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.
FT_Size_Metrics structure.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.
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).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.linearHoriAdvance
              but for the glyph's vertical advance height.  Its value
              is only reliable if the font face contains vertical
              metrics.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
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.
glyph, of
              type FT_Glyph.
              This is a handle (pointer) to an individual glyph
              image.FT_LOAD_RENDER
              because we want to grab a scalable glyph image that we
              can transform later on.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.
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.
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 of
            pseudo-code.
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
            it with the possible exception of rotated glyphs.]
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.
FT_RENDER_MODE_DEFAULT for an 8-bit
              anti-aliased pixmap, or FT_RENDER_MODE_MONO
              for a 1-bit monochrome bitmap.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).
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.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 field in a glyph slot.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.
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 an 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.
bbox.yMax.bbox.yMin.  This field is
              negative for values below the baseline.max_advance_width but for
              vertical text layout.Notice that the values of the ascender and the descender are not reliable (due to various discrepancies in font formats), unfortunately.
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 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 but
            rounded for historical reasons.
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. It is rounded to an integer value.
Be careful not to confuse it with the
                ‘height’ field in
                the FT_Glyph_Metrics
                structure.
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.
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.
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.
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.
FT_Load_Glyph
              instead of FT_Load_Char.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.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.FT_Get_Kerning.  This is because the
              function always sets the content of delta
              to (0,0) if an error occurs.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
            (cf. dropout
            control).
/* 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.
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.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.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.
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.
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 );
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.
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).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.
Another very useful demo program
            is ftdiff.c,
            demonstrating the use and the optical results of the
            various rendering and hinting modes provided by FreeType.
            In particular, it also demonstrates how to do sub-pixel
            positioning (for unhinted glyphs and ‘light’
            hinting mode) – all code in this tutorial assumes
            integer coordinates.
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.
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 integer values
            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.  Here, the scaling
            factors can correspond to fractional ppem values.
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.
pixel_size_x in the above
              example.pixel_size_y in the above
              example.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 by using doubles.
FT_Size_Metrics* metrics = &face->size->metrics; /* shortcut */ double pixels_x, pixels_y; double x_scale, y_scale; /* compute floating point scale factors */ x_scale = face->size->metrics.x_scale / 65536.0; y_scale = face->size->metrics.y_scale / 65536.0; /* convert design distances to floating point pixels */ pixels_x = design_x * x_scale; pixels_y = design_y * y_scale;
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.
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.
Last update: 13-Feb-2018