FreeType Tutorial / II

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.

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. Be careful not to confuse it with the ‘height’ field in the FT_Size_Metrics structure.

horiBearingX

For horizontal text layouts, this is the horizontal distance from the current cursor position to the leftmost 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 topmost border of the glyph image's bounding box.

horiAdvance

For horizontal text layouts, this is the horizontal distance 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 leftmost 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 topmost 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 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.

advance

This field is a FT_Vector that holds the transformed advance for the glyph. That is useful when you are using a transformation through FT_Set_Transform, as shown in the rotated text example of part I. Other than that, its value is by default (metrics.horiAdvance,0), unless you specify FT_LOAD_VERTICAL when loading the glyph image; it is then (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., being a multiple of 64) by the font driver that actually loads the glyph image. linearHoriAdvance is a 16.16 fixed-point number that gives the value of the original glyph advance width in 1/65536th of pixels. It can be use to perform pseudo device-independent text layouts.

linearVertAdvance

This is the similar to linearHoriAdvance but 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. 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).

left

Just like the bitmap_left field of a glyph slot, this is the horizontal distance from the glyph origin (0,0) to the leftmost 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 topmost pixel of the glyph bitmap (more precise, 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 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.

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 in the last section of this tutorial part. Its value usually is 2048 (for TrueType) or 1000 (for Type 1 or CFF), but other values are possible, too. It is set to 1 for fixed-size formats like FNT, FON, PCF, or BDF.

bbox

The global bounding box is defined as the smallest rectangle that can enclose all the glyphs in a font face.

ascender

The ascender is the vertical distance from the horizontal baseline to the highest ‘character’ coordinate in a font face. Unfortunately, font formats don't define the ascender in a uniform way. For some formats, 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 bbox.yMax.

descender

The descender is the vertical distance from the horizontal baseline to the lowest ‘character’ coordinate in a font face. Unfortunately, font formats don't define the descender in a uniform way. For some formats, 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 bbox.yMin. This field is negative for values below the baseline.

height

This field represents 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 extend above or below subsequent baselines when using this distance – think of it as a value the designer of the font finds appropriate.

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.

underline_position

When displaying or rendering underlined text, this value corresponds to the vertical position, relative to the baseline, of the underline bar's center. It is negative if it is below the baseline.

underline_thickness

When displaying or rendering underlined text, this value corresponds to the vertical thickness of the underline.

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.

ascender

The scaled version of the original design ascender.

descender

The scaled version of the original design descender.

height

The scaled version of the original design text height (the vertical distance from one baseline to the next). This is probably the only field you should really use in this structure.

Be careful not to confuse it with the ‘height’ field in the FT_Glyph_Metrics structure.

max_advance

The scaled version of the original design maximum advance.

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.

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

/* 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, 0 );
  if ( !error )
  {
    FT_BitmapGlyph  bit = (FT_BitmapGlyph)image;


    my_draw_bitmap( bit->bitmap,
                    bit->left,
                    my_target_height - bit->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 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.

x_ppem

The field name stands for ‘x pixels per EM’; this is the horizontal size in integer pixels of the EM square, which also is the horizontal character pixel size, called pixel_size_x in the above example.

y_ppem

The field name stands for ‘y pixels per EM’; this is the vertical size in integer pixels of the EM square, which also is the vertical character pixel size, called pixel_size_y in the above example.

x_scale

This is a 16.16 fixed-point scale to directly scale horizontal distances from design space to 1/64th of device pixels.

y_scale

This is a 16.16 fixed-point scale 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 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.