# FreeType 2.0 Tutorial

### © 2000 David Turner (david@freetype.org)
© 2000 The FreeType Development Team (www.freetype.org)

## Introduction:

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

### 1. Header files :

You only need to include the public header file named `freetype.h` in your source code. Depending on how the library was installed on your system, you might need to use :

`#include <freetype.h>

`

or

`#include <freetype2/freetype.h>

`

in your application to include the public interface to FreeType.

### 2. Initialise 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.h>

           FT_Library  library;

           ....

           {
             ..
             error = FT_Init_FreeType( &library );
             if (error) { .. an error occured during library initialisation .. }
           }

This function is in charge of the following:

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

- Load each font driver that FreeType knows about in the library. This means that by default, your new `library` object is able to handle TrueType and Type 1 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 succesful; 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, read or simply that it is broken..
              }

As you certainly imagine, `FT_New_Face` opens a font file then tries to extract one face from it. Its parameters are :

`library` | handle to the FreeType library instance where the face object is created
`filepathname` | the font file pathname (standard C string).
`face_index` | Certain font formats allow several font faces to be embedded in a single file. This index tells which face you want to load. An error will be returned if its value is too large. Index 0 always work though.
`face` | A pointer to the handle that will be set to describe the new face object. It is set to NULL in case of error.

To known how many faces a given font file contains, simply load its first face (use `face_index`=0), then see the value of `face->num_faces` which indicates how many faces are embedded in the font file.

#### 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 than `FT_New_Face`.

#### c. From other sources:

There are cases where using a filepathname 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 through the `FT_Stream` type.

Basically, one has to set up a `FT_Stream` object, according to the rules defined in the document named FreeType 2 System Interface, then pass it to the function `FT_Open_Face` as in:

              FT_Library   library;   /* handle to library     */
              FT_Face      face;      /* handle to face object */

              error = FT_Init_FreeType( &library );
              if (error) { ..... }

              ... set up stream object, with handle "stream" ...

              error = FT_Open_Face( library,
                                    stream,    /* handle to stream objects */
                                    0,         /* face_index               */
                                    &face );
              if (error) { ... }

custom implementations of `FT_Stream` are great to provide advanced features like automatic support of compressed files, network transparency, using UTF-16 file pathnames, etc..

### 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 :

`face->num_glyphs` | gives the number of glyphs available in the font face. A glyph is simply a character image. It doesn't necessarily correspond to a character code though.
`face->flags` | a 32-bit integer containing bit flags used to describe some face properties. For example, the flag `FT_FACE_FLAG_SCALABLE` is used to indicate that the face's font format is scalable and that glyph images can be rendered for all character pixel sizes. For more information on face flags, please read the FreeType 2 API Reference
`face->units_per_EM` | This field is only valid for scalable formats (it is set to 0 otherwise). It indicates the number of font units covered by the EM.
`face->num_fixed_sizes` | this field gives the number of embedded bitmap strikes in the current face. A strike is simply a series of glyph images for a given character pixel size. For example, a font face could include strikes for pixel sizes 10, 12 and 14. Note that even scalable font formats can embedded bitmap strikes !
`face->fixed_sizes` | this is a pointer to an array of `FT_Bitmap_Size` element. each `FT_Bitmap_Size` indicates the horizontal and vertical pixel sizes for each of the strikes that are present in the face.

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

### 5. Setting the current pixel size:

A face object also holds a handle to a size object in its `face->size` field. The size object is used to model all information for the face that is relative to a given character size.

When a new face object is created, its size object defaults to the character size of 10 pixels (both horizontall and vertically) for scalable formats. For fixed-sizes formats, the size is more or less undefined, which is why you must set it before trying to load a glyph.

To do that, simply call `FT_Set_Char_Size`. Here's an example where the character size is set to 16 pts for a 300x300 dpi device:

            error = FT_Set_Char_Size( face,    /* handle to face object           */
                                      0,       /* char_width in 1/64th of points  */
                                      16*64,   /* char_height in 1/64th of points */
                                      300,     /* horizontal device resolution    */
                                      300 );   /* vertical device resolution      */

You'll notice that:

- The character width and heights are specified in 1/64th of points.

- 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.

- 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 width and height.

- Using a value of 0 for the horizontal or vertical resolution means "same value since last call". By default a new face object has a size whose resolutions are set to 72x72 dpi.

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 the 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'll describe later how to look for specific charmaps in a face. For now, we'll assume that the face contains at least a Unicode charmap that was selected during `FT_New_Face`. To convert a Unicode character code to a font glyph index, we use `FT_Get_Char_Index` as in:

             glyph_index = FT_Get_Char_Index( face, charcode );

This will look the glyph index corresponding to the given `charcode` in the charmap that is currently selected for the face. If charmap is selected, the function simply returns the charcode.

Note that this is one of the rare FreeType functions that do not return an error code. However, when a given character code has no glyph image in the face, the value 0 is returned. By convention, it always correspond to a special glyph image called the missing glyph, which usually is represented as a box or a space.

#### 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 it names suggests, a glyph slot is simply a container that is able to hold one glyph image at a time, be it a bitmap, an outline, or something else. Each face object has a single glyph object that can be accessed as `face->glyph`.

Loading a glyph image into the slot is performed by calling `FT_Load_Glyph` as in:

            error = FT_Load_Glyph( face,          /* handle to face object */
                                   glyph_index,   /* glyph index           */
                                   load_flags );  /* load flags, see below */

The `load_flags` value is a set of bit flags used to indicate some special operations. The default value `FT_LOAD_DEFAULT` is 0. The function performs the following :

- if there is a bitmap for the corresponding glyph and size, load it in the glyph slot, unless the `FT_LOAD_NO_BITMAP` flag is set. This is even true for scalable formats (embedded bitmaps are favored over outlines as they usually correspond to higher-quality images of the same glyph).

- if there is an outline for the corresponding glyph, load it unless `FT_LOAD_NO_OUTLINE` is set. Otherwise, scale it to the current size, unless the `FT_LOAD_NO_SCALE` flag is set.

- if the outline was loaded and scaled, try to grid-fit it (which dramatically improves its quality) unless the flag `FT_LOAD_NO_HINTING` is set.

There are a few others `FT_LOAD_xxx` flags defined. For more details see the FreeType 2 API Reference.

#### 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.

The field `face->num_charmaps` and `face->charmaps` (notice the 's') can be used by client applications to look at what charmaps are available in a given face.

`face-charmaps` is an array of pointers to the `face->num_charmaps` charmaps contained in the font face.

Each charmap has a few visible fields used to describe it in more details. For example, `charmap->encoding` is an enumeration type that describes the charmap with FreeType codes. One can also look at `charmap->platform_id` and `charmap->encoding_id` for more exotic needs.

Here's an example code that looks for a chinese Big5 charmap then selects it via `FT_Set_CharMap`:

            FT_CharMap  found = 0;
            FT_CharMap  charmap;
            int         n;

            for ( n = 0; n < face->num_charmaps; n++ )
            {
              charmap = face>charmaps[n];
              if (charmap->encoding == ft_encoding_big5)
              {
                found = charmap;
                break;
              }
            }

            if (!found) { ... }

            /* now, select the charmap for the face object */
            error = FT_Set_CharMap( face, found );
            if (error) { .... }

One might now call `FT_Get_Char_Index` with Big5 character codes to retrieve glyph indices.

### 7. Accessing glyph image data:

Glyph image data is accessible through `face-glyph`. See the definition of the `FT_GlyphSlot` type on more details. You can perfectly create a shortcut to the glyph slot as in:

         {
           FT_GlyphSlot  glyph;

           .... load glyph ...

           glyph = face->glyph;   /* shortcut to glyph data */

           .... access glyph data as glyph->xxxx
         }

For example, one can access the following fields:

`glyph->format` | Indicates the type of the loaded glyph image. Can be either `ft_glyph_format_bitmap`, `ft_glyph_format_outline` or other values.
`glyph->metrics` | A simple structure used to hold the glyph image's metrics. Note that all distances are expressed in 1/64th of pixels ! See the API reference or User Guide for a description of the `FT_Glyph_Metrics` structure.
`glyph->bitmap` | When the glyph slot contains a bitmap, a simple `FT_Bitmap` that describes it. See the API reference or user guide for a description of the `FT_Bitmap` structure.
`glyph->outline` | When the glyph slot contains a scalable outline, this structure describes it. See the definition of the `FT_Outline` structure.

### 8. Rendering glyph outlines into bitmaps:

When the glyph image loaded in a glyph slot is a bitmap, you can use your favorite graphics library to blit it to your own surfaces.

On the other hand, when the image is a scalable outline, or something else, FreeType provides a function to convert the glyph image into a pre-existing bitmap that you'll handle to it, named `FT_Get_Glyph_Bitmap`. Here's a simple example code that renders an outline into a monochrome bitmap :

         {
           FT_GlyphSlot  glyph;

           .... load glyph ...

           glyph = face->glyph;   /* shortcut to glyph data */
           if (glyph->format == ft_glyph_format_outline )
           {
             FT_Bitmap  bit;

             /* set-up a bitmap descriptor for our target bitmap */
             bit.rows       = bitmap_height;
             bit.width      = bitmap_width;
             bit.pitch      = bitmap_row_bytes;
             bit.pixel_mode = ft_pixel_mode_mono;  /* render into a mono bitmap */
             bit.buffer     = bitmap_buffer;

             /* render the outline directly into the bitmap */
             error = FT_Get_Glyph_Bitmap( face, &bit );
             if (error) { ... }
           }
         }

You should note that `FT_Get_Glyph_Bitmap` doesn't create the bitmap. It only needs a descriptor, of type `FT_Bitmap`, and writes directly into it.

Note that the FreeType scan-converter for outlines can also generate anti-aliased glyph bitmaps with 128 level of grays. For now, it is restricted to rendering to 8-bit gray-level bitmaps, though this may change in the future. Here's some code to do just that:

         {
           FT_GlyphSlot  glyph;

           .... load glyph ...

           glyph = face->glyph;   /* shortcut to glyph data */
           if (glyph->format == ft_glyph_format_outline )
           {
             FT_Bitmap  bit;

             /* set-up a bitmap descriptor for our target bitmap */
             bit.rows       = bitmap_height;
             bit.width      = bitmap_width;
             bit.pitch      = bitmap_row_bytes;
             bit.pixel_mode = ft_pixel_mode_gray;  /* 8-bit gray-level bitmap */
             bit.grays      = 128;                 /* MUST be 128 for now     */
             bit.buffer     = bitmap_buffer;

             /* clean the bitmap - IMPORTANT */
             memset( bit.buffer, 0, bit.rows*bit.pitch );

             /* render the outline directly into the bitmap */
             error = FT_Get_Glyph_Bitmap( face, &bit );
             if (error) { ... }
           }
         }

You'll notice that :

- 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.