expressed in 1/64th of a pixel, for a character size of
12 points.</p>
<p>When the <tt>FT_New_Face</tt> function is called (or one
of its siblings), it <em>automatically</em> creates a new
size object for the returned face. This size object is
directly accessible as
<tt>face−>size</tt>.</p>
<p class="note">NOTE: A single face object can deal with one
or more size objects at a time; however, this is something
that few programmers really need to do. We have thus
decided to simplify the API for the most common use (i.e.,
one size per face) while keeping this feature available
through additional functions.</p>
<p>When a new face object is created, all elements are set
to 0 during initialization. To populate the
structure with sensible values, you should
call <a href="../reference/ft2-base_interface.html#FT_Set_Char_Size"><tt>FT_Set_Char_Size</tt></a>.
Here is an example, setting the character size to 16pt for
a 300×300dpi device:</p>
<pre>
error = FT_Set_Char_Size(
face, <span class="comment">/* handle to face object */</span>
0, <span class="comment">/* char_width in 1/64th of points */</span>
16*64, <span class="comment">/* char_height in 1/64th of points */</span>
300, <span class="comment">/* horizontal device resolution */</span>
300 ); <span class="comment">/* vertical device resolution */</span></pre>
<p>Some notes.</p>
<ul>
<li>The character widths and heights are specified in
1/64th of points. A point is a <em>physical</em>
distance, equaling 1/72th of an inch. Normally, it is
not equivalent to a pixel.</li>
<li>Value of 0 for the character width means
‘same as character height’, value of 0
for the character height means ‘same as character
width’. Otherwise, it is possible to specify
different character widths and heights.</li>
<li>The horizontal and vertical device resolutions are
expressed in <em>dots-per-inch</em>, or <em>dpi</em>.
Standard values are 72 or 96 dpi for display
devices like the screen. The resolution is used to
compute the character pixel size from the character
point size.</li>
<li>Value of 0 for the horizontal resolution means
‘same as vertical resolution’, value
of 0 for the vertical resolution means ‘same
as horizontal resolution’. If both values are
zero, 72 dpi is used for both dimensions.</li>
<li>The first argument is a handle to a face object, not a
size object.</li>
</ul>
<p>This function computes the character pixel size that
corresponds to the character width and height and device
resolutions. However, if you want to specify the pixel
sizes yourself, you can call
<a href="../reference/ft2-base_interface.html#FT_Set_Pixel_Sizes"><tt>FT_Set_Pixel_Sizes</tt></a>.</p>
<pre>
error = FT_Set_Pixel_Sizes(
face, <span class="comment">/* handle to face object */</span>
0, <span class="comment">/* pixel_width */</span>
16 ); <span class="comment">/* pixel_height */</span></pre>
<p>This example sets the character pixel sizes to
16×16 pixels. As previously, a value of 0
for one of the dimensions means ‘same as the
other’.</p>
<p>Note that both functions return an error code. Usually,
an error occurs with a fixed-size font format (like FNT or
PCF) when trying to set the pixel size to a value that is
not listed in the <tt>face->fixed_sizes</tt> array.</p>
<h3 id="section-6">6. Loading a Glyph Image</h3>
<h4>a. Converting a Character Code Into a Glyph Index</h4>
<p>Normally, an application wants to load a glyph image
based on its <em>character code</em>, which is a unique
value that defines the character for a
given <em>encoding</em>. For example, code 65 (0x41)
represents character ‘A’ in ASCII
encoding.</p>
<p>A face object contains one or more tables, called
<em>charmaps</em>, to convert character codes to glyph
indices. For example, most older TrueType fonts contain
two charmaps: One is used to convert Unicode character
codes to glyph indices, the other one is used to convert
Apple Roman encoding to glyph indices. Such fonts can
then be used either on Windows (which uses Unicode) and
old MacOS versions (which use Apple Roman). Note also
that a given charmap might not map to all the glyphs
present in the font.</p>
<p>By default, when a new face object is created, it selects
a Unicode charmap. FreeType tries to emulate a Unicode
charmap if the font doesn't contain such a charmap, based
on glyph names. Note that it is possible that the
emulation misses glyphs if glyph names are non-standard.
For some fonts like symbol fonts, no Unicode emulation is
possible at all.</p>
<p>Later on we will describe how to look for specific
charmaps in a face. For now, we assume that the face
contains at least a Unicode charmap that was selected
during a call to <tt>FT_New_Face</tt>. To convert a
Unicode character code to a font glyph index, we use
<a href="../reference/ft2-base_interface.html#FT_Get_Char_Index"><tt>FT_Get_Char_Index</tt></a>.</p>
<pre>
glyph_index = FT_Get_Char_Index( face, charcode );</pre>
<p>This code line looks up the glyph index corresponding to
the given <tt>charcode</tt> in the charmap that is
currently selected for the face. You should use the
UTF-32 representation form of Unicode; for example, if you
want to load character U+1F028, use value 0x1F028 as the
value for <tt>charcode</tt>.
<p>If no charmap was selected, the function returns
the charcode.</p>
<p>Note that this is one of the rare FreeType functions that
do not return an error code. However, when a given
character code has no glyph image in the face,
value 0 is returned. By convention, it always
corresponds to a special glyph image called
the <em>missing glyph</em>, which is commonly displayed as
a box or a space.</p>
<h4>b. Loading a Glyph From the Face</h4>
<p>Once you have a glyph index, you can load the
corresponding glyph image. The latter can be stored in
various formats within the font file. For fixed-size
formats like FNT or PCF, each image is a bitmap. Scalable
formats like TrueType or CFF use vectorial shapes
(<em>outlines</em>) to describe each glyph. Some formats
may have even more exotic ways of representing glyphs
(e.g., MetaFont – but this format is not supported).
Fortunately, FreeType 2 is flexible enough to support
any kind of glyph format through a simple API.</p>
<p>The glyph image is always stored in a special object called a
<em>glyph slot</em>. As its name suggests, a glyph slot
is a container that is able to hold one glyph image at a
time, be it a bitmap, an outline, or something else. Each
face object has a single glyph slot object that can be
accessed as <tt>face->glyph</tt>. Its fields are
explained by
the <a href="../reference/ft2-base_interface.html#FT_GlyphSlotRec"><tt>FT_GlyphSlotRec</tt></a>
structure documentation.</p>
<p>Loading a glyph image into the slot is performed by
calling <a href="../reference/ft2-base_interface.html#FT_Load_Glyph"><tt>FT_Load_Glyph</tt></a>.</p>
<pre>
error = FT_Load_Glyph(
face, <span class="comment">/* handle to face object */</span>
glyph_index, <span class="comment">/* glyph index */</span>
load_flags ); <span class="comment">/* load flags, see below */</span></pre>
<p>The <tt>load_flags</tt> value is a set of bit flags to
indicate some special operations. The default value
<tt>FT_LOAD_DEFAULT</tt> is 0.</p>
<p>This function tries to load the corresponding glyph image
from the face.</p>
<ul>
<li>If a bitmap is found for the corresponding glyph and
pixel size, it is loaded into the slot. Embedded
bitmaps are always favoured over native image formats,
because we assume that they are higher-quality versions
of the same glyph. This can be changed by using
the <tt>FT_LOAD_NO_BITMAP</tt> flag.</li>
<li>Otherwise, a native image for the glyph is loaded. It
is also scaled to the current pixel size, as well as
hinted for certain formats like TrueType and
Type 1.</li>
</ul>
<p>The field <tt>face−>glyph−>format</tt>
describes the format used for storing the glyph image in
the slot. If it is not <tt>FT_GLYPH_FORMAT_BITMAP</tt>,
one can immediately convert it to a bitmap
through <a href="../reference/ft2-base_interface.html#FT_Render_Glyph"><tt>FT_Render_Glyph</tt></a>.</p>
<pre>
error = FT_Render_Glyph( face->glyph, <span class="comment">/* glyph slot */</span>
render_mode ); <span class="comment">/* render mode */</span></pre>
<p>The parameter <tt>render_mode</tt> is a set of bit flags
to specify how to render the glyph image.
<tt>FT_RENDER_MODE_NORMAL</tt>, the default, renders an
anti-aliased coverage bitmap with 256 gray levels (also
called a <em>pixmap</em>), as this is the default. You
can alternatively use <tt>FT_RENDER_MODE_MONO</tt> if you
want to generate a 1-bit monochrome bitmap. More values
are available for
the <a href="../reference/ft2-base_interface.html#FT_Render_Mode"><tt>FT_Render_Mode</tt></a>
enumeration value.</p>
<p>Once you have a bitmapped glyph image, you can access it
directly through <tt>glyph->bitmap</tt> (a simple
descriptor for bitmaps or pixmaps), and position it
through <tt>glyph->bitmap_left</tt> and
<tt>glyph->bitmap_top</tt>. For optimal rendering on a
screen the bitmap should be used as an alpha channel in
linear blending with gamma correction.</p>
<p>Note that <tt>bitmap_left</tt> is the horizontal distance
from the current pen position to the leftmost border of
the glyph bitmap, while <tt>bitmap_top</tt> is the
vertical distance from the pen position (on the baseline)
to the topmost border of the glyph bitmap. <em>It is
positive to indicate an upwards distance</em>.</p>
<h4>c. Using Other Charmaps</h4>
<p>As said before, when a new face object is created, it
looks for a Unicode charmap and select it. The currently
selected charmap can be accessed
via <tt>face->charmap</tt>. This field is NULL if no
charmap is selected, which typically happens when you
create a new <tt>FT_Face</tt> object from a font file that
doesn't contain a Unicode charmap (which is rather
infrequent today).</p>
<p>There are two ways to select a different charmap with
FreeType. It's easiest if the encoding you need already
has a corresponding enumeration defined
in <tt>FT_FREETYPE_H</tt>, for
example <tt>FT_ENCODING_BIG5</tt>. In this case, you can
call
<a href="../reference/ft2-base_interface.html#FT_Select_Charmap"><tt>FT_Select_Charmap</tt></a>.</p>
<pre>
error = FT_Select_Charmap(
face, <span class="comment">/* target face object */</span>
FT_ENCODING_BIG5 ); <span class="comment">/* encoding */</span></pre>
<p>Another way is to manually parse the list of charmaps for
the face; this is accessible through the
fields <tt>num_charmaps</tt> and
<tt>charmaps</tt> (notice the ‘s’) of the face
object. As you could expect, the first is the number of
charmaps in the face, while the second is <em>a table of
pointers to the charmaps</em> embedded in the face.</p>
<p>Each charmap has a few visible fields to describe it more
precisely. The most important ones are
<tt>charmap->platform_id</tt>
and <tt>charmap->encoding_id</tt>, defining a pair of
values that describe the charmap in a rather generic way:
Each value pair corresponds to a given encoding. For
example, the pair (3,1) corresponds to Unicode. The list
is defined in the TrueType specification; you can also use
the file <tt>FT_TRUETYPE_IDS_H</tt>, which defines several
helpful constants to deal with them.</p>
<p>To select a specific encoding, you need to find a
corresponding value pair in the specification, then look
for it in the charmaps list. Don't forget that there are
encodings that correspond to several value pairs due to
historical reasons.</p>
<pre>
FT_CharMap found = 0;
FT_CharMap charmap;
int n;
for ( n = 0; n < face->num_charmaps; n++ )
{
charmap = face->charmaps[n];
if ( charmap->platform_id == my_platform_id &&
charmap->encoding_id == my_encoding_id )
{
found = charmap;
break;
}
}
if ( !found ) { ... }
<span class="comment">/* now, select the charmap for the face object */</span>
error = FT_Set_Charmap( face, found );
if ( error ) { ... }</pre>
<p>Once a charmap has been selected, either through
<tt>FT_Select_Charmap</tt>
or <a href="../reference/ft2-base_interface.html#FT_Set_Charmap"><tt>FT_Set_Charmap</tt></a>,
it is used by all subsequent calls
to <tt>FT_Get_Char_Index</tt>.</p>
<h4>d. Glyph Transformations</h4>
<p>It is possible to specify an affine transformation with
<a href="../reference/ft2-base_interface.html#FT_Set_Transform"><tt>FT_Set_Transform</tt></a>,
to be applied to glyph images when they are loaded. Of
course, this only works for scalable (vectorial) font
formats.</p>
<pre>
error = FT_Set_Transform(
face, <span class="comment">/* target face object */</span>
&matrix, <span class="comment">/* pointer to 2x2 matrix */</span>
&delta ); <span class="comment">/* pointer to 2d vector */</span></pre>
<p>This function sets the current transformation for a given
face object. Its second parameter is a pointer to an
<a href="../reference/ft2-basic_types.html#FT_Matrix"><tt>FT_Matrix</tt></a>
structure that describes a 2×2 affine matrix. The
third parameter is a pointer to
an <a href="../reference/ft2-basic_types.html#FT_Vector"><tt>FT_Vector</tt></a>
structure, describing a two-dimensional vector that
translates the glyph image <em>after</em> the 2×2
transformation.</p>
<p>Note that the matrix pointer can be set to NULL, in which
case the identity transformation is used. Coefficients of
the matrix are otherwise in 16.16 fixed-point units.</p>
<p>The vector pointer can also be set to NULL (in which case
a delta of (0,0) is used). The vector coordinates are
expressed in 1/64th of a pixel (also known as 26.6
fixed-point numbers).</p>
<p class="warning">The transformation is applied to every
glyph that is loaded through <tt>FT_Load_Glyph</tt> and
is <em>completely independent of any hinting process</em>.
This means that you won't get the same results if you load
a glyph at the size of 24 pixels, or a glyph at the
size of 12 pixels scaled by 2 through a
transformation, because the hints are computed differently
(except if you have disabled hints).</p>
<p>If you ever need to use a non-orthogonal transformation
with optimal hints, you first have to decompose your
transformation into a scaling part and a rotation/shearing
part. Use the scaling part to compute a new character
pixel size, then the other one to call
<tt>FT_Set_Transform</tt>. This is explained in more
detail in part II of this tutorial.</p>
<p class="warning">Rotation usually disables hinting.</p>
<p>Loading a glyph bitmap with a non-identity transformation
works; the transformation is ignored in this case.</p>
<h3 id="section-7">7. Simple Text Rendering</h3>
<p>We now present a simple example to render a string of
8-bit Latin-1 text, assuming a face that contains a
Unicode charmap.</p>
<p>The idea is to create a loop that loads one glyph image
on each iteration, converts it to a pixmap, draws it on
the target surface, then increments the current pen
position.</p>
<h4 id="basic-code">a. Basic Code</h4>
<p>The following code performs our simple text rendering
with the functions previously described.</p>
<pre>
FT_GlyphSlot slot = face->glyph; <span class="comment">/* a small shortcut */</span>
int pen_x, pen_y, n;
... initialize library ...
... create face object ...
... set character size ...
pen_x = 300;
pen_y = 200;
for ( n = 0; n < num_chars; n++ )
{
FT_UInt glyph_index;
<span class="comment">/* retrieve glyph index from character code */</span>
glyph_index = FT_Get_Char_Index( face, text[n] );
<span class="comment">/* load glyph image into the slot (erase previous one) */</span>
error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT );
if ( error )
continue; <span class="comment">/* ignore errors */</span>
<span class="comment">/* convert to an anti-aliased bitmap */</span>
error = FT_Render_Glyph( face->glyph, FT_RENDER_MODE_NORMAL );
if ( error )
continue;
<span class="comment">/* now, draw to our target surface */</span>
my_draw_bitmap( &slot->bitmap,
pen_x + slot->bitmap_left,
pen_y - slot->bitmap_top );
<span class="comment">/* increment pen position */</span>
pen_x += slot->advance.x >> 6;
pen_y += slot->advance.y >> 6; <span class="comment">/* not useful for now */</span>
}</pre>
<p>This code needs a few explanations.</p>
<ul>
<li>We define a handle named <tt>slot</tt> that points to
the face object's glyph slot. (The
type <a href="../reference/ft2-base_interface.html#FT_GlyphSlot"><tt>FT_GlyphSlot</tt></a>
is a pointer). That is a convenience to avoid using
<tt>face->glyph->XXX</tt> every time.</li>
<li>We increment the pen position with the vector
<tt>slot->advance</tt>, which correspond to the
glyph's <em>advance width</em> (also known as
its <em>escapement</em>). The advance vector is
expressed in 1/64th of pixels, and is truncated to
integer pixels on each iteration.</li>
<li>The function <tt>my_draw_bitmap</tt> is not part of
FreeType but must be provided by the application to draw
the bitmap to the target surface. In this example, it
takes a pointer to
an <a href="../reference/ft2-basic_types.html#FT_Bitmap"><tt>FT_Bitmap</tt></a>
descriptor and the position of its top-left corner as
arguments. For ideal rendering on a screen this
function should perform linear blending with gamma
correction, using the bitmap as an alpha channel.</li>
<li>The value of <tt>slot->bitmap_top</tt> is positive
for an <em>upwards</em> vertical distance. Assuming
that the coordinates taken by <tt>my_draw_bitmap</tt>
use the opposite convention (increasing Y
corresponds to downwards scanlines), we subtract it
from <tt>pen_y</tt>, instead of adding to it.</li>
</ul>
<h4>b.Refined code</h4>
<p>The following code is a refined version of the example
above. It uses features and functions of FreeType that
have not yet been introduced, and which are explained
below.</p>
<pre>
FT_GlyphSlot slot = face->glyph; <span class="comment">/* a small shortcut */</span>
FT_UInt glyph_index;
int pen_x, pen_y, n;
... initialize library ...
... create face object ...
... set character size ...
pen_x = 300;
pen_y = 200;
for ( n = 0; n < num_chars; n++ )
{
<span class="comment">/* load glyph image into the slot (erase previous one) */</span>
error = FT_Load_Char( face, text[n], FT_LOAD_RENDER );
if ( error )
continue; <span class="comment">/* ignore errors */</span>
<span class="comment">/* now, draw to our target surface */</span>
my_draw_bitmap( &slot->bitmap,
pen_x + slot->bitmap_left,
pen_y - slot->bitmap_top );
<span class="comment">/* increment pen position */</span>
pen_x += slot->advance.x >> 6;
}</pre>
<p>We have reduced the size of our code, but it does exactly
the same thing.</p>
<ul>
<li>We use the
function <a href="../reference/ft2-base_interface.html#FT_Load_Char"><tt>FT_Load_Char</tt></a>
instead of <tt>FT_Load_Glyph</tt>. As you probably
imagine, it is equivalent to
calling <tt>FT_Get_Char_Index</tt>, then
<tt>FT_Load_Glyph</tt>.</li>
<li>
<p>We do not use <tt>FT_LOAD_DEFAULT</tt> for the
loading mode, but the bit
flag <tt>FT_LOAD_RENDER</tt>. It indicates that the
glyph image must be immediately converted to an
anti-aliased bitmap. This is of course a shortcut
that avoids calling <tt>FT_Render_Glyph</tt>
explicitly but is strictly equivalent.</p>
<p>Note that you can also specify that you want a
monochrome bitmap instead by using the
additional <tt>FT_LOAD_MONOCHROME</tt> load flag.</p>
</li>
</ul>
<h4 id="transformed-text">c. More Advanced Rendering</h4>
<p>Let us try to render transformed text now (for example
through a rotation). We can do this
using <tt>FT_Set_Transform</tt>.</p>
<pre>
FT_GlyphSlot slot;
FT_Matrix matrix; <span class="comment">/* transformation matrix */</span>
FT_UInt glyph_index;
FT_Vector pen; <span class="comment">/* untransformed origin */</span>
int n;
... initialize library ...
... create face object ...
... set character size ...
slot = face->glyph; <span class="comment">/* a small shortcut */</span>
<span class="comment">/* set up matrix */</span>
matrix.xx = (FT_Fixed)( cos( angle ) * 0x10000L );
matrix.xy = (FT_Fixed)(-sin( angle ) * 0x10000L );
matrix.yx = (FT_Fixed)( sin( angle ) * 0x10000L );
matrix.yy = (FT_Fixed)( cos( angle ) * 0x10000L );
<span class="comment">/* the pen position in 26.6 cartesian space coordinates */</span>
<span class="comment">/* start at (300,200) */</span>
pen.x = 300 * 64;
pen.y = ( my_target_height - 200 ) * 64;
for ( n = 0; n < num_chars; n++ )
{
<span class="comment">/* set transformation */</span>
FT_Set_Transform( face, &matrix, &pen );
<span class="comment">/* load glyph image into the slot (erase previous one) */</span>
error = FT_Load_Char( face, text[n], FT_LOAD_RENDER );
if ( error )
continue; <span class="comment">/* ignore errors */</span>
<span class="comment">/* now, draw to our target surface (convert position) */</span>
my_draw_bitmap( &slot->bitmap,
slot->bitmap_left,
my_target_height - slot->bitmap_top );
<span class="comment">/* increment pen position */</span>
pen.x += slot->advance.x;
pen.y += slot->advance.y;
}</pre>
<p>Some remarks.</p>
<ul>
<li>We now use a vector of type <tt>FT_Vector</tt> to
store the pen position, with coordinates expressed as
1/64th of pixels, hence a multiplication. The position
is expressed in cartesian space.</li>
<li>Glyph images are always loaded, transformed, and
described in the cartesian coordinate system within
FreeType (which means that increasing Y corresponds
to upper scanlines), unlike the system typically used
for bitmaps (where the topmost scanline has
coordinate 0). We must thus convert between the
two systems when we define the pen position, and when we
compute the topleft position of the bitmap.</li>
<li>We set the transformation on each glyph to indicate
the rotation matrix as well as a delta that moves the
transformed image to the current pen position (in
cartesian space, not bitmap space). As a consequence,
the values of <tt>bitmap_left</tt>
and <tt>bitmap_top</tt> correspond to the bitmap origin
in target space pixels. We thus don't
add <tt>pen.x</tt> or <tt>pen.y</tt> to their values
when calling <tt>my_draw_bitmap</tt>.</li>
<li>The advance width is always returned transformed,
which is why it can be directly added to the current pen
position. Note that it is <em>not</em> rounded this
time.</li>
</ul>
<p>A complete source code example can be found
<a href="example1.c">here</a>.</p>
<p>It is important to note that, while this example is a bit
more complex than the previous one, it is strictly
equivalent for the case where the transformation is the
identity. Hence it can be used as a replacement (but a
more powerful one).</p>
<p>The still present few shortcomings will be explained, and
solved, in the next part of this tutorial.</p>
</div>
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<div class="updated">
<p>Last update: 8-Dec-2015</p>
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