FreeType 2 Tutorial
Author: David Turner

# FreeType 2.0 Tutorial
Step 1 -- simple glyph loading

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

## Introduction This is the first part of the FreeType 2 tutorial. It will teach you to do the following: initialize 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 To include the main FreeType header file, say
        #include <freetype/freetype.h>
in your application code. Note that other files are available in the FreeType include directory, most of them being included by `freetype.h` and other (internal) files. Some of them will be described later in this tutorial. ### 2. Initialize the library Simply create a variable of type `FT_Library` named, for example, `library`, and call the function `FT_Init_FreeType()` as in
        #include <freetype/freetype.h>

        FT_Library  library;

        ...

        {
          ...
          error = FT_Init_FreeType( &library );
          if ( error )
          {
            ... an error occurred during library initialization ...
          }
        }
This function is in charge of the following: Creating a new instance of the FreeType 2 library, and set the handle `library` to it. Load each module that FreeType knows about in the library. This means that by default, your new `library` object is able to handle TrueType, Type 1, Windows FON, 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 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
`library` | A handle to the FreeType library instance where the face object is created
`filepathname` | The font file pathname (a 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 know how many faces a given font file contains, load its first face (use `face_index`=0), 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 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()` 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 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 refer to the FreeType 2 API reference 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 contents 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 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 have embedded bitmap strikes!
`face->fixed_sizes` | This is a pointer to an array of `FT_Bitmap_Size` elements. 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 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`. 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 functions. 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 is not a pixel. 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. This behaviour 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 in ASCII encoding represents letter `A'. 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 usually). 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 up the glyph index corresponding to the given `charcode` in the charmap that is currently selected for the face. Note that this is one of the rare FreeType functions that do not return an error code. If a given character code has no glyph image in the face, the value 0 is returned. By convention, it always corresponds 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. 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 glyphs (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 a container that is able to hold one glyph image at a time, be it a bitmap, an outline, or something else. Each face object has a single glyph slot object that can be accessed as `face->glyph`. Loading a glyph image into the slot is performed by calling `FT_Load_Glyph()` as in
        error = FT_Load_Glyph(
                  face,          /* handle to face object */
                  glyph_index,   /* glyph index           */
                  load_flags );  /* load flags, see below */
The `load_flags` value is a set of bit flags used to indicate some special operations. The default value `FT_LOAD_DEFAULT` is 0. This function will try to load the corresponding glyph image from the face. Basically, this means that If a bitmap is found for the corresponding glyph and pixel size, it will 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 changed 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 Type 1. The field `glyph->format` describes the format used to store the glyph image in the slot. If it is not `ft_glyph_format_bitmap`, it is possible to immedialy 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` specifies how to render the glyph image. Set it `ft_render_mode_normal` to render a high-quality anti-aliased (256 gray levels) bitmap. 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 with `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 second part of the tutorial describes the contents 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` if 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 if the encoding you need already has a corresponding enumeration defined in `freetype/freetype.h`, as `ft_encoding_big5`. In this case, you can simply call `FT_Select_CharMap()` as in
        error = FT_Select_CharMap(
                  face,                 /* target face object */
                  ft_encoding_big5 );   /* encoding           */
Another way is to manually parse the list of charmaps for the face; this is accessible through the fields `num_charmaps` and `charmaps` (notice the final 's') of the face object. As expected, 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` which 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 (on the Windows platform). A list of such pairs is defined in the TrueType specification, but you can also use the file `<freetype/ttnameid.h>` which defines several helpful constants to deal with them. Note that some pid/eid pairs are artificial; such values have been created by FreeType to identify platforms resp. encodings not covered by the original TrueType specification. To look up a specific encoding you need to find a corresponding value pair in the specification, then look for it in the `charmaps` list. Bear in mind that some encodings correspond to several values pairs (yes, it's a real mess, but blame Apple and Microsoft on such stupidity). Here 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 transformations It is possible to specify an affine transformation to be applied to glyph images when they are 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 transformation for a given face object. Its second parameter is a pointer to an `FT_Matrix` structure that describes a 2x2 affine matrix. The third parameter is a pointer to an `FT_Vector` structure that describes a simple 2d vector that is used to translate the glyph image after the 2x2 transformation. Note that the matrix pointer can be set to `NULL`, in which case the identity transformation 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 vector of (0,0) will be used. The vector coordinates are expressed in 1/64th of a pixel (also known as 26.6 fixed floats). 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 at 12 pixels scaled by 2 through a transformation, because hints will have been computed differently (unless hints have been disabled, of course). If you ever need to use a non-orthogonal transformation with optimal hints, you first need 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 details in a later section of this tutorial. Note also that loading a glyph bitmap with a non-identity transformation will produce an error. ### 7. Simple text rendering We will now present 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;

        .. 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.) This is a convenience to avoid using `face->glyph->XXX` every time. We increment the pen position with the vector `slot->advance`, which corresponds 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. 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 which will be 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()` followed by `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()` explicitly but is strictly equivalent. Note that you can also specify that you want a monochrome bitmap by using the `FT_LOAD_MONOCHROME` load flag instead. #### c. more advanced rendering We now render transformed text (for example through a rotation). To do that we use `FT_Set_Transform()`:
        FT_GlyphSlot  slot = face->glyph;  /* a small shortcut */
        FT_Matrix     matrix;              /* transformation matrix */
        FT_UInt       glyph_index;
        FT_Vector     pen;                 /* untransformed origin */
        int           n;

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

        /* 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 */
        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;
        }
Notes: 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. In FreeType, glyph images are always loaded, transformed, and described in the cartesian coordinate system (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 apply the transformation matrix on each glyph to indicate rotation as well as a delta vector that will move the transformed image to the current pen position (in cartesian space, not bitmap space). 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. 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). It has, however, a few shortcomings 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 how to render rotated text. The next part will dive into more details of the API in order to let you access glyph metrics and images directly, how to deal with scaling, hinting, kerning, etc. The third part will discuss issues like modules, caching, and a few other advanced topics like how to use multiple size objects with a single face.