FreeType 2 Tutorial

Copyright …

• I. Simple Glyph Loading
  • 1. Header Files
  • 2. Library Initialization
  • 3. Loading a Font Face
    • a. From a Font File
    • b. From Memory
    • c. From Other Sources (Compressed Files, Network, etc.)
  • 4. Accessing the Face Data
  • 5. Setting the Current Pixel Size
  • 6. Loading a Glyph Image
    • a. Converting a Character Code Into a Glyph Index
    • b. Loading a Glyph From the Face
    • c. Using Other Charmaps
    • d. Glyph Transformations
  • 7. Simple Text Rendering
    • a. Basic Code
    • b.Refined code
    • c. More Advanced Rendering
• II. Managing Glyphs
  • 1. Glyph Metrics
  • 2. Managing Glyph Images
    • a.Extracting the Glyph Image
    • b. Transforming & Copying the Glyph Image
    • c. Measuring the Glyph Image
    • d. Converting the Glyph Image to a Bitmap
  • 3. Global Glyph Metrics
    • a. Design global metrics
    • b. Scaled Global Metrics
    • c. Kerning
  • 4. Simple Text Rendering: Kerning and Centering
    • a. Kerning Support
    • b. Centering
  • 5. Advanced Text Rendering: Transformation and Centering and Kerning
    • a. Packing and Translating Glyphs
    • b. Rendering a Transformed Glyph Sequence
  • 6. Accessing Metrics in Design Font Units, and Scaling Them
    • a. Scaling Distances to Device Space
    • b. Accessing Design Metrics (Glyph & Global)
  • Conclusion
• III. Examples

I. Simple Glyph Loading

1. Header Files

The following are instructions required to compile an application that uses the FreeType 2 library.

1. Locate the FreeType 2 include directory.

   You have to add it to your compilation include path.

   In Unix-like environments you can 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 FreeType 2 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
   

   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.

• It avoids conflicts with (deprecated) FreeType 1.x public header files.
• The macro names are not limited to the DOS 8.3 file naming limit; names like 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.
• It allows special installation tricks that will not be discussed here.

2. Library Initialization

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

• creating a new instance of the FreeType 2 library and setting the handle library to it, and
• loading each module that FreeType knows about in the library. Among others, 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 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.

3. Loading 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 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.

To know how many faces a given font file contains, load its first face (this is, face_index should be set to zero), then check 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 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 memory before calling FT_Done_Face.

c. From Other Sources (Compressed Files, Network, etc.)

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.

4. Accessing the Face Data

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.

Note that, generally speaking, these are not the cell size of the bitmap strikes.

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 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.

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 simplify the API for the most common use (i.e., one size per face) while keeping 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.

• The character widths and heights are specified in 1/64th of points. A point is a physical distance, equaling 1/72th of an inch. Normally, it is not equivalent to a pixel.
• 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.
• The horizontal and vertical device resolutions are expressed in dots-per-inch, or dpi. 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.
• 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.
• The first argument is a handle to a face object, not a size object.

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 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.

6. Loading a Glyph Image

a. Converting a Character Code Into a Glyph Index

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, 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.

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.

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 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.

• 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 FT_LOAD_NO_BITMAP flag.
• 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.

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.

c. Using Other Charmaps

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 TrueType specification; you can also use the file FT_TRUETYPE_IDS_H, which defines several helpful constants to deal with them.

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.

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 Transformations

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.

7. Simple Text Rendering

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.

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;

... 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.

• We define a handle named 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.
• 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 1/64th 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 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.
• 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 subtract it from pen_y, instead of adding to it.

b.Refined code

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.

• We use the function 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.

c. More Advanced Rendering

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.

• 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 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.
• 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 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.
• The advance width 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.

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.

II. Managing Glyphs

1. Glyph Metrics

Glyph metrics are, as the name suggests, certain distances associated with each glyph that describe how to position this glyph while creating a text layout.

There are usually two sets of metrics for a single glyph: Those used to represent glyphs in horizontal text layouts (Latin, Cyrillic, Arabic, Hebrew, etc.), and those used to represent glyphs in vertical text layouts (Chinese, Japanese, Korean, Mongolian, etc.).

Note that only a few font formats provide vertical metrics. You can test whether a given face object contains them by using the macro FT_HAS_VERTICAL, which returns true if appropriate.

Individual glyph metrics can be accessed by first loading the glyph in a face’s glyph slot, then accessing them through the face->glyph->metrics structure, whose type is FT_Glyph_Metrics. We will discuss this in more detail below; for now, we only note that it contains the following fields.

As not all fonts do contain vertical metrics, the values of vertBearingX, vertBearingY and vertAdvance should not be considered reliable if FT_HAS_VERTICAL returns false.

The following graphics illustrate the metrics more clearly. In case a distance is directed, it is marked with a single arrow, indicating a positive value. The first image displays horizontal metrics, where the baseline is the horizontal axis.

For vertical text layouts, the baseline is vertical, identical to the vertical axis. Contrary to all other arrows, bearingX shows a negative value in this image.

The metrics found in face->glyph->metrics are normally expressed in 26.6 pixel format (i.e., 1/64th of pixels), unless you use the FT_LOAD_NO_SCALE flag when calling FT_Load_Glyph or FT_Load_Char. In this case, the metrics are expressed in original font units.

The glyph slot object has also a few other interesting fields that eases a developer’s work. You can access them through face->glyph->xxx, where xxx is one of the following fields.

2. Managing Glyph Images

The glyph image that is loaded in a glyph slot can be converted into a bitmap, either by using FT_LOAD_RENDER when loading it, or by calling FT_Render_Glyph. Each time you load a new glyph image, the previous one is erased from the glyph slot.

There are situations, however, where you may need to extract this image from the glyph slot in order to cache it within your application, and even perform additional transformations and measures on it before converting it to a bitmap.

The FreeType 2 API has a specific extension that is capable of dealing with glyph images in a flexible and generic way. To use it, you first need to include the FT_GLYPH_H header file.

#include FT_GLYPH_H

a.Extracting the Glyph Image

You can extract a single glyph image very easily. Here some code that shows how to do it.

FT_Glyph  glyph; /* a handle to the glyph image */

...
error = FT_Load_Glyph( face, glyph_index, FT_LOAD_NORMAL );
if ( error ) { ... }

error = FT_Get_Glyph( face->glyph, &glyph );
if ( error ) { ... }

The following steps are performed.

• Create a variable named glyph, of type FT_Glyph. This is a handle (pointer) to an individual glyph image.
• Load the glyph image in the normal way into the face’s glyph slot. We don’t use FT_LOAD_RENDER because we want to grab a scalable glyph image that we can transform later on.
• Copy the glyph image from the slot into a new FT_Glyph object by calling FT_Get_Glyph. This function returns an error code and sets glyph.

It is important to note that the extracted glyph is in the same format as the original one that is still in the slot. For example, if we are loading a glyph from a TrueType font file, the glyph image is really a scalable vector outline. You can access the field glyph->format if you want to know exactly how the glyph is modeled and stored.

A new glyph object can be destroyed with a call to FT_Done_Glyph.

The glyph object contains exactly one glyph image and a 2D vector representing the glyph’s advance in 16.16 fixed-point coordinates. The latter can be accessed directly as glyph->advance

Note that unlike other FreeType objects, the library doesn’t keep a list of all allocated glyph objects. This means you have to destroy them yourself instead of relying on FT_Done_FreeType doing all the clean-up.

b. Transforming & Copying the Glyph Image

If the glyph image is scalable (i.e., if glyph->format is not equal to FT_GLYPH_FORMAT_BITMAP), it is possible to transform the image anytime by a call to FT_Glyph_Transform.

You can also copy a single glyph image with FT_Glyph_Copy.

FT_Glyph   glyph, glyph2;
FT_Matrix  matrix;
FT_Vector  delta;

... load glyph image in `glyph' ...

/* copy glyph to glyph2 */

error = FT_Glyph_Copy( glyph, &glyph2 );
if ( error ) { ... could not copy (out of memory) ... }

/* translate `glyph' */

delta.x = -100 * 64; /* coordinates are in 26.6 pixel format */
delta.y =   50 * 64;

FT_Glyph_Transform( glyph, 0, &delta );

/* transform glyph2 (horizontal shear) */

matrix.xx = 0x10000L;
matrix.xy = 0.12 * 0x10000L;
matrix.yx = 0;
matrix.yy = 0x10000L;

FT_Glyph_Transform( glyph2, &matrix, 0 );

Note that the 2×2 transformation matrix is always applied to the 16.16 advance vector in the glyph; you thus don’t need to recompute it.

c. Measuring the Glyph Image

You can also retrieve the control (bounding) box of any glyph image (scalable or not) through the FT_Glyph_Get_CBox function.

FT_BBox  bbox;

...
FT_Glyph_Get_CBox( glyph, bbox_mode, &bbox );

Coordinates are relative to the glyph origin (0,0), using the y upwards convention. This function takes a special argument, the bbox mode, to indicate how box coordinates are expressed.

If the glyph has been loaded with FT_LOAD_NO_SCALE, bbox_mode must be set to FT_GLYPH_BBOX_UNSCALED to get unscaled font units in 26.6 pixel format. The value FT_GLYPH_BBOX_SUBPIXELS is another name for this constant.

Note that the box’s maximum coordinates are exclusive, which means that you can always compute the width and height of the glyph image (regardless of using integer or 26.6 coordinates) with a simple subtraction.

width  = bbox.xMax - bbox.xMin;
height = bbox.yMax - bbox.yMin;

Note also that for 26.6 coordinates, if FT_GLYPH_BBOX_GRIDFIT is used as the bbox mode, the coordinates are also grid-fitted, which corresponds to the following four lines.

bbox.xMin = FLOOR( bbox.xMin )
bbox.yMin = FLOOR( bbox.yMin )
bbox.xMax = CEILING( bbox.xMax )
bbox.yMax = CEILING( bbox.yMax )

To get the bbox in integer pixel coordinates, set bbox_mode to FT_GLYPH_BBOX_TRUNCATE.

Finally, to get the bounding box in grid-fitted pixel coordinates, set bbox_mode to FT_GLYPH_BBOX_PIXELS.

[Computing exact bounding boxes can be done with function FT_Outline_Get_BBox, at the cost of slower execution. You probably don’t need with the possible exception of rotated glyphs.]

d. Converting the Glyph Image to a Bitmap

You may need to convert the glyph object to a bitmap once you have conveniently cached or transformed it. This can be done easily with the FT_Glyph_To_Bitmap function, which handles any glyph object.

FT_Vector  origin;

origin.x = 32; /* 1/2 pixel in 26.6 format */
origin.y = 0;

error = FT_Glyph_To_Bitmap( &glyph,
                            render_mode,
                            &origin,
                            1 );          /* destroy original image == true */

Some notes.

• The first parameter is the address of the source glyph’s handle. When the function is called, it reads it to access the source glyph object. After the call, the handle points to a new glyph object that contains the rendered bitmap.
• The second parameter is a standard render mode to specify what kind of bitmap we want. For example, it can be FT_RENDER_MODE_DEFAULT for an 8-bit anti-aliased pixmap, or FT_RENDER_MODE_MONO for a 1-bit monochrome bitmap.
• The third parameter is a pointer to a two-dimensional vector to translate the source glyph image before the conversion. After the call, the source image is translated back to its original position (and is thus left unchanged). If you do not need to translate the source glyph before rendering, set this pointer to NULL.
• The last parameter is a boolean that indicates whether the source glyph object should be destroyed by the function. If false, the original glyph object is never destroyed, even if its handle is lost (it is up to client applications to keep it).

The new glyph object always contains a bitmap (if no error is returned), and you must typecast its handle to the FT_BitmapGlyph type in order to access its content. This type is a sort of ‘subclass’ of FT_Glyph that contains additional fields (see FT_BitmapGlyphRec).

3. Global Glyph Metrics

Unlike glyph metrics, global metrics are used to describe distances and features of a whole font face. They can be expressed either in 26.6 pixel format or in (unscaled) font units for scalable formats.

a. Design global metrics

For scalable formats, all global metrics are expressed in font units in order to be later scaled to the device space, according to the rules described in the last section of this tutorial part. You can access them directly as fields of a FT_Face handle.

However, you need to check that the font face’s format is scalable before using them. One can do it with macro FT_IS_SCALABLE, which returns true when appropriate.

Here a table of the global design metrics for scalable faces.

Notice that the values of the ascender and the descender are not reliable (due to various discrepancies in font formats), unfortunately.

b. Scaled Global Metrics

Each size object also contains a scaled version of some of the global metrics described above, to be directly accessed through the face->size->metrics structure (of type FT_Size_Metrics). No rounding or grid-fitting is performed for those values. They are also completely independent of any hinting process. In other words, don’t rely on them to get exact metrics at the pixel level. They are expressed in 26.6 pixel format.

Note that the face->size->metrics structure contains other fields that are used to scale design coordinates to device space. They are described in the last section.

c. Kerning

Kerning is the process of adjusting the position of two subsequent glyph images in a string of text in order to improve the general appearance of text. For example, if a glyph for an uppercase ‘A’ is followed by a glyph for an uppercase ‘V’, the space between the two glyphs can be slightly reduced to avoid extra ‘diagonal whitespace’.

Note that in theory kerning can happen both in the horizontal and vertical direction between two glyphs; however, it only happens in a single direction in nearly all cases.

Not all font formats contain kerning information, and not all kerning formats are supported by FreeType; in particular, for TrueType fonts, the API can only access kerning via the ‘kern’ table. OpenType kerning via the ‘GPOS’ table is not supported! You need a higher-level library like HarfBuzz, Pango, or ICU, since GPOS kerning requires contextual string handling.

Sometimes, the font file is associated with an additional file that contains various glyph metrics, including kerning, but no glyph images. A good example is the Type 1 format where glyph images are stored in files with extension .pfa or .pfb, while kerning metrics can be found in files with extension .afm or .pfm.

FreeType 2 allows you to deal with this, by providing the FT_Attach_File and FT_Attach_Stream APIs. Both functions are used to load additional metrics into a face object by reading them from an additional format-specific file. Here an example, opening a Type 1 font.

error = FT_New_Face( library, "/usr/share/fonts/cour.pfb",
                     0, &face );
if ( error ) { ... }

error = FT_Attach_File( face, "/usr/share/fonts/cour.afm" );
if ( error )
{ ... could not read kerning and additional metrics ... }

Note that FT_Attach_Stream is similar to FT_Attach_File except that it doesn’t take a C string to name the extra file but an FT_Stream handle. Also, reading a metrics file is in no way mandatory.

Finally, the file attachment APIs are very generic and can be used to load any kind of extra information for a given face. The nature of the additional content is entirely font format specific.

FreeType 2 allows you to retrieve the kerning information between two glyphs through the FT_Get_Kerning function.

FT_Vector  kerning;

...
error = FT_Get_Kerning( face,          /* handle to face object */
                        left,          /* left glyph index      */
                        right,         /* right glyph index     */
                        kerning_mode,  /* kerning mode          */
                        &kerning );    /* target vector         */

This function takes a handle to a face object, the indices of the left and right glyph for which the kerning value is desired, an integer, called the kerning mode, and a pointer to a destination vector that receives the corresponding distances.

The kerning mode is very similar to the bbox mode described in a previous section. It is a enumeration that indicates how the kerning distances are expressed in the target vector.

The default value is FT_KERNING_DEFAULT, which has value 0. It corresponds to kerning distances expressed in 26.6 grid-fitted pixels (which means that the values are multiples of 64). For scalable formats, this means that the design kerning distance is scaled, then rounded.

The value FT_KERNING_UNFITTED corresponds to kerning distances expressed in 26.6 unfitted pixels (i.e., that do not correspond to integer coordinates). It is the design kerning distance that is scaled without rounding.

Finally, the value FT_KERNING_UNSCALED indicates to return the design kerning distance, expressed in font units. You can later scale it to the device space using the computations explained in the last section of this part.

Note that the ‘left’ and ‘right’ positions correspond to the visual order of the glyphs in the string of text. This is important for bidirectional or right-to-left text.

4. Simple Text Rendering: Kerning and Centering

In order to show off what we have just learned, we now demonstrate how to modify the example code that was provided in part I to render a string of text, and enhance it to support kerning and delayed rendering.

a. Kerning Support

Adding support for kerning to our code is trivial, as long as we consider that we are still dealing with a left-to-right script like Latin. We simply need to retrieve the kerning distance between two glyphs in order to alter the pen position appropriately.

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

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

pen_x = 300;
pen_y = 200;

use_kerning = FT_HAS_KERNING( face );
previous    = 0;

for ( n = 0; n < num_chars; n++ )
{
  /* convert character code to glyph index */
  glyph_index = FT_Get_Char_Index( face, text[n] );

  /* retrieve kerning distance and move pen position */
  if ( use_kerning && previous && glyph_index )
  {
    FT_Vector  delta;


    FT_Get_Kerning( face, previous, glyph_index,
                    FT_KERNING_DEFAULT, &delta );

    pen_x += delta.x >> 6;
  }

  /* load glyph image into the slot (erase previous one) */
  error = FT_Load_Glyph( face, glyph_index, 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;

  /* record current glyph index */
  previous = glyph_index;
}

We are done. Some notes.

• As kerning is determined by glyph indices, we need to explicitly convert our character codes into glyph indices, then later call FT_Load_Glyph instead of FT_Load_Char.
• We use a boolean named use_kerning, which is set to the result of the macro FT_HAS_KERNING. It is certainly faster not to call FT_Get_Kerning when we know that the font face does not contain kerning information.
• We move the position of the pen before a new glyph is drawn.
• We initialize the variable previous with the value 0, which always corresponds to the ‘missing glyph’ (also called .notdef in the PostScript world). There is never any kerning distance associated with this glyph.
• We do not check the error code returned by FT_Get_Kerning. This is because the function always sets the content of delta to (0,0) if an error occurs.

b. Centering

Our code begins to become interesting but it is still a bit too simple for normal use. For example, the position of the pen is determined before we do the rendering; normally, you would rather determine the layout of the text and measure it before computing its final position (centering, etc.), or perform things like word-wrapping.

Let us now decompose our text rendering function into two distinct but successive parts: The first one positions individual glyph images on the baseline, while the second one renders the glyphs. As we will see, this has many advantages.

We thus start by storing individual glyph images, as well as their position on the baseline.

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

FT_Glyph      glyphs[MAX_GLYPHS];   /* glyph image    */
FT_Vector     pos   [MAX_GLYPHS];   /* glyph position */
FT_UInt       num_glyphs;

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

pen_x = 0;   /* start at (0,0) */
pen_y = 0;

num_glyphs  = 0;
use_kerning = FT_HAS_KERNING( face );
previous    = 0;

for ( n = 0; n < num_chars; n++ )
{
  /* convert character code to glyph index */
  glyph_index = FT_Get_Char_Index( face, text[n] );

  /* retrieve kerning distance and move pen position */
  if ( use_kerning && previous && glyph_index )
  {
    FT_Vector  delta;


    FT_Get_Kerning( face, previous, glyph_index,
                    FT_KERNING_DEFAULT, &delta );

    pen_x += delta.x >> 6;
  }

  /* store current pen position */
  pos[num_glyphs].x = pen_x;
  pos[num_glyphs].y = pen_y;

  /* load glyph image into the slot without rendering */
  error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT );
  if ( error )
    continue;  /* ignore errors, jump to next glyph */

  /* extract glyph image and store it in our table */
  error = FT_Get_Glyph( face->glyph, &glyphs[num_glyphs] );
  if ( error )
    continue;  /* ignore errors, jump to next glyph */

  /* increment pen position */
  pen_x += slot->advance.x >> 6;

  /* record current glyph index */
  previous = glyph_index;

  /* increment number of glyphs */
  num_glyphs++;
}

This is a very slight variation of our previous code; we extract each glyph image from the slot, then store it, along with the corresponding position, in our tables.

Note also that pen_x contains the total advance for the string of text. We can now compute the bounding box of the text string with a simple function.

void  compute_string_bbox( FT_BBox  *abbox )
{
  FT_BBox  bbox;
  FT_BBox  glyph_bbox;


  /* initialize string bbox to "empty" values */
  bbox.xMin = bbox.yMin =  32000;
  bbox.xMax = bbox.yMax = -32000;

  /* for each glyph image, compute its bounding box, */
  /* translate it, and grow the string bbox          */
  for ( n = 0; n < num_glyphs; n++ )
  {
    FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels,
                       &glyph_bbox );

    glyph_bbox.xMin += pos[n].x;
    glyph_bbox.xMax += pos[n].x;
    glyph_bbox.yMin += pos[n].y;
    glyph_bbox.yMax += pos[n].y;

    if ( glyph_bbox.xMin < bbox.xMin )
      bbox.xMin = glyph_bbox.xMin;

    if ( glyph_bbox.yMin < bbox.yMin )
      bbox.yMin = glyph_bbox.yMin;

    if ( glyph_bbox.xMax > bbox.xMax )
      bbox.xMax = glyph_bbox.xMax;

    if ( glyph_bbox.yMax > bbox.yMax )
      bbox.yMax = glyph_bbox.yMax;
  }

  /* check that we really grew the string bbox */
  if ( bbox.xMin > bbox.xMax )
  {
    bbox.xMin = 0;
    bbox.yMin = 0;
    bbox.xMax = 0;
    bbox.yMax = 0;
  }

  /* return string bbox */
  *abbox = bbox;
}

The resulting bounding box dimensions are expressed in integer pixels and can then be used to compute the final pen position before rendering the string.

In general, the above function does not compute an exact bounding box of a string! As soon as hinting is involved, glyph dimensions must be derived from the resulting outlines. For anti-aliased pixmaps, FT_Outline_Get_BBox then yields proper results. In case you need 1-bit monochrome bitmaps, it is even necessary to actually render the glyphs because the rules for the conversion from outline to bitmap can also be controlled by hinting instructions.

void  compute_string_bbox( FT_BBox  *abbox )
{
  FT_BBox  bbox;
  FT_BBox  glyph_bbox;


  /* initialize string bbox to "empty" values */
  bbox.xMin = bbox.yMin =  32000;
  bbox.xMax = bbox.yMax = -32000;

  /* for each glyph image, compute its bounding box, */
  /* translate it, and grow the string bbox          */
  for ( n = 0; n < num_glyphs; n++ )
  {
    FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels,
                       &glyph_bbox );

    glyph_bbox.xMin += pos[n].x;
    glyph_bbox.xMax += pos[n].x;
    glyph_bbox.yMin += pos[n].y;
    glyph_bbox.yMax += pos[n].y;

    if ( glyph_bbox.xMin < bbox.xMin )
      bbox.xMin = glyph_bbox.xMin;

    if ( glyph_bbox.yMin < bbox.yMin )
      bbox.yMin = glyph_bbox.yMin;

    if ( glyph_bbox.xMax > bbox.xMax )
      bbox.xMax = glyph_bbox.xMax;

    if ( glyph_bbox.yMax > bbox.yMax )
      bbox.yMax = glyph_bbox.yMax;
  }

  /* check that we really grew the string bbox */
  if ( bbox.xMin > bbox.xMax )
  {
    bbox.xMin = 0;
    bbox.yMin = 0;
    bbox.xMax = 0;
    bbox.yMax = 0;
  }

  /* return string bbox */
  *abbox = bbox;
}

Some remarks.

• The pen position is expressed in the Cartesian space (i.e., y upwards).
• We call FT_Glyph_To_Bitmap with the destroy parameter set to 0 (false), in order to avoid destroying the original glyph image. The new glyph bitmap is accessed through image after the call and is typecast to FT_BitmapGlyph.
• We use translation when calling FT_Glyph_To_Bitmap. This ensures that the left and top fields of the bitmap glyph object are already set to the correct pixel coordinates in the Cartesian space.
• Of course, we still need to convert pixel coordinates from Cartesian to device space before rendering, hence the my_target_height - bitmap->top in the call to my_draw_bitmap.

The same loop can be used to render the string anywhere on our display surface, without the need to reload our glyph images each time.

5. Advanced Text Rendering: Transformation and Centering and Kerning

We are now going to modify our code in order to be able to easily transform the rendered string, for example, to rotate it. First, some minor improvements.

a. Packing and Translating Glyphs

We start by packing the information related to a single glyph image into a single structure instead of parallel arrays.

typedef struct  TGlyph_
{
  FT_UInt    index;  /* glyph index                  */
  FT_Vector  pos;    /* glyph origin on the baseline */
  FT_Glyph   image;  /* glyph image                  */

} TGlyph, *PGlyph;

We also translate each glyph image directly after it is loaded to its position on the baseline at load time. As we will see, this has several advantages. Here is our new glyph sequence loader.

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

TGlyph        glyphs[MAX_GLYPHS];  /* glyphs table */
PGlyph        glyph;               /* current glyph in table */
FT_UInt       num_glyphs;


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

pen_x = 0;   /* start at (0,0) */
pen_y = 0;

num_glyphs  = 0;
use_kerning = FT_HAS_KERNING( face );
previous    = 0;

glyph = glyphs;
for ( n = 0; n < num_chars; n++ )
{
  glyph->index = FT_Get_Char_Index( face, text[n] );

  if ( use_kerning && previous && glyph->index )
  {
    FT_Vector  delta;


    FT_Get_Kerning( face, previous, glyph->index,
                    FT_KERNING_MODE_DEFAULT, &delta );

    pen_x += delta.x >> 6;
  }

  /* store current pen position */
  glyph->pos.x = pen_x;
  glyph->pos.y = pen_y;

  error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT );
  if ( error ) continue;

  error = FT_Get_Glyph( face->glyph, &glyph->image );
  if ( error ) continue;

  /* translate the glyph image now */
  FT_Glyph_Transform( glyph->image, 0, &glyph->pos );

  pen_x   += slot->advance.x >> 6;
  previous = glyph->index;

  /* increment number of glyphs */
  glyph++;
}

/* count number of glyphs loaded */
num_glyphs = glyph - glyphs;

Note that translating glyphs now has several advantages. The first one is that we don’t need to translate the glyph bbox when we compute the string’s bounding box.

void  compute_string_bbox( FT_BBox  *abbox )
{
  FT_BBox  bbox;


  bbox.xMin = bbox.yMin =  32000;
  bbox.xMax = bbox.yMax = -32000;

  for ( n = 0; n < num_glyphs; n++ )
  {
    FT_BBox  glyph_bbox;


    FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels,
                       &glyph_bbox );

    if (glyph_bbox.xMin < bbox.xMin)
      bbox.xMin = glyph_bbox.xMin;

    if (glyph_bbox.yMin < bbox.yMin)
      bbox.yMin = glyph_bbox.yMin;

    if (glyph_bbox.xMax > bbox.xMax)
      bbox.xMax = glyph_bbox.xMax;

    if (glyph_bbox.yMax > bbox.yMax)
      bbox.yMax = glyph_bbox.yMax;
  }

  if ( bbox.xMin > bbox.xMax )
  {
    bbox.xMin = 0;
    bbox.yMin = 0;
    bbox.xMax = 0;
    bbox.yMax = 0;
  }

  *abbox = bbox;
}

With the above modifications, the compute_string_bbox function can now compute the bounding box of a transformed glyph string, which allows further code simplications.

FT_BBox    bbox;
FT_Matrix  matrix;
FT_Vector  delta;


... load glyph sequence ...
... set up `matrix' and `delta' ...

/* transform glyphs */
for ( n = 0; n < num_glyphs; n++ )
  FT_Glyph_Transform( glyphs[n].image, &matrix, &delta );

/* compute bounding box of transformed glyphs */
compute_string_bbox( &bbox );

b. Rendering a Transformed Glyph Sequence

However, directly transforming the glyphs in our sequence is not a good idea if we want to reuse them in order to draw the text string with various angles or transformations. It is better to perform the affine transformation just before the glyph is rendered.

FT_Vector  start;
FT_Matrix  matrix;

FT_Glyph   image;
FT_Vector  pen;
FT_BBox    bbox;


/* get bbox of original glyph sequence */
compute_string_bbox( &string_bbox );

/* compute string dimensions in integer pixels */
string_width  = (string_bbox.xMax - string_bbox.xMin) / 64;
string_height = (string_bbox.yMax - string_bbox.yMin) / 64;

/* set up start position in 26.6 Cartesian space */
start.x = ( ( my_target_width  - string_width  ) / 2 ) * 64;
start.y = ( ( my_target_height - string_height ) / 2 ) * 64;

/* set up transform (a rotation here) */
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 );

pen = start;

for ( n = 0; n < num_glyphs; n++ )
{
  /* create a copy of the original glyph */
  error = FT_Glyph_Copy( glyphs[n].image, &image );
  if ( error ) continue;

  /* transform copy (this will also translate it to the */
  /* correct position                                   */
  FT_Glyph_Transform( image, &matrix, &pen );

  /* check bounding box; if the transformed glyph image      */
  /* is not in our target surface, we can avoid rendering it */
  FT_Glyph_Get_CBox( image, ft_glyph_bbox_pixels, &bbox );
  if ( bbox.xMax <= 0 || bbox.xMin >= my_target_width  ||
       bbox.yMax <= 0 || bbox.yMin >= my_target_height )
    continue;

  /* convert glyph image to bitmap (destroy the glyph copy!) */
  error = FT_Glyph_To_Bitmap(
            &image,
            FT_RENDER_MODE_NORMAL,
            0,                  /* no additional translation */
            1 );                /* destroy copy in "image"   */
  if ( !error )
  {
    FT_BitmapGlyph  bit = (FT_BitmapGlyph)image;


    my_draw_bitmap( bit->bitmap,
                    bit->left,
                    my_target_height - bit->top );

    /* increment pen position --                       */
    /* we don't have access to a slot structure,       */
    /* so we have to use advances from glyph structure */
    /* (which are in 16.16 fixed float format)         */
    pen.x += image.advance.x >> 10;
    pen.y += image.advance.y >> 10;

    FT_Done_Glyph( image );
  }
}

There are a few changes compared to the original version of this code.

• We keep the original glyph images untouched; instead, we transform a copy.
• We perform clipping computations in order to avoid rendering and drawing glyphs that are not within our target surface.
• We always destroy the copy when calling FT_Glyph_To_Bitmap in order to get rid of the transformed scalable image. Note that the image is not destroyed if the function returns an error code (which is why FT_Done_Glyph is only called within the compound statement).
• The translation of the glyph sequence to the start pen position is integrated into the call to FT_Glyph_Transform instead of FT_Glyph_To_Bitmap.

It is possible to call this function several times to render the string with different angles, or even change the way start is computed in order to move it to different place.

This code is the basis of the FreeType 2 demonstration program named ftstring.c. It could be easily extended to perform advanced text layout or word-wrapping in the first part, without changing the second one.

Note, however, that a normal implementation would use a glyph cache in order to reduce memory needs. For example, let us assume that our text string is ‘FreeType’. We would store three identical glyph images in our table for the letter ‘e’, which isn’t optimal (especially when you consider longer lines of text, or even whole pages).

A FreeType demo program that shows how glyph caching can be implemented is ftview.c. In general, ‘ftview’ is the main program used by the FreeType developer team to check the validity of loading, parsing, and rendering fonts.

6. Accessing Metrics in Design Font Units, and Scaling Them

Scalable font formats usually store a single vectorial image, called an outline, for each glyph in a face. Each outline is defined in an abstract grid called the design space, with coordinates expressed in font units. When a glyph image is loaded, the font driver usually scales the outline to device space according to the current character pixel size found in an FT_Size object. The driver may also modify the scaled outline in order to significantly improve its appearance on a pixel-based surface (a process known as hinting or grid-fitting).

This section describes how design coordinates are scaled to the device space, and how to read glyph outlines and metrics in font units. This is important for a number of things.

• ‘True’ WYSIWYG text layout.
• Accessing font content for conversion or analysis purposes.

a. Scaling Distances to Device Space

Design coordinates are scaled to the device space using a simple scaling transformation whose coefficients are computed with the help of the character pixel size.

device_x = design_x * x_scale
device_y = design_y * y_scale

x_scale  = pixel_size_x / EM_size
y_scale  = pixel_size_y / EM_size

Here, the value EM_size is font-specific and corresponds to the size of an abstract square of the design space (called the EM), which is used by font designers to create glyph images. It is thus expressed in font units. It is also accessible directly for scalable font formats as face->units_per_EM. You should check that a font face contains scalable glyph images by using the FT_IS_SCALABLE macro, which returns true if appropriate.

When you call the function FT_Set_Pixel_Sizes, you are specifying the value of pixel_size_x and pixel_size_y FreeType shall use. The library will immediately compute the values of x_scale and y_scale.

When you call the function FT_Set_Char_Size, you are specifying the character size in physical points, which is used, along with the device’s resolutions, to compute the character pixel size and the corresponding scaling factors.

Note that after calling any of these two functions, you can access the values of the character pixel size and scaling factors as fields of the face->size->metrics structure.

You can scale a distance expressed in font units to 26.6 pixel format directly with the help of the FT_MulFix function.

/* convert design distances to 1/64th of pixels */
pixels_x = FT_MulFix( design_x, face->size->metrics.x_scale );
pixels_y = FT_MulFix( design_y, face->size->metrics.y_scale );

Alternatively, you can also scale the value directly with more accuracy by using doubles.

FT_Size_Metrics*  metrics = &face->size->metrics; /* shortcut */
double            pixels_x, pixels_y;
double            em_size, x_scale, y_scale;


/* compute floating point scale factors */
em_size = 1.0 * face->units_per_EM;
x_scale = metrics->x_ppem / em_size;
y_scale = metrics->y_ppem / em_size;

/* convert design distances to floating point pixels */
pixels_x = design_x * x_scale;
pixels_y = design_y * y_scale;

b. Accessing Design Metrics (Glyph & Global)

You can access glyph metrics in font units simply by specifying the FT_LOAD_NO_SCALE bit flag in FT_Load_Glyph or FT_Load_Char. The metrics returned in face->glyph->metrics will all be in font units.

You can access unscaled kerning data using the FT_KERNING_MODE_UNSCALED mode.

Finally, a few global metrics are available directly in font units as fields of the FT_Face handle, as described in section 3 of this part.

Conclusion

This is the end of the second part of the FreeType tutorial. You are now able to access glyph metrics, manage glyph images, and render text much more intelligently (kerning, measuring, transforming & caching); this is sufficient knowledge to build a pretty decent text service on top of FreeType.

The demo programs in the ‘ft2demos’ bundle (especially ‘ftview’) are a kind of reference implementation, and are a good resource to turn to for answers. They also show how to use additional features, such as the glyph stroker and cache.

III. Examples

For completeness, here again a link to the example used and explained in the first part of the tutorial.

Erik Möller contributed a very nice C++ example that shows renderer callbacks in action to draw a coloured glyph with a differently coloured outline. The source code can be found here.

Another example demonstrates how to use FreeType’s stand-alone rasterizer, ftraster.c, both in B/W and 5-levels gray mode. You need files from FreeType version 2.3.10 or newer.

Róbert Márki contributed a small Qt demonstration program (together with its qmake file) that shows both direct rendering with a callback and rendering with a buffer, yielding the same result. You need FreeType 2.4.3 or newer.