FreeType 2.0 Tutorial
Step 1 - simple glyph loading

Introduction

This is the first section of the FreeType 2 tutorial. It will teach you to do the following:

• initialise the library
• open a font file by creating a new face object
• select a character size in points or in pixels
• load a single glyph image and convert it to a bitmap
• render a very simple string of text
• render a rotated string of text easily

1. Header files

The following are instructions required to compile an application that uses the FreeType 2 library. Please read them carefully, we've changed a few things since the last release:

1. locate the FreeType 2 "include" directory
   and add it to your compilation include path.

   Note that on Unix systems, you can now run the "freetype-config" script with the "--cflags" 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.

---

2. include the file named "ft2build.h"
   it contains various macro declarations that are later used to #include the appropriate public FT2 header files.

---

3. 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
       

   As you can see, FT_FREETYPE_H is a special macro defined in the file <ftbuild.h>. The latter contains some installation-specific macros used to name other public header files of the FreeType 2 API.

   You can read this this section of the FreeType 2 API Reference for a complete listing of macros.

Note that the use of macros in #include statements is ANSI-compliant. It is now used for several reasons:

• It avoids some painful conflicts with the FreeType 1.x public header files.

• The macro names are not limited to the DOS 8.3 file naming limit, hence, FT_MULTIPLE_MASTERS_H or FT_SFNT_NAMES_H are a lot more readable and explanatory than the equivalent ftmm.h and ftsnames.h, to give a small example.

• It also special installation tricks that will not be discussed here.

NOTA BENE: The old header file inclusion scheme is still supported in order to limit the number of applications that this new release of FT2 will break. This means that you can still do something like the following (provided you use freetype-config to determine the correct include path):

	   #include <freetype/freetype.h>
	   #include <freetype/ftglyph.h>
	   ..etc..

However, we will not support this inclusion scheme in our next release, a.k.a FreeType 2.1. We thus strongly encourage you to change your applications' sources now to match the new inclusion scheme to avoid painful compilation breakages in the future.

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 <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 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 in "face−>num_glyphs".

The complete list of available fields in in the FT_FaceRec structure description. However, we'll describe here a few of them in more details:

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 no charmap was 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 is commonly displayed 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. 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 Type 1 use vectorial shapes, named "outlines" to describe each glyph. Some formats may have even more exotic ways of representing glyph (e.g. MetaFont). 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 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. Its fields are documented by the FT_GlyphSlotRec structure documentation.

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 be loaded into the slot (embedded bitmaps are always favored over native image formats, because we assume that they are higher-quality versions of the same glyph. 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 face−>glyph−>format describes 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, or the equivalent ft_render_mode_normal to render a high-quality anti-aliased (256 gray levels) bitmap, as this is the default. You can alternatively use ft_render_mode_mono if you want to generate a 1-bit monochrome bitmap.

Once you have a bitmapped 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 FT_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 FT_TRUETYPE_IDS_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 that is used to translate the glyph image after the 2x2 transform.

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

If you ever need to use a non-orthogonal transform with optimal hints, you first need to decompose your transform 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 details in a later section of this tutorial.

Note also that loading a glyph bitmap with a non-identity transform will produce an error..

7. Simple Text Rendering:

We will now present you with a very simple example used 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 will, on each iteration, load one glyph image, convert it to an anti-aliased bitmap, draw it on the target surface, then increment the current pen position

a. basic code :

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;

       .. initialise 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;   // unuseful for now..
       }
    

This code needs a few explanations:

• we define a handle named slot that points to the face object's glyph slot. (the type FT_GlyphSlot is a pointer). That's a convenience to avoid using face->glyph->XXX every time.

• we increment the pen position with the vector slot->advance, which correspond to the glyph's advance width (also known as its escapement). The advance vector is expressed in 64/th of pixels, and is truncated to integer pixels on each iteration.

• The function 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 a FT_Bitmap descriptor and the position of its top-left corner as arguments.

• The value of 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 substract it to pen_y, instead of adding it..

b. refined code:

The following code is a refined version of the example above. It uses features and functions of FreeType 2 that have not yet been introduced, and they'll be explained below:

       FT_GlyphSlot  slot = face->glyph;  // a small shortcut
       FT_UInt       glyph_index;
       int           pen_x, pen_y, n;

       .. initialise 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've reduced the size of our code, but it does exactly the same thing, as:

• We use the function FT_Load_Char instead of FT_Load_Glyph. As you probably imagine, it's equivalent to calling FT_Get_Char_Index then FT_Get_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 explicitely but is strictly equivalent.

  Note that you can also specify that you want a monochrome bitmap instead by using the addition FT_LOAD_MONOCHROME load flag.

c. more advanced rendering:

Let's try to render transformed text now (for example through a rotation). We can do this using FT_Set_Transform. Here's how to do it:

       FT_GlyphSlot  slot = face->glyph;  // a small shortcut
       FT_Matrix     matrix;              // transformation matrix
       FT_UInt       glyph_index;
       FT_Vector     pen;                 // untransformed origin
       int           pen_x, pen_y, n;

       .. initialise library ..
       .. create face object ..
       .. set character size ..

       // set up matrix
       matrix.xx = (FT_Fixed)( cos(angle)*0x10000);
       matrix.xy = (FT_Fixed)(-sin(angle)*0x10000);
       matrix.yx = (FT_Fixed)( sin(angle)*0x10000);
       matrix.yy = (FT_Fixed)( cos(angle)*0x10000);
              
       // the pen position in 26.6 cartesian space coordinates
       pen.x = 300 * 64;
       pen.y = ( my_target_height - 200 ) * 64;
       
       for ( n = 0; n < num_chars; n++ )
       {
         // set transform
         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;
       }
    

You'll notice that:

• we now use a vector, of type 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.

• glyph images are always loaded, transformed and described in the cartesian coordinate system in FreeType (which means that increasing Y corresponds to upper scanlines), unlike the system typically used for bitmaps (where the top-most scanline has coordinate 0). We must thus convert between the two systems when we define the pen position, and when we compute the top-left position of the bitmap.

• we set the transform on each glyph, to indicate the rotation matrix, as well as a delta that will move the transformed image to the current pen position (in cartesian space, not bitmap space).

  as a consequence, the values of 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 valus when calling my_draw_bitmap.

• the advance is always returned transformed, which is why it can be directly added to the current pen position. Note that it is not rounded this time.

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 transform is the identity.. Hence it can be used as a replacement (but a more powerful one).

It has however a few short comings that we will explain, and solve, in the next part of this tutorial.

Conclusion

In this first section, you have learned the basics of FreeType 2, as well as sufficient knowledge to know how to render rotated text. Woww ! Congratulations..

The next section will dive into more details of the API in order to let you access glyph metrics and images directly, as well as how to deal with scaling, hinting, kerning, etc..

The third section will discuss issues like modules, caching and a few other advanced topics like how to use multiple size objects with a single face.