  
  
  
  
  FreeType 2 Tutorial
  Step 2 — managing glyphs
  © 2003 David Turner
    (
mailto:david@freetype.orgdavid@freetype.org )  © 2003 The FreeType Development Team
    (
http://www.freetype.orgwww.freetype.org )
  
  
    Introduction
  
  
This is the second section of the FreeType 2 tutorial. It describes
  how to
  
    
retrieve glyph metrics    
easily manage glyph images    
retrieve global metrics (including kerning)    
render a simple string of text, with kerning    
render a centered string of text (with kerning)    
render a transformed string of text (with centering)    
access metrics in design font units when needed,
        and how to scale them to device space
  
    
    
      1. Glyph metrics
    
    
Glyph metrics are, as their name suggests, certain distances
    associated with 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 (Latin, Cyrillic, Arabic,
    Hebrew, etc.), and those used to layout the glyph in vertical text
    layouts (Chinese, Japanese, Korean, 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 is true when 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 ../reference/ft2-base_interface.html#FT_Glyph_Metrics    
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.
      
    
    
      
        
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 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 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.
      
    
    
    
    
NOTE: As not all fonts do contain vertical
    metrics, the values of 
vertBearingX, vertBearingY and
    
vertAdvance should not be considered reliable when
    
FT_HAS_VERTICAL is false.
    
The following graphics illustrate the metrics more clearly.  First,
    for horizontal metrics, where the baseline is the horizontal axis:
    
      
horizontal layout    
    
For vertical text layouts, the baseline is vertical, identical to the
    vertical axis:
    
      
vertical layout    
    
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 will be expressed
    in original font units.
    
The glyph slot object has also a few other interesting fields that
    will ease 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 which holds the transformed
        advance for the glyph.  That is useful when you are using a transform
        through 
FT_Set_Transform, as shown in the rotated text
        example of section I.  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 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 which is capable of
    dealing with glyph images in a flexible and generic way.  To use it, you
    first need to include the 
../reference/ft2-header_file_macros.html#FT_GLYPH_H    
FT_GLYPH_H  header file, as in:
    
  #include FT_GLYPH_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; 
/* 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 ) { ... }
      
      
As you see, we have:
      
        
          
Created a variable, named glyph, of type ../reference/ft2-glyph_management.html#FT_Glyph          
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 later transform it.
        
        
          
Copy the glyph image from the slot into a new FT_Glyph          object, by calling 
../reference/ft2-glyph_management.html#FT_Get_Glyph          
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 than 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 
../reference/ft2-glyph_management.html#FT_Done_Glyph      
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 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 
../reference/ft2-glyph_management.html#FT_Glyph_Transform      
FT_Glyph_Transform .
      
You can also copy a single glyph image with ../reference/ft2-glyph_management.html#FT_Glyph_Copy      
FT_Glyph_Copy .  Here is 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 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 transform 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 
../reference/ft2-glyph_management.html#FT_Glyph_Get_CBox      
FT_Glyph_Get_CBox  function, as in:
      
  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,
      be in 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 used as the 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 )
      
      
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.
      
        d. Converting the glyph image to a bitmap
      
      
You may need to convert the glyph object to a bitmap once you have
      convienently cached or transformed it.  This can be done easily with
      the 
../reference/ft2-glyph_management.html      
FT_Glyph_To_Bitmap  function. It is in charge of
      converting any glyph object into a bitmap, as in:
      
  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 its 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 two-dimensional 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 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 
../reference/ft2-glyph_management.html#FT_BitmapGlyphRec      
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 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 the device space, according to
      the rules described in the last chapter of this section of the
      tutorial.  You can access them directly as simple 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 by using the macro
      
FT_IS_SCALABLE which returns true when appropriate.
      
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 in the last chapter of this section.  Its
          value usually is 2048 (for TrueType) or 1000 (for Type 1),
          but others 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.
        
      
      
        
          
ascender        
        
          
The ascender is the vertical distance from the horizontal
          baseline to the highest ‘character’ coordinate 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 lowest ‘character’ coordinate 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
          negative for values below the baseline.
        
      
      
        
          
text_height        
        
          
This field is simply 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 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 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 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 a 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        
        
          
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.  This
          is probably the only field you should really use in this
          structure.
        
      
      
        
          
max_advance        
        
          
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 in the last chapter.
      
        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 is
      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 ../reference/ft2-base_interface.html#FT_Attach_File      
FT_Attach_File  and ../reference/ft2-base_interface.html#FT_Attach_Stream      
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 content is entirely font format specific.
      
FreeType 2 allows you to retrieve the kerning information for
      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 you see, the function takes a handle to a face object, the
      indices of the left and right glyph 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 the bbox mode      described in a previous chapter.  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 is used 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 chapter of this section.
      
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 simply when
      writing right-to-left text.
    
    
      4. Simple text rendering: kerning + centering
    
    
In order to show off what we just learned, we will now demonstrate
    how to modify the example code that was provided in section 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.  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.  Some notes:
      
        
          
As kerning is determined from glyph indices, we need to
          explicitely convert our character codes into a glyph indices, 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.  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) when 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
      layout 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 will position individual glyph
      images on the baseline, while the second one will render the glyphs. 
      As we will see, this has many advantages.
      
We will thus 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_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 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], 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 as in:
      
  
/* 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 typecasted 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.  We could also decide to implement word wrapping, and only
      draw
    
    
      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 also translate each glyph image directly after it is loaded to
      its position on the baseline at load time.  As we will see, this as
      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;
      
      
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.  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 > 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;
  }
      
      
Now take a closer look: The compute_string_bbox function
      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
      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, 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 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 );
  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 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( bitmap->bitmap,
                      bitmap->left,
                      my_target_height - bitmap->top );
      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
          & 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 instead
          of 
FT_Glyph_To_Bitmap.
        
      
      
It is possible to call this function several times to render the
      string width 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&rsquo'.  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 chapter 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 when
      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.  These fields
      are:
      
      
      
        
          
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 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 pixel
      format 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:
      
  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
      chapter 3 of this section.
    
    
      Conclusion
    
    
This is the end of the second section 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).
    
You have now sufficient knowledge to build a pretty decent text
    service on top of FreeType 2, and you could possibly stop here 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).  [This section has not been written yet.]
  
step1.htmlFreeType 2 Tutorial Step 1 
