FreeType 2.0 Tutorial

Introduction

This short tutorial will teach you how to use the FreeType 2 library in your own applications.

1. Header files

To include the main FreeType header file, simply say

    #include <freetype/freetype.h>

in your application code. Note that other files are available in the FreeType include directory, most of them being included by "freetype.h". They will be described later in this tutorial.

2. Initialize the library

Simply create a variable of type FT_Library named, for example, library, and call the function FT_Init_FreeType() as in

    #include <freetype/freetype.h>

    FT_Library  library;

    ...

    {
      ...
      error = FT_Init_FreeType( &library );
      if ( error )
      {
        ... an error occurred during library initialization ...
      }
    }

This function is in charge of the following:

• Creating a new instance of the FreeType 2 library, and set the handle library to it.

• Load each modules that FreeType knows about in the library. This means that by default, your new library 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 others in the FreeType API. An error code of 0 always means that the operation was successful; otherwise, the value describes the error, and library is set to NULL.

3. Load a font face

a. From a font file

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 simply 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

To know 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.

b. From memory

In the case where you have already loaded the font file in memory, you can similarly create a new face object for it by calling FT_New_Memory_Face as in

    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 as FT_New_Face().

c. From other sources (compressed files, network, etc.)

There are cases where using a file pathname or preloading the file in memory is simply not enough. 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 refer to the FreeType 2 reference manual in order to learn how to use it.

Note that providing a custom stream might also be used to access a TrueType font embedded in a Postscript Type 42 wrapper.

4. Accessing face content

A face object models all information that globally describes the face. Usually, this data can be accessed directly by dereferencing a handle, like

For a complete listing of all face properties and fields, please read the FreeType 2 API Reference.

5. Setting the current pixel size

FreeType 2 uses "size objects" to model all information related to a given character size for a given face. For example, a size object will hold 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.

When the FT_New_Face function is called (or one of its cousins), it automatically creates a new size object for the returned face. This size object is directly accessible as face->size.

NOTA BENE: 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 have decided to simplify the API for the most common use (i.e. one size per face), while keeping this feature available through additional fuctions.

When a new face object is created, its size object defaults to the character size of 10 pixels (both horizontally 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 is an example where the character size is set to 16pt 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 will notice that:

• The character width and heights are specified in 1/64th of points. A point is a physical distance, equaling 1/72th of an inch, it's not a pixel..

• The horizontal and vertical device resolutions are expressed in dots-per-inch, or dpi. You can use 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.

• A value of 0 for the character width means "same as character height", a value of 0 for the character height means "same as character width". Otherwise, it is possible to specify different char widths and heights.

• Using a value of 0 for the horizontal or vertical resolution means 72 dpi, which is the default.

• The first argument is a handle to a face object, not a size object. That's normal, and must be seen as a convenience.

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.

6. Loading a glyph image

a. Converting a character code into a glyph index

Usually, 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, the character code 65 represents the `A' in ASCII encoding.

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 will describe later how to look for specific charmaps in a face. For now, we will 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.

b. Loading a glyph from the face

Once you have a glyph index, you can load the corresponding glyph image. Note that the glyph image can be in several formats. For example, it will be a bitmap for fixed-size formats like FNT, FON, or PCF. It will also be a scalable vector outline for formats like TrueType or Type 1. The glyph image can also be stored in an alternate way that is not known at the time of writing this documentation.

The glyph image is always stored in a special object called a glyph slot. As its name 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 slot 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.

This function will try to load the corresponding glyph image from the face. Basically, this means that:

• If a bitmap is found for the corresponding glyph and pixel size, it will in the slot (embedded bitmaps are always favored over native image formats, because we assume that they are higher-quality versions of the same image. This can be ignored by using the FT_LOAD_NO_BITMAP flag)

• Otherwise, a native image for the glyph will be loaded. It will also be scaled to the current pixel size, as well as hinted for certain formats like TrueType and Type1.

The field glyph->format describe the format used to store 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, as in:

   error = FT_Render_Glyph(
                  face->glyph,      /* glyph slot  */
		  render_mode );    /* render mode */
      

The parameter render_mode is a set of bit flags used to specify how to render the glyph image. Set it to 0 to render a monochrome bitmap, or to ft_render_mode_antialias to generate a high-quality (256 gray levels) anti-aliased bitmap from the glyph image.

Once you have a bitmap glyph image, you can access it directly through glyph->bitmap (a simple bitmap descriptor), and position it through glyph->bitmap_left and glyph->bitmap_top.

Note that bitmap_left is the horizontal distance from the current pen position to the left-most border of the glyph bitmap, while bitmap_top is the vertical distance from the pen position (on the baseline) to the top-most border of the glyph bitmap. It is positive to indicate an upwards distance.

The next section will detail the content of a glyph slot and how to access specific glyph information (including metrics).

c. Using other charmaps

As said before, when a new face object is created, it will look for a Unicode, Latin-1, or ASCII charmap and select it. The currently selected charmap is accessed via face->charmap. This field is NULL when no charmap is selected, which typically happens when you create a new FT_Face object from a font file that doesn't contain an ASCII, Latin-1, or Unicode charmap (rare stuff).

There are two ways to select a different charmap with FreeType 2. The easiest is when the encoding you need already has a corresponding enumeration defined in <freetype/freetype.h>, as ft_encoding_big5. In this case, you can simply call FT_Select_CharMap as in:

    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 used to describe it more precisely. Mainly, one will look at charmap->platform_id and charmap->encoding_id that define a pair of values that can be used to 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. Their list is defined in the TrueType specification but you can also use the file <freetype/ftnameid.h> which defines several helpful constants to deal with them..

To look for 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 some encoding correspond to several values pair (yes it's a real mess, but blame Apple and Microsoft on such stupidity..). Here's some code to do it:

    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().

d. Glyph Transforms:

It is possible to specify an affine transformation to be applied to glyph images when they're loaded. Of course, this will only work for scalable (vectorial) font formats.

To do that, simply call FT_Set_Transform, as in:

   error = FT_Set_Transform(
                    face,           /* target face object    */
		    &matrix,    /* pointer to 2x2 matrix */
		    &delta );   /* pointer to 2d vector  */
     

This function will set the current transform for a given face object. Its second parameter is a pointer to a simple FT_Matrix structure that describes a 2x2 affine matrix. The third parameter is a pointer to a FT_Vector structure that describe a simple 2d vector.

Note that the matrix pointer can be set to NULL, (in which case the identity transform will be used). Coefficients of the matrix are in 16.16 fixed float units.

The vector pointer can also be set to NULL (in which case a delta of (0,0) will be used). The vector coordinates are expressed in 1/64th of a pixel (also known as 26.6 fixed floats).

NOTA BENE: The transform is applied every glyph that is loaded through FT_Load_Glyph. Note that loading a glyph bitmap with a non-trivial transform will produce an error..

7. Accessing glyph image data

Glyph image data is accessible through face->glyph. See the definition of the FT_GlyphSlot type for more details. As stated previously, each face has a single glyph slot, where one glyph image at a time can be loaded. Each time you call FT_Load_Glyph(), you erase the content of the glyph slot with a new glyph image.

Note however that the glyph slot object itself doesn't change, only its content, which means that you can perfectly create a "shortcut" to access it as in

    {
      /* shortcut to glyph slot */
      FT_GlyphSlot  glyph = face->glyph;

      for ( n = 0; n < face->num_glyphs; n++ )
      {
        ... load glyph n ...
        ... access glyph data as glyph->xxxx
      }
    }

The glyph variable will be valid until its parent face is destroyed. Here are a few important fields of the glyph slot:

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 most distances are expressed in 1/64th of pixels! See the API reference or the user guide for a description of the FT_Glyph_Metrics structure.
glyph->bitmap	If 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.

8. Rendering glyph outlines into bitmaps

You can easily test the format of the glyph image by inspecting the face->glyph->format variable. If its value is ft_glyph_format_bitmap, the glyph image that was loaded is a bitmap that can be directly blit to your own surfaces through your favorite graphics library (FreeType 2 doesn't provide bitmap blitting routines, as you may imagine :-)

If the format is ft_glyph_format_outline or something else, the library provides a means to convert such glyph images to bitmaps through what are called rasters.

On the other hand, if 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 will 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;
        /* render into a mono bitmap */
        bit.pixel_mode = ft_pixel_mode_mono;
        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 is 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;
        /* 8-bit gray-level bitmap */
        bit.pixel_mode = ft_pixel_mode_gray;
        /* MUST be 128 for now     */
        bit.grays      = 128;
        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 will notice that

• As previously, FT_Get_Glyph_Bitmap() doesn't generate the bitmap, it simply renders to it.

• The target bitmap must be cleaned before calling the function. This is a limitation of our current anti-aliasing algorithm and is EXTREMELY important.

• The anti-aliaser uses 128 levels of grays exclusively for now (this will probably change in a near future). This means that you must set bit.grays to 128. The generated image uses values from 0 (back color) to 127 (foreground color).

• It is not possible to render directly an anti-aliased outline into a pre-existing gray-level bitmap, or even any colored-format one (like RGB16 or paletted 8-bits). We will not discuss this issue in great details here, but the reason is that we do not want to deal with graphics composition (or alpha-blending) within FreeType.