#include <freetype.h> FT_Library library; .... { .. error = FT_Init_FreeType( &library ); if (error) { .. an error occured during library initialisation .. } }
This function is in charge of the following:
As you can see, the function returns an error code, like most others in the FreeType API. An error code of 0 always means that the operation was succesful; otherwise, the value describes the error, and library is set to NULL.
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, read or simply that it is broken..
}
As you certainly imagine, FT_New_Face opens a font file then tries to extract one face from it. Its parameters are :
library | handle to the FreeType library instance where the face object is created |
filepathname | the font file pathname (standard C string). |
face_index | 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 will be returned if its value is too large. Index 0 always work though. |
face | A pointer to the handle that will be set to
describe the new face object. It is set to NULL in case of error. |
To known how many faces a given font file contains, simply load its first face (use face_index=0), then see the value of face->num_faces which indicates how many faces are embedded in the font file.
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 simply 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 than FT_New_Face.
Basically, one has to set up a FT_Stream object, according to the rules defined in the document named FreeType 2 System Interface, then pass it to the function FT_Open_Face as in:
FT_Library library; /* handle to library */
FT_Face face; /* handle to face object */
error = FT_Init_FreeType( &library );
if (error) { ..... }
... set up stream object, with handle "stream" ...
error = FT_Open_Face( library,
stream, /* handle to stream objects */
0, /* face_index */
&face );
if (error) { ... }
custom implementations of FT_Stream are great to provide advanced features like automatic support of compressed files, network transparency, using UTF-16 file pathnames, etc..
face->num_glyphs | gives the number of glyphs available in the font face. A glyph is simply a character image. It doesn't necessarily correspond to a character code though. |
face->flags | a 32-bit integer containing bit flags used to describe some face properties. For example, the flag FT_FACE_FLAG_SCALABLE is used to indicate 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 |
face->units_per_EM | 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. |
face->num_fixed_sizes | this field gives the number of embedded bitmap strikes in the current face. A strike is simply 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 embedded bitmap strikes ! |
face->fixed_sizes | this is a pointer to an array of FT_Bitmap_Size element. each FT_Bitmap_Size indicates the horizontal and vertical pixel sizes for each of the strikes that are present in the face. |
For a complete listing of all face properties and fields, please read the FreeType 2 API Reference.
When a new face object is created, its size object defaults to the character size of 10 pixels (both horizontall and vertically) for scalable formats. For fixed-sizes formats, the size is more or less undefined, which is why you must set it before trying to load a glyph.
To do that, simply call FT_Set_Char_Size. Here's an example where the character size is set to 16 pts for a 300x300 dpi 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 */
You'll notice that:
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 simply call FT_Set_Pixel_Sizes, as in:
error = FT_Set_Pixel_Sizes( face, /* handle to face object */
0, /* pixel_width */
16 ); /* pixel_height */
This example will set the character pixel sizes to 16x16 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.
A face object contains one or more tables, called charmaps, that are used to convert character codes to glyph indices. For example, most TrueType fonts contain two charmaps. One is used to convert Unicode character codes to glyph indices, the other is used to convert Apple Roman encoding into glyph indices. Such fonts can then be used either on Windows (which uses Unicode) and Macintosh (which uses Apple Roman, bwerk..). 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, it lists all the charmaps contained in the font face and selects the one that supports Unicode character codes if it finds one. Otherwise, it tries to find support for Latin-1, then ASCII.
We'll describe later how to look for specific charmaps in a face. For now, we'll assume that the face contains at least a Unicode charmap that was selected during FT_New_Face. To convert a Unicode character code to a font glyph index, we use FT_Get_Char_Index as in:
glyph_index = FT_Get_Char_Index( face, charcode );
This will look the glyph index corresponding to the given charcode in the charmap that is currently selected for the face. If charmap is selected, the function simply 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, the value 0 is returned. By convention, it always correspond to a special glyph image called the missing glyph, which usually is represented as a box or a space.
The glyph image is always stored in a special object called a glyph slot. As it names suggests, a glyph slot is simply 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 object that can be accessed as face->glyph.
Loading a glyph image into the slot is performed by calling FT_Load_Glyph as in:
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 used to indicate some special operations. The default value FT_LOAD_DEFAULT is 0. The function performs the following :
There are a few others FT_LOAD_xxx flags defined. For more details see the FreeType 2 API Reference.
The field face->num_charmaps and face->charmaps (notice the 's') can be used by client applications to look at what charmaps are available in a given face.
face-charmaps is an array of pointers to the face->num_charmaps charmaps contained in the font face.
Each charmap has a few visible fields used to describe it in more details. For example, charmap->encoding is an enumeration type that describes the charmap with FreeType codes. One can also look at charmap->platform_id and charmap->encoding_id for more exotic needs.
Here's an example code that looks for a chinese Big5 charmap then selects it via FT_Set_CharMap:
FT_CharMap found = 0;
FT_CharMap charmap;
int n;
for ( n = 0; n < face->num_charmaps; n++ )
{
charmap = face>charmaps[n];
if (charmap->encoding == ft_encoding_big5)
{
found = charmap;
break;
}
}
if (!found) { ... }
/* now, select the charmap for the face object */
error = FT_Set_CharMap( face, found );
if (error) { .... }
One might now call FT_Get_Char_Index with Big5 character codes to retrieve glyph indices.
{
FT_GlyphSlot glyph;
.... load glyph ...
glyph = face->glyph; /* shortcut to glyph data */
.... access glyph data as glyph->xxxx
}
For example, one can access the following fields:
glyph->format | Indicates the type of the loaded glyph image. Can be either ft_glyph_format_bitmap, ft_glyph_format_outline or other values. |
glyph->metrics | A simple structure used to hold the glyph image's metrics. Note that all distances are expressed in 1/64th of pixels ! See the API reference or User Guide for a description of the FT_Glyph_Metrics structure. |
glyph->bitmap | When the glyph slot contains a bitmap, a simple FT_Bitmap that describes it. See the API reference or user guide for a description of the FT_Bitmap structure. |
glyph->outline | When the glyph slot contains a scalable outline, this structure describes it. See the definition of the FT_Outline structure. |
On the other hand, when the image is a scalable outline, or something else, FreeType provides a function to convert the glyph image into a pre-existing bitmap that you'll handle to it, named FT_Get_Glyph_Bitmap. Here's a simple example code that renders an outline into a monochrome bitmap :
{
FT_GlyphSlot glyph;
.... load glyph ...
glyph = face->glyph; /* shortcut to glyph data */
if (glyph->format == ft_glyph_format_outline )
{
FT_Bitmap bit;
/* set-up a bitmap descriptor for our target bitmap */
bit.rows = bitmap_height;
bit.width = bitmap_width;
bit.pitch = bitmap_row_bytes;
bit.pixel_mode = ft_pixel_mode_mono; /* render into a mono bitmap */
bit.buffer = bitmap_buffer;
/* render the outline directly into the bitmap */
error = FT_Get_Glyph_Bitmap( face, &bit );
if (error) { ... }
}
}
You should note that FT_Get_Glyph_Bitmap doesn't create the bitmap. It only needs a descriptor, of type FT_Bitmap, and writes directly into it.
Note that the FreeType scan-converter for outlines can also generate anti-aliased glyph bitmaps with 128 level of grays. For now, it is restricted to rendering to 8-bit gray-level bitmaps, though this may change in the future. Here's some code to do just that:
{
FT_GlyphSlot glyph;
.... load glyph ...
glyph = face->glyph; /* shortcut to glyph data */
if (glyph->format == ft_glyph_format_outline )
{
FT_Bitmap bit;
/* set-up a bitmap descriptor for our target bitmap */
bit.rows = bitmap_height;
bit.width = bitmap_width;
bit.pitch = bitmap_row_bytes;
bit.pixel_mode = ft_pixel_mode_gray; /* 8-bit gray-level bitmap */
bit.grays = 128; /* MUST be 128 for now */
bit.buffer = bitmap_buffer;
/* clean the bitmap - IMPORTANT */
memset( bit.buffer, 0, bit.rows*bit.pitch );
/* render the outline directly into the bitmap */
error = FT_Get_Glyph_Bitmap( face, &bit );
if (error) { ... }
}
}
You'll notice that :