The following are instructions required to compile an application that uses the FreeType 2 library.
Locate the FreeType 2 include
directory.
You have to add it to your compilation include path.
In Unix-like environments you can use the
pkg-config program to retrieve the
appropriate compilation flags; say.
pkg-config --cflags freetype2
to get the compilation flags.
This program 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.
Another solution is the freetype-config
script. However, its use is deprecated since it can't
be used reliably for cross compilation.
Include the file named ft2build.h.
It contains various macro declarations that are later
used to #include the appropriate public
FreeType 2 header files.
Include the main FreeType 2 API header file.
You should do that using the
macro FT_FREETYPE_H, like in the
following example.
#include <ft2build.h> #include FT_FREETYPE_H
FT_FREETYPE_H is a special macro defined
in file ftheader.h. It contains some
installation-specific macros to name other public
header files of the FreeType 2 API.
You can read this section of the FreeType 2 API Reference for a complete listing of the header macros.
The use of macros in #include statements is
ANSI-compliant. It is used for several reasons.
FT_MULTIPLE_MASTERS_H
or FT_SFNT_NAMES_H are a lot more readable
and explanatory than the real file names
ftmm.h and ftsnames.h.To initialize the FreeType library, create a variable of
type FT_Library
named, for example, library, and call the
function FT_Init_FreeType.
#include <ft2build.h>
#include FT_FREETYPE_H
FT_Library library;
...
error = FT_Init_FreeType( &library );
if ( error )
{
... an error occurred during library initialization ...
}
This function is in charge of
library to it,
andlibrary
object is able to handle TrueType, Type 1,
CID-keyed & OpenType/CFF fonts gracefully.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 FT_Err_Ok) always means that the
operation was successful; otherwise, the value describes
the error, and library is set to NULL.
A list of all FreeType error codes can be found in
file fterrdef.h.
Create a new face object by
calling FT_New_Face.
A face describes a given typeface and style. For
example, ‘Times New Roman Regular’ and
‘Times New Roman Italic’ correspond to two
different faces.
FT_Library library; /* handle to library */ FT_Face face; /* handle to face object */ 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... }
As you can certainly imagine, FT_New_Face
opens a font file, then tries to extract one face from it.
Its parameters are as follows.
Certain font formats allow several font faces to be embedded in a single file.
This index tells which face you want to load. An error is returned if its value is too large.
Index 0 always works, though.
A pointer to the handle that is set to describe the new face object.
It is set to NULL in case of error.
To know how many faces a given font file contains, set
face_index to -1, then
check the value of face->num_faces, which
indicates how many faces are embedded in the font
file.
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 FT_New_Memory_Face.
FT_Library library; /* handle to library */ FT_Face face; /* handle to face object */ error = FT_Init_FreeType( &library ); if ( error ) { ... } error = FT_New_Memory_Face( library, buffer, /* first byte in memory */ size, /* size in bytes */ 0, /* face_index */ &face ); if ( error ) { ... }
As you can see, FT_New_Memory_Face 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
FT_New_Face.
Note that you must not deallocate the font file buffer
before calling
FT_Done_Face.
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.
This is done through
the FT_Open_Face
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 FreeType 2
reference manual in order to learn how to use it.
A face object models all information that
globally describes the face. Usually, this data can be
accessed directly by dereferencing a handle, like
in face−>num_glyphs.
The complete list of available fields is in
the FT_FaceRec
structure description. However, we describe here a few of
them in more detail.
FT_FACE_FLAG_SCALABLE 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 FreeType 2
API Reference.A pointer to an array
of FT_Bitmap_Size
elements. Each FT_Bitmap_Size indicates
the horizontal and vertical character pixel
sizes for each of the strikes that are present in
the face.
Note that, generally speaking, these are not the cell size of the bitmap strikes.
FreeType 2 uses size objects 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 (however, those values are rounded to integers, i.e., multiples of 64).
When the FT_New_Face function is called (or
one of its siblings), it automatically creates a
new size object for the returned face. This size object
is directly accessible as
face−>size.
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 make this feature available through additional functions.
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 FT_Set_Char_Size.
Here is an example, setting the character size to 16pt for
a 300×300dpi device:
error = FT_Set_Char_Size(
face, /* handle to face object */
0, /* char_width in 1/64th of points */
16*64, /* char_height in 1/64th of points */
300, /* horizontal device resolution */
300 ); /* vertical device resolution */
Some notes.
This function computes the (possibly fractional) character pixel size that corresponds to the character width and height and device resolutions. A common acronym for the pixel size is ppem (pixel per em).
If you want to specify the (integer) pixel sizes
yourself, you can call
FT_Set_Pixel_Sizes.
error = FT_Set_Pixel_Sizes(
face, /* handle to face object */
0, /* pixel_width */
16 ); /* pixel_height */
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’.
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 face->fixed_sizes
array.
Be aware that fractional ppem values are not always supported. For example, the native bytecode engine for hinting TrueType fonts (TTFs) only supports integer ppem values, and FreeType rounds fractional ppem values accordingly.
Normally, an application wants to load a glyph image based on its character code, which is a unique value that defines the character for a given encoding. For example, code 65 (0x41) represents character ‘A’ in ASCII encoding.
A face object contains one or more tables, called charmaps, 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.
By default, when a new face object is created, FreeType tries to select a Unicode charmap. It emulates 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.
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 FT_New_Face. To convert a
Unicode character code to a font glyph index, we use
FT_Get_Char_Index.
glyph_index = FT_Get_Char_Index( face, charcode );
This code line looks up the glyph index corresponding to
the given charcode 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 charcode.
If no charmap was selected, the function returns the charcode.
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 missing glyph, which is commonly displayed as a box or a space.
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 (outlines) 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.
The glyph image is always stored in a special object called a
glyph slot. 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 face->glyph. Its fields are
explained by
the FT_GlyphSlotRec
structure documentation.
Loading a glyph image into the slot is performed by
calling FT_Load_Glyph.
error = FT_Load_Glyph(
face, /* handle to face object */
glyph_index, /* glyph index */
load_flags ); /* load flags, see below */
The load_flags value is a set of bit flags
to indicate some special operations. The default value
FT_LOAD_DEFAULT is 0.
This function tries to load the corresponding glyph image from the face.
FT_LOAD_NO_BITMAP flag.The
field face−>glyph−>format
describes the format used for storing the glyph image in
the slot. If it is
not FT_GLYPH_FORMAT_BITMAP, one can
immediately convert it to a bitmap
through FT_Render_Glyph.
error = FT_Render_Glyph( face->glyph, /* glyph slot */ render_mode ); /* render mode */
The parameter render_mode is a set of bit
flags to specify how to render the glyph image.
FT_RENDER_MODE_NORMAL, the default, renders
an anti-aliased coverage bitmap with 256 gray levels (also
called a pixmap), as this is the default. You
can alternatively use FT_RENDER_MODE_MONO if
you want to generate a 1-bit monochrome bitmap. More
values are available for
the FT_Render_Mode
enumeration value.
Once you have a bitmapped glyph image, you can access it
directly through glyph->bitmap (a simple
descriptor for bitmaps or pixmaps), and position it
through glyph->bitmap_left and
glyph->bitmap_top. For optimal rendering
on a screen the bitmap should be used as an alpha channel
in linear blending with gamma correction.
Note that bitmap_left is the horizontal
distance from the current pen position to the leftmost
border of the glyph bitmap, while bitmap_top
is the vertical distance from the pen position (on the
baseline) to the topmost border of the glyph
bitmap. It is positive to indicate an upwards
distance.
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 face->charmap. This field is NULL if
no charmap is selected, which typically happens when you
create a new FT_Face object from a font file
that doesn't contain a Unicode charmap (which is rather
infrequent today).
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 FT_FREETYPE_H, for
example FT_ENCODING_BIG5. In this case, you
can call
FT_Select_Charmap.
error = FT_Select_Charmap(
face, /* target face object */
FT_ENCODING_BIG5 ); /* encoding */
Another way is to manually parse the list of charmaps for
the face; this is accessible through the
fields num_charmaps and
charmaps (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 a table
of pointers to the charmaps embedded in the face.
Each charmap has a few visible fields to describe it more
precisely. The most important ones are
charmap->platform_id
and charmap->encoding_id, 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 OpenType specification; you can also use
the file FT_TRUETYPE_IDS_H, which defines
several helpful constants to deal with them. Note that we
use the OpenType enumeration values for non-OpenType fonts
also (by defining additional constants where
necessary).
To select a specific encoding, you need to find a corresponding value pair in the specification, then look for it in the charmaps list.
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 ) { ... }
/* now, select the charmap for the face object */
error = FT_Set_Charmap( face, found );
if ( error ) { ... }
Once a charmap has been selected, either through
FT_Select_Charmap
or FT_Set_Charmap,
it is used by all subsequent calls
to FT_Get_Char_Index.
It is possible to specify an affine transformation with
FT_Set_Transform,
to be applied to glyph images when they are loaded. Of
course, this only works for scalable (vectorial) font
formats.
error = FT_Set_Transform(
face, /* target face object */
&matrix, /* pointer to 2x2 matrix */
&delta ); /* pointer to 2d vector */
This function sets the current transformation for a given
face object. Its second parameter is a pointer to an
FT_Matrix
structure that describes a 2×2 affine matrix. The
third parameter is a pointer to
an FT_Vector
structure, describing a two-dimensional vector that
translates the glyph image after the 2×2
transformation.
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.
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).
The transformation is applied to every
glyph that is loaded through FT_Load_Glyph
and is completely independent of any hinting
process. 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).
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
FT_Set_Transform. This is explained in more
detail in part II of this tutorial.
Rotation usually disables hinting.
Loading a glyph bitmap with a non-identity transformation works; the transformation is ignored in this case.
We now present a simple example to render a string of 8-bit Latin-1 text, assuming a face that contains a Unicode charmap.
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.
The following code performs our simple text rendering with the functions previously described.
FT_GlyphSlot slot = face->glyph; /* a small shortcut */ 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; /* retrieve glyph index from character code */ glyph_index = FT_Get_Char_Index( face, text[n] ); /* load glyph image into the slot (erase previous one) */ error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT ); if ( error ) continue; /* ignore errors */ /* convert to an anti-aliased bitmap */ error = FT_Render_Glyph( face->glyph, FT_RENDER_MODE_NORMAL ); if ( error ) continue; /* now, draw to our target surface */ my_draw_bitmap( &slot->bitmap, pen_x + slot->bitmap_left, pen_y - slot->bitmap_top ); /* increment pen position */ pen_x += slot->advance.x >> 6; pen_y += slot->advance.y >> 6; /* not useful for now */ }
This code needs a few explanations.
slot that points
to the face object's glyph slot. (The
type FT_GlyphSlot
is a pointer). That is a convenience to avoid using
face->glyph->XXX every time.slot->advance, which correspond to the
glyph's advance width (also known as
its escapement). The advance vector is
expressed in 1/64th of pixels, and is truncated to
integer pixels on each iteration.my_draw_bitmap 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 FT_Bitmap
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.slot->bitmap_top is
positive for an upwards vertical distance.
Assuming that the coordinates taken
by my_draw_bitmap use the opposite
convention (increasing Y corresponds to downwards
scanlines), we subtract it from pen_y,
instead of adding to it.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.
FT_GlyphSlot slot = face->glyph; /* a small shortcut */ 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++ ) { /* load glyph image into the slot (erase previous one) */ error = FT_Load_Char( face, text[n], FT_LOAD_RENDER ); if ( error ) continue; /* ignore errors */ /* now, draw to our target surface */ my_draw_bitmap( &slot->bitmap, pen_x + slot->bitmap_left, pen_y - slot->bitmap_top ); /* increment pen position */ pen_x += slot->advance.x >> 6; }
We have reduced the size of our code, but it does exactly the same thing.
FT_Load_Char
instead of FT_Load_Glyph. As you probably
imagine, it is equivalent to
calling FT_Get_Char_Index, then
FT_Load_Glyph.We do not use FT_LOAD_DEFAULT for the
loading mode, but the bit
flag FT_LOAD_RENDER. It indicates that
the glyph image must be immediately converted to an
anti-aliased bitmap. This is of course a shortcut
that avoids calling FT_Render_Glyph
explicitly but is strictly equivalent.
Note that you can also specify that you want a
monochrome bitmap instead by using the
additional FT_LOAD_MONOCHROME load
flag.
Let us try to render transformed text now (for example
through a rotation). We can do this
using FT_Set_Transform.
FT_GlyphSlot slot; FT_Matrix matrix; /* transformation matrix */ FT_UInt glyph_index; FT_Vector pen; /* untransformed origin */ int n; ... initialize library ... ... create face object ... ... set character size ... slot = face->glyph; /* a small shortcut */ /* set up matrix */ 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 ); /* the pen position in 26.6 cartesian space coordinates */ /* start at (300,200) */ pen.x = 300 * 64; pen.y = ( my_target_height - 200 ) * 64; for ( n = 0; n < num_chars; n++ ) { /* set transformation */ FT_Set_Transform( face, &matrix, &pen ); /* load glyph image into the slot (erase previous one) */ error = FT_Load_Char( face, text[n], FT_LOAD_RENDER ); if ( error ) continue; /* ignore errors */ /* now, draw to our target surface (convert position) */ my_draw_bitmap( &slot->bitmap, slot->bitmap_left, my_target_height - slot->bitmap_top ); /* increment pen position */ pen.x += slot->advance.x; pen.y += slot->advance.y; }
Some remarks.
FT_Vector to
store the pen position, with coordinates expressed as
1/64th of pixels, hence a multiplication. The position
is expressed in cartesian space.bitmap_left
and bitmap_top correspond to the bitmap
origin in target space pixels. We thus don't
add pen.x or pen.y to their
values when calling my_draw_bitmap.A complete source code example can be found here.
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).
The still present few shortcomings will be explained, and solved, in the next part of this tutorial.
Last update: 13-Feb-2018