FreeType 2 Tutorial
Author: David Turner

# FreeType 2.0 Tutorial
Step 2 -- managing glyphs

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

## Introduction This is the second part of the FreeType 2 tutorial. It will teach you the following: how to retrieve glyph metrics how to easily manage glyph images how to retrieve global metrics (including kerning) how to render a simple string of text, with kerning how to render a centered string of text (with kerning) how to render a transformed string of text (with centering) finally, how to access metrics in design font units if needed, and how to scale them to device space ### 1. Glyph metrics Glyph metrics are, as their name suggests, certain distances associated to each glyph in order to describe how to use it to layout text. There are usually two sets of metrics for a single glyph: those used to layout the glyph in horizontal text layouts (like Latin, Cyrillic, Arabic, Hebrew, etc.), and those used to layout the glyph in vertical text layouts (like some layouts of Chinese, Japanese, Korean, and others). Note that only a few font formats provide vertical metrics. You can test wether a given face object contains them by using the macro `FT_HAS_VERTICAL(face)`, which is true if vertical metrics are available. Individual glyph metrics can be accessed by first loading the glyph in a face's glyph slot, then using the `face->glyph->metrics` structure. This will be described later; for now, we observe that it contains the following fields:
`width` | This is the width of the glyph image's bounding box. It is independent of the layout direction.
`height` | This is the height of the glyph image's bounding box. It is independent of the layout direction.
`horiBearingX` | For horizontal text layouts, this is the horizontal distance from the current cursor position to the left-most border of the glyph image's bounding box.
`horiBearingY` | For horizontal text layouts, this is the vertical distance from the current cursor position (on the baseline) to the top-most border of the glyph image's bounding box.
`horiAdvance` | For horizontal text layouts, this is the horizontal distance used to increment the pen position when the glyph is drawn as part of a string of text.
`vertBearingX` | For vertical text layouts, this is the horizontal distance from the current cursor position to the left-most border of the glyph image's bounding box.
`vertBearingY` | For vertical text layouts, this is the vertical distance from the current cursor position (on the baseline) to the top-most border of the glyph image's bounding box.
`vertAdvance` | For vertical text layouts, this is the vertical distance used to increment the pen position when the glyph is drawn as part of a string of text.
As not all fonts do contain vertical metrics, the values of `vertBearingX`, `vertBearingY`, and `vertAdvance` should not be considered reliable if `FT_HAS_VERTICAL(face)` is false. The following graphics illustrate the metrics more clearly. First horizontal metrics, where the baseline is the horizontal axis: For vertical text layouts, the baseline is vertical and is the vertical axis: 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 that case, the metrics will be expressed in original font units. The glyph slot object has a few other interesting fields that will ease a developer's work. You can access them through `face->glyph->???`:
`advance` | This field is an `FT_Vector` which holds the transformed advance value for the glyph. This is useful if you are using a transformation through `FT_Set_Transform()`, as shown in the rotated text example of the previous part. Other than that, its value is by default (metrics.horiAdvance,0), unless you specify `FT_LOAD_VERTICAL` when loading the glyph image; it will then be (0,metrics.vertAdvance).
`linearHoriAdvance` | This field contains the linearly-scaled value of the glyph's horizontal advance width. Indeed, the value of `metrics.horiAdvance` that is returned in the glyph slot is normally rounded to integer pixel coordinates (i.e., it will be a multiple of 64) by the font driver used to load the glyph image. `linearHoriAdvance` is a 16.16 fixed float number that gives the value of the original glyph advance width in 1/65536th of pixels. It can be used to perform pseudo device-independent text layouts.
`linearVertAdvance` | This is the same thing as `linearHoriAdvance` for the glyph's vertical advance height. Its value is only reliable if the font face contains vertical metrics.
### 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()` afterwards. Each time you load a new glyph image, the previous one is erased from the glyph slot. There are times, however, where you may need to extract this image from the glyph slot. For example, you want to cache images within your application, or you want to apply additional transformations and measures on it before converting it to a bitmap. The FreeType 2 API has a specific extension which is capable of dealing with glyph images in a flexible and generic way. To use it, you first need to include the `ftglyph.h` header file:
        #include <freetype/ftglyph.h>
We will now explain how to use the functions defined in this file. #### 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;    /* handle to glyph image */

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

        error = FT_Get_Glyph( face->glyph, &glyph );
        if ( error ) { .... }
As can be seen, we have created a variable, named `glyph`, of type `FT_Glyph`. This is a handle (pointer) to an individual glyph image, loaded the glyph image normally in the face's glyph slot. We did not use `FT_LOAD_RENDER` because we want to grab a scalable glyph image in order to transform it later, copied 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 will really be 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 float 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 will need to destroy them yourself, instead of relying on `FT_Done_FreeType()` to do all the clean-up. #### b. Transforming & copying the glyph image If the glyph image is scalable (i.e., if `glyph->format` is not `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()`. Here some example code:
        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 pixels */
        delta.y =   50 * 64;

        FT_Glyph_Transform( glyph, 0, &delta );

        /* transform glyph2 (horizontal shear) */
        matrix.xx = 0x10000L;
        matrix.xy = 0;
        matrix.yx = 0.12 * 0x10000L;
        matrix.yy = 0x10000L;

        FT_Glyph_Transform( glyph2, &matrix, 0 );
Note that the 2x2 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), using the `FT_Glyph_Get_CBox()` function:
        FT_BBox  bbox;

        ...
        FT_Glyph_Get_CBox( glyph, bbox_mode, &bbox );
Coordinates are relative to the glyph origin, i.e. (0,0), using the Y upwards convention. This function takes a special argument, `bbox_mode`, to indicate how box coordinates are expressed. If `bbox_mode` is set to `ft_glyph_bbox_subpixels`, the coordinates are returned in 26.6 pixels (i.e. 1/64th of pixels). Note that the box's maximum coordinates are exclusive, which means that you can always compute the width and height of the glyph image, be it in integer or 26.6 pixels with
        width  = bbox.xMax - bbox.xMin;
        height = bbox.yMax - bbox.yMin;
Note also that for 26.6 coordinates, if `ft_glyph_bbox_gridfit` is set in `bbox_mode`, the coordinates will also be grid-fitted, which corresponds to
        bbox.xMin = FLOOR(bbox.xMin)
        bbox.yMin = FLOOR(bbox.yMin)
        bbox.xMax = CEILING(bbox.xMax)
        bbox.yMax = CEILING(bbox.yMax)
The default value for `bbox_mode` is `ft_glyph_bbox_pixels` (i.e. integer, grid-fitted pixel coordinates). Please check the API reference of `FT_Glyph_Get_CBox()` for other possible values. #### 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:
        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 orig. image == true */
Some details on this function's parameters: 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 will point to a new glyph object that contains the rendered bitmap. The second parameter is a standard render mode that is used to specify what kind of bitmap we want. 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 2d vector that is used to translate the source glyph image before the conversion. Note that the source image will be translated back to its original position (and will thus be left unchanged) after the call. If you do not need to translate the source glyph before rendering, set this pointer to 0. The last parameter is a Boolean to indicate whether the source glyph object should be destroyed by the function. By default, the original glyph object is never destroyed, even if its handle is lost (it's up to client applications to keep it). The new glyph object always contains a bitmap (when no error is returned), and you must typecast its handle to the `FT_BitmapGlyph` type in order to access its contents. This type is a sort of subclass of `FT_Glyph` that contains additional fields:
`left` | Just like the `bitmap_left` field of a glyph slot, this is the horizontal distance from the glyph origin (0,0) to the left-most pixel of the glyph bitmap. It is expressed in integer pixels.
`top` | Just like the `bitmap_top` field of a glyph slot, this is the vertical distance from the glyph origin (0,0) to the top-most pixel of the glyph bitmap (more exactly, to the pixel just above the bitmap). This distance is expressed in integer pixels, and is positive for upwards Y.
`bitmap` | This is a bitmap descriptor for the glyph object, just like the `bitmap` field in a glyph slot.
### 3. Global glyph metrics Unlike glyph metrics, global ones are used to describe distances and features of a whole font face. They can be expressed either in 26.6 pixel format or in design 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 device space, according to the rules described in the last chapter of this part of the tutorial. You can access them directly as fields of an `FT_Face` handle. However, you need to check that the font face's format is scalable before using them. This can be done with the macro `FT_IS_SCALABLE(face)` which returns true if we have a scalable format. In this case, you can access the global design metrics as
`units_per_EM` | This is the size of the EM square for the font face. It is used by scalable formats to scale design coordinates to device pixels, as described by the last chapter of this part. Its value usually is 2048 (for TrueType) or 1000 (for Type 1), but other values are possible too. It is set to 1 for fixed-size formats like FNT/FON/PCF/BDF.
`global_bbox` | The global bounding box is defined as the largest rectangle that can enclose all the glyphs in a font face. It is defined for horizontal layouts only. This is not necessarily the smallest bounding box which is possible.
`ascender` | The ascender is the vertical distance from the horizontal baseline to the height of the highest character in a font face. Unfortunately, font formats define the ascender differently. For some, it represents the ascent of all capital Latin characters, without accents, for others it is the ascent of the highest accented character, and finally, other formats define it as being equal to `global_bbox.yMax`.
`descender` | The descender is the vertical distance from the horizontal baseline to the depth of the lowest character in a font face. Unfortunately, font formats define the descender differently. For some, it represents the descent of all capital Latin characters, without accents, for others it is the ascent of the lowest accented character, and finally, other formats define it as being equal to `global_bbox.yMin`. This field is usually negative.
`text_height` | This field is used to compute a default line spacing (i.e. the baseline-to-baseline distance) when writing text with this font. Note that it usually is larger than the sum of the ascender and descender taken as absolute values. There is also no guarantee that no glyphs can extend above or below subsequent baselines when using this distance.
`max_advance_width` | This field gives the maximum horizontal cursor advance for all glyphs in the font. It can be used to quickly compute the maximum advance width of a string of text. It doesn't correspond to the maximum glyph image width!
`max_advance_height` | Same as `max_advance_width` but for vertical text layout. It is only available in fonts providing vertical glyph metrics.
`underline_position` | When displaying or rendering underlined text, this value corresponds to the vertical position, relative to the baseline, of the underline bar. It normally is negative (as it is below the baseline).
`underline_thickness` | When displaying or rendering underlined text, this value corresponds to the vertical thickness of the underline.
Notice how, unfortunately, the values of the ascender and the descender are not reliable (due to various discrepancies in font formats). #### b. Scaled global metrics Each size object also contains scaled versions of some of the global metrics described above. They can be accessed directly through the `face->size->metrics` structure. Note that these values correspond to scaled versions of the design global metrics, with no rounding/grid-fitting performed. 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.
`ascender` | This is the scaled version of the original design ascender.
`descender` | This is the scaled version of the original design descender.
`height` | The scaled version of the original design text height. This is probably the only field you should really use in this structure.
`max_advance` | This is the scaled version of the original design max advance.
Note that the `face->size->metrics` structure contains other fields that are used to scale design coordinates to device space. They are described below. #### 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. Basically, it means that when the glyph for an "A" is followed by the glyph for a "V", the space between them 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 the horizontal direction in nearly all cases except really extreme ones. Not all font formats contain kerning information. Instead, they sometimes rely on an additional file that contains various glyph metrics, including kerning, but no glyph images. A good example would be the Type 1 format, where glyph images are stored in a file with extension `.pfa` or `.pfb`, and where kerning metrics can be found in an additional file 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. For example, you could open a Type 1 font by doing the following:
        error = FT_New_Face( library,
                             "/usr/shared/fonts/cour.pfb",
                             0,
                             &face );
        if ( error ) { ... }

        error = FT_Attach_File( face, "/usr/shared/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 a `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 data is entirely font format specific. FreeType 2 allows you to retrieve the kerning information between two glyphs through the `FT_Get_Kerning()` function, whose interface looks like
        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         */
As can be seen, the function takes a handle to a face object, the indices of the left and right glyphs for which the kerning value is desired, as well as 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 `bbox_mode` described in a previous part. It is an enumeration value that indicates how the kerning distances are expressed in the target vector. The default value `ft_kerning_mode_default` (which has value 0) 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_mode_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_mode_unscaled` is used to return the design kerning distance, expressed in font units. You can later scale it to device space using the computations explained in the last chapter 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 text, or when writing right-to-left text. ### 4. Simple text rendering: kerning + centering In order to show off what we have just learned, we will now modify the example code that was provided in the first part 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 need to retrieve the kerning distance between two glyphs in order to alter the pen position appropriately. The code looks like
        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_mode_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. Notice that As kerning is determined from glyph indices, we need to explicitly convert our character code into a glyph index, then later call `FT_Load_Glyph()` instead of `FT_Load_Char()`. We use a Boolean named `use_kerning` which is set with the result of the macro `FT_HAS_KERNING(face)`. It is certainly faster not to call `FT_Get_Kerning()` if 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 did initialize the variable `previous` with the value 0, which always correspond 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 set `delta` to (0,0) when an error occurs. #### b. Centering Our code becomes more interesting but it is still a bit too simple for normal uses. For example, the position of the pen is determined before we do the rendering if in a real-life situation; you would want to layout the text and measure it before computing its final position (e.g. centering) or perform things like word-wrapping. As a consequence we are now going to decompose our text rendering function into two distinct but successive parts: The first one will position individual glyph images on the baseline, while the second one will render the glyphs. As will be shown, this has many advantages. We start by storing individual glyph images, as well as their position on the baseline. This can be done with code like
        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_mode_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.  DO NOT RENDER IT! */
          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++;
        }
As you see, this is a very slight variation of our previous code where we extract each glyph image from the slot, and 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 like
        void  compute_string_bbox( FT_BBox*  abbox )
        {
          FT_BBox  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_BBox  glyph_bbox;

            FT_Glyph_Get_CBox( glyphs[n], &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 &gy; 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 can then be used to compute the final pen position before rendering the string as in:
        /* compute string dimensions in integer pixels */
        string_width  = ( string_bbox.xMax - string_bbox.xMin ) / 64;
        string_height = ( string_bbox.yMax - string_bbox.yMin ) / 64;

        /* compute start pen position in 26.6 cartesian pixels */
        start_x = ( ( my_target_width  - string_width  ) / 2 ) * 64;
        start_y = ( ( my_target_height - string_height ) / 2 ) * 64;

        for ( n = 0; n < num_glyphs; n++ )
        {
          FT_Glyph  image;
          FT_Vector pen;

          image = glyphs[n];

          pen.x = start_x + pos[n].x;
          pen.y = start_y + pos[n].y;

          error = FT_Glyph_To_Bitmap( &image, ft_render_mode_normal,
                                      &pen.x, 0 );
          if ( !error )
          {
            FT_BitmapGlyph  bit = (FT_BitmapGlyph)image;

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

            FT_Done_Glyph( image );
          }
        }
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 an `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 + centering + 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. We will start by performing a few minor improvements: #### a. packing & translating glyphs We start by packing the information related to a single glyph image into a single structure instead of parallel arrays. We thus define the following structure type:
        typedef struct  TGlyph_
        {
          FT_UInt    index;    /* glyph index                  */
          FT_Vector  pos;      /* glyph origin on the baseline */
          FT_Glyph   image;    /* glyph image                  */

         } TGlyph, *PGlyph;
We will also translate each glyph image directly after it is loaded to its position on the baseline at load time, which has several advantages. Our glyph sequence loader thus becomes:
        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;
Translating glyphs now has several advantages, as mentioned earlier. The first one is that we don't need to translate the glyph bounding box when we compute the string's bounding box. The code becomes:
        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], &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 &gy; 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;
        }
`compute_string_bbox()` can now compute the bounding box of a transformed glyph string. For example, we can do something like
        FT_BBox    bbox;
        FT_Matrix  matrix;
        FT_Vector  delta;

        ... load glyph sequence

        ... setup "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 useful idea if we want to reuse them in order to draw the text string with various angles or transforms. It is better to perform the affine transformation just before the glyph is rendered, as in the following code:
        FT_Vector  start;
        FT_Matrix  transform;

        /* 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 transformation (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 );

        for ( n = 0; n < num_glyphs; n++ )
        {
          FT_Glyph  image;
          FT_Vector pen;
          FT_BBox   bbox;

          /* 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, &start );

          /* check bounding box -- if the transformed glyph image */
          /* is not in our target surface, we can avoid rendering */
          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( bitmap->bitmap,
                            bitmap->left,
                            my_target_height - bitmap->top );

            FT_Done_Glyph( image );
          }
        }
There are a few changes compared to the previous version of this code: We keep the original glyph images untouched, by transforming a copy. We perform clipping computations in order to avoid rendering & 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 destroyed even when 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 in the call to `FT_Glyph_Transform()` intead 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 a 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 consumption. For example, let us assume that our text string to render 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). ### 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 nominal 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 a `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 device space, and how to read glyph outlines and metrics in font units. This is important for a number of things: to perform "true" WYSIWYG text layout. to access font data for conversion or analysis purposes #### a. Scaling distances to device space Design coordinates are scaled to 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(face)` macro, which returns true when the font is scalable. When you call the function `FT_Set_Pixel_Sizes()`, you are specifying the value of `pixel_size_x` and `pixel_size_y`; FreeType will then 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, then the 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. These fields are:
`x_ppem` | This is the size in integer pixels of the EM square, which also is the horizontal character pixel size, called `pixel_size_x` in the above example. `x_ppem` means "x pixels per EM".
`y_ppem` | This is the size in integer pixels of the EM square, which also is the vertical character pixel size, called `pixel_size_y` in the above example. `y_ppem` means "y pixels per EM".
`x_scale` | This is a 16.16 fixed float scale that is used to directly scale horizontal distances from design space to 1/64th of device pixels.
`y_scale` | This is a 16.16 fixed float scale that is used to directly scale vertical distances from design space to 1/64th of device pixels.
You can scale a distance expressed in font units to 26.6 pixels directly with the help of the `FT_MulFix()` function, as in:
        /* 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 );
However, you can also scale the value directly with more accuracy by using doubles and the equations:
        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 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 then all be in font units. Unscaled kerning data can be retrieved 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 tutorial part. ### Conclusion This is the end of the second part of the FreeType 2 tutorial; you are now able to access glyph metrics, manage glyph images, and render text much more intelligently (kerning, measuring, transforming & caching). With this knowledge you can build a pretty decent text service on top of FreeType 2, and you could possibly stop there if you want. The next section will deal with FreeType 2 internals (like modules, vector outlines, font drivers, renderers), as well as a few font format specific issues (mainly, how to access certain TrueType or Type 1 tables).