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Code Editor : step2.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
 "http://www.w3.org/TR/html4/loose.dtd">

<title>FreeType Tutorial / II</title>
</head>

<body>

<div id="top"
 class="bar">
 <h1><a href="http://freetype.org/index.html">FreeType</a>
 Tutorial&nbsp;/&nbsp;II</h1>
</div>

<div id="simple-glyph-loading">
 <h2>II. Managing Glyphs</h2>

<h3 id="section-1">1. Glyph Metrics</h3>

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 <a href="../reference/ft2-base_interface.html#FT_HAS_VERTICAL"><code>FT_HAS_VERTICAL</code></a>,
 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 <code>face-&gt;glyph-&gt;metrics</code>
 structure, whose type
 is <a href="../reference/ft2-base_interface.html#FT_Glyph_Metrics"><code>FT_Glyph_Metrics</code></a>.
 We will discuss this in more detail below; for now, we
 only note that it contains the following fields.

<dl>
 <dt>width</dt>
 <dd>This is the width of the glyph image's bounding box.
 It is independent of the layout direction.</dd>

<dt>height</dt>
 <dd>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 &lsquo;height&rsquo; field in
 the <a href="../reference/ft2-base_interface.html#FT_Size_Metrics">
 <code>FT_Size_Metrics</code></a> structure.</dd>

<dt>horiBearingX</dt>
 <dd>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.</dd>

<dt>horiBearingY</dt>
 <dd>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.</dd>

<dt>horiAdvance</dt>
 <dd>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.</dd>

<dt>vertBearingX</dt>
 <dd>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.</dd>

<dt>vertBearingY</dt>
 <dd>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.</dd>

<dt>vertAdvance</dt>
 <dd>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.</dd>
 </dl>

As not all fonts do contain vertical
 metrics, the values of <code>vertBearingX</code>,
 <code>vertBearingY</code> and <code>vertAdvance</code>
 should not be considered reliable
 if <code>FT_HAS_VERTICAL</code> 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, <code>bearingX</code> shows a negative value in
 this image.

The metrics found
 in <code>face-&gt;glyph-&gt;metrics</code> are normally
 expressed in 26.6 pixel format (i.e., 1/64th of pixels),
 unless you use the <code>FT_LOAD_NO_SCALE</code> flag when
 calling <code>FT_Load_Glyph</code>
 or <code>FT_Load_Char</code>. 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 <code>face-&gt;glyph-&gt;xxx</code>,
 where <code>xxx</code> is one of the following fields.

<dl>
 <dt>advance</dt>
 <dd>This field is a <code>FT_Vector</code> that holds the
 transformed advance for the glyph. That is useful when
 you are using a transformation
 through <code>FT_Set_Transform</code>, as shown in the
 <a href="step1.html#transformed-text">rotated text
 example</a> of part&nbsp;I. Other than that, its value
 is by default (metrics.horiAdvance,0), unless you
 specify <code>FT_LOAD_VERTICAL</code> when loading the
 glyph image; it is then (0,metrics.vertAdvance).</dd>

<dt>linearHoriAdvance</dt>
 <dd>This field contains the linearly scaled value of the
 glyph's horizontal advance width. Indeed, the value of
 <code>metrics.horiAdvance</code> that is returned in the
 glyph slot is normally rounded to integer pixel
 coordinates (i.e., being a multiple of&nbsp;64) by the
 font driver that actually loads the glyph
 image. <code>linearHoriAdvance</code> 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.</dd>

<dt>linearVertAdvance</dt>
 <dd>This is the similar to <code>linearHoriAdvance</code>
 but for the glyph's vertical advance height. Its value
 is only reliable if the font face contains vertical
 metrics.</dd>
 </dl>

<h3 id="section-2">2. Managing Glyph Images</h3>

The glyph image that is loaded in a glyph slot can be
 converted into a bitmap, either by
 using <code>FT_LOAD_RENDER</code> when loading it, or by
 calling <a href="../reference/ft2-base_interface.html#FT_Render_Glyph"><code>FT_Render_Glyph</code></a>.
 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&nbsp;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 <a href="../reference/ft2-header_file_macros.html#FT_GLYPH_H"><code>FT_GLYPH_H</code></a>
 header file.

<pre>
#include FT_GLYPH_H</pre>

<h4>a.Extracting the Glyph Image</h4>

You can extract a single glyph image very easily. Here
 some code that shows how to do it.

<pre>
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-&gt;glyph, &amp;glyph );
if ( error ) { ... }</pre>

The following steps are performed.

<ul>
 <li>Create a variable named <code>glyph</code>, of
 type <a href="../reference/ft2-glyph_management.html#FT_Glyph"><code>FT_Glyph</code></a>.
 This is a handle (pointer) to an individual glyph
 image.</li>

<li>Load the glyph image in the normal way into the face's
 glyph slot. We don't use <code>FT_LOAD_RENDER</code>
 because we want to grab a scalable glyph image that we
 can transform later on.</li>

<li>Copy the glyph image from the slot into a
 new <code>FT_Glyph</code> object by
 calling <a href="../reference/ft2-glyph_management.html#FT_Get_Glyph"><code>FT_Get_Glyph</code></a>.
 This function returns an error code and
 sets <code>glyph</code>.</li>
 </ul>

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 <code>glyph-&gt;format</code> if you want to know
 exactly how the glyph is modeled and stored.

A new glyph object can be destroyed with a call
 to <a href="../reference/ft2-glyph_management.html#FT_Done_Glyph"><code>FT_Done_Glyph</code></a>.

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 <code>glyph-&gt;advance</code>

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 <code>FT_Done_FreeType</code> doing
 all the clean-up.

<h4>b. Transforming &amp; Copying the Glyph Image</h4>

If the glyph image is scalable (i.e.,
 if <code>glyph-&gt;format</code> is not equal
 to <code>FT_GLYPH_FORMAT_BITMAP</code>), it is possible to
 transform the image anytime by a call
 to <a href="../reference/ft2-glyph_management.html#FT_Glyph_Transform"><code>FT_Glyph_Transform</code></a>.

You can also copy a single glyph image
 with <a href="../reference/ft2-glyph_management.html#FT_Glyph_Copy"><code>FT_Glyph_Copy</code></a>.

<pre>
FT_Glyph glyph, glyph2;
FT_Matrix matrix;
FT_Vector delta;

... load glyph image in `glyph' ...

/* copy glyph to glyph2 */

error = FT_Glyph_Copy( glyph, &amp;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, &amp;delta );

/* transform glyph2 (horizontal shear) */

matrix.xx = 0x10000L;
matrix.xy = 0.12 * 0x10000L;
matrix.yx = 0;
matrix.yy = 0x10000L;

FT_Glyph_Transform( glyph2, &amp;matrix, 0 );</pre>

Note that the 2&times;2 transformation matrix is always
 applied to the 16.16 advance vector in the glyph; you thus
 don't need to recompute it.

<h4>c. Measuring the Glyph Image</h4>

You can also retrieve the control (bounding) box of any
 glyph image (scalable or not) through
 the <a href="../reference/ft2-glyph_management.html#FT_Glyph_Get_CBox"><code>FT_Glyph_Get_CBox</code></a>
 function.

<pre>
FT_BBox bbox;

...
FT_Glyph_Get_CBox( glyph, bbox_mode, &amp;bbox );</pre>

Coordinates are relative to the glyph origin (0,0), using
 the y&nbsp;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 <code>FT_LOAD_NO_SCALE</code>, <code>bbox_mode</code>
 must be set to <code>FT_GLYPH_BBOX_UNSCALED</code> to get
 unscaled font units in 26.6 pixel format. The
 value <code>FT_GLYPH_BBOX_SUBPIXELS</code> 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.

<pre class="example">
width = bbox.xMax - bbox.xMin;
height = bbox.yMax - bbox.yMin;</pre>

Note also that for 26.6 coordinates, if
 <code>FT_GLYPH_BBOX_GRIDFIT</code> is used as the bbox
 mode, the coordinates are also grid-fitted, which
 corresponds to the following four lines of
 pseudo-code.

<pre class="example">
bbox.xMin = FLOOR( bbox.xMin )
bbox.yMin = FLOOR( bbox.yMin )
bbox.xMax = CEILING( bbox.xMax )
bbox.yMax = CEILING( bbox.yMax )</pre>

To get the bbox in integer pixel coordinates,
 set <code>bbox_mode</code>
 to <code>FT_GLYPH_BBOX_TRUNCATE</code>.

Finally, to get the bounding box in grid-fitted pixel
 coordinates, set <code>bbox_mode</code>
 to <code>FT_GLYPH_BBOX_PIXELS</code>.

[Computing exact bounding boxes can be done with
 function <a href="../reference/ft2-outline_processing.html#FT_Outline_Get_BBox"><code>FT_Outline_Get_BBox</code></a>,
 at the cost of slower execution. You probably don't need
 it with the possible exception of rotated glyphs.]

<h4>d. Converting the Glyph Image to a Bitmap</h4>

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 <a href="../reference/ft2-glyph_management.html"><code>FT_Glyph_To_Bitmap</code></a>
 function, which handles any glyph object.

<pre>
FT_Vector origin;

origin.x = 32; /* 1/2 pixel in 26.6 format */
origin.y = 0;

error = FT_Glyph_To_Bitmap(
 &amp;glyph,
 render_mode,
 &amp;origin,
 1 ); /* destroy original image == true */</pre>

Some notes.

<ul>
 <li>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.</li>

<li>The second parameter is a standard render mode to
 specify what kind of bitmap we want. For example, it
 can be <code>FT_RENDER_MODE_DEFAULT</code> for an 8-bit
 anti-aliased pixmap, or <code>FT_RENDER_MODE_MONO</code>
 for a 1-bit monochrome bitmap.</li>

<li>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.</li>

<li>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).</li>
 </ul>

The new glyph object always contains a bitmap (if no
 error is returned), and you must typecast its
 handle to the <code>FT_BitmapGlyph</code> type in order to
 access its content. This type is a sort of
 &lsquo;subclass&rsquo; of <code>FT_Glyph</code> that
 contains additional fields
 (see <a href="../reference/ft2-glyph_management.html#FT_BitmapGlyphRec"><code>FT_BitmapGlyphRec</code></a>).

<dl>
 <dt>left</dt>
 <dd>Just like the <code>bitmap_left</code> 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.</dd>

<dt>top</dt>
 <dd>Just like the <code>bitmap_top</code> 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&nbsp;y.</dd>

<dt>bitmap</dt>
 <dd>This is a bitmap descriptor for the glyph object, just
 like the <code>bitmap</code> field in a glyph slot.</dd>
 </dl>

<h3 id="section-3">3. Global Glyph Metrics</h3>

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.

<h4> a. Design global metrics</h4>

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 <code>FT_Face</code> handle.

However, you need to check that the font face's format is
 scalable before using them. One can do it with
 macro <code>FT_IS_SCALABLE</code>, which returns true when
 appropriate.

Here a table of the global design metrics for scalable
 faces.

<dl>
 <dt>units_per_EM</dt>
 <dd>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&nbsp;1 or CFF),
 but other values are possible, too. It is set to&nbsp;1
 for fixed-size formats like FNT, FON, PCF, or BDF.</dd>

<dt>bbox</dt>
 <dd>The global bounding box is defined as the smallest
 rectangle that can enclose all the glyphs in a font
 face.</dd>

<dt>ascender</dt>
 <dd>The ascender is the vertical distance from the
 horizontal baseline to the highest
 &lsquo;character&rsquo; 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 <code>bbox.yMax</code>.</dd>

<dt>descender</dt>
 <dd>The descender is the vertical distance from the
 horizontal baseline to the lowest
 &lsquo;character&rsquo; 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 <code>bbox.yMin</code>. This field is
 negative for values below the baseline.</dd>

<dt>height</dt>
 <dd>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 &ndash; think of it as a value the
 designer of the font finds appropriate.</dd>

<dt>max_advance_width</dt>
 <dd>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!</dd>

<dt>max_advance_height</dt>
 <dd>Same as <code>max_advance_width</code> but for
 vertical text layout.</dd>

<dt>underline_position</dt>
 <dd>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.</dd>

<dt>underline_thickness</dt>
 <dd>When displaying or rendering underlined text, this
 value corresponds to the vertical thickness of the
 underline.</dd>
 </dl>

Notice that the values of the ascender and the descender
 are not reliable (due to various discrepancies in font
 formats), unfortunately.

<h4>b. Scaled Global Metrics</h4>

Each size object also contains a scaled version of some
 of the global metrics described above, to be directly
 accessed through
 the <code>face-&gt;size-&gt;metrics</code> structure (of
 type <a href="../reference/ft2-base_interface.html#FT_Size_Metrics">
 <code>FT_Size_Metrics</code></a>). 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.

<dl>
 <dt>ascender</dt>
 <dd>The scaled version of the original design ascender;
 rounded up to an integer value.</dd>

<dt>descender</dt>
 <dd>The scaled version of the original design descender,
 rounded down to an integer value.</dd>

<dt>height</dt>
 <dd>
 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
 &lsquo;height&rsquo; field in
 the <a href="../reference/ft2-base_interface.html#FT_Glyph_Metrics"><code>FT_Glyph_Metrics</code></a>
 structure.
 </dd>

<dt>max_advance</dt>
 <dd>The scaled version of the original design maximum
 advance, rounded to an integer value.</dd>
 </dl>

Note that the <code>face-&gt;size-&gt;metrics</code>
 structure contains other fields that are used to scale
 design coordinates to device space. They are described in
 the last section.

<h4>c. Kerning</h4>

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 &lsquo;A&rsquo; is followed by a
 glyph for an uppercase &lsquo;V&rsquo;, the space between
 the two glyphs can be slightly reduced to avoid extra
 &lsquo;diagonal whitespace&rsquo;.

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 &lsquo;kern&rsquo;
 table. OpenType kerning via the
 &lsquo;GPOS&rsquo; table is not supported! You
 need a higher-level library
 like <a href="http://www.harfbuzz.org">HarfBuzz</a>,
 <a href="http://www.pango.org">Pango</a>,
 or <a href="http://www.icu-project.org">ICU</a>, 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&nbsp;1 format where glyph images are stored in files
 with extension <code>.pfa</code> or <code>.pfb</code>,
 while kerning metrics can be found in files with extension
 <code>.afm</code> or <code>.pfm</code>.

FreeType&nbsp;2 allows you to deal with this, by
 providing
 the <a href="../reference/ft2-base_interface.html#FT_Attach_File"><code>FT_Attach_File</code></a>
 and <a href="../reference/ft2-base_interface.html#FT_Attach_Stream"><code>FT_Attach_Stream</code></A>
 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&nbsp;1 font.

<pre>
error = FT_New_Face( library, "/usr/share/fonts/cour.pfb",
 0, &amp;face );
if ( error ) { ... }

error = FT_Attach_File( face, "/usr/share/fonts/cour.afm" );
if ( error )
{ ... could not read kerning and additional metrics ... }</pre>

Note that <code>FT_Attach_Stream</code> is similar to
 <code>FT_Attach_File</code> except that it doesn't take a
 C&nbsp;string to name the extra file but
 an <a href="../reference/ft2-system_interface.html#FT_StreamRec"><code>FT_Stream</code></a>
 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&nbsp;2 allows you to retrieve the kerning
 information between two glyphs through
 the <a href="../reference/ft2-base_interface.html#FT_Get_Kerning"><code>FT_Get_Kerning</code></a>
 function.

<pre>
FT_Vector kerning;

...
error = FT_Get_Kerning( face, /* handle to face object */
 left, /* left glyph index */
 right, /* right glyph index */
 kerning_mode, /* kerning mode */
 &amp;kerning ); /* target vector */</pre>

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 <code>FT_KERNING_DEFAULT</code>,
 which has value&nbsp;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 <code>FT_KERNING_UNFITTED</code> 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 <code>FT_KERNING_UNSCALED</code>
 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 &lsquo;left&rsquo; and &lsquo;right&rsquo;
 positions correspond to the visual order of the
 glyphs in the string of text. This is important for
 bidirectional or right-to-left text.

<h3 id="section-4">4. Simple Text Rendering: Kerning and Centering</h3>

In order to show off what we have just learned, we now
 demonstrate how to modify
 the <a href="step1.html#basic-code">example code</a> that
 was provided in part&nbsp;I to render a string of text,
 and enhance it to support kerning and delayed
 rendering.

<h4>a. Kerning Support</h4>

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.

<pre>
FT_GlyphSlot slot = face-&gt;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 &lt; 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 &amp;&amp; previous &amp;&amp; glyph_index )
 {
 FT_Vector delta;

FT_Get_Kerning( face, previous, glyph_index,
                    FT_KERNING_DEFAULT, &amp;delta );

pen_x += delta.x &gt;&gt; 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( &amp;slot-&gt;bitmap,
 pen_x + slot-&gt;bitmap_left,
 pen_y - slot-&gt;bitmap_top );

/* increment pen position */
 pen_x += slot->advance.x &gt;&gt; 6;

/* record current glyph index */
 previous = glyph_index;
}</pre>

We are done. Some notes.

<ul>
 <li>As kerning is determined by glyph indices, we need to
 explicitly convert our character codes into glyph
 indices, then later call <code>FT_Load_Glyph</code>
 instead of <code>FT_Load_Char</code>.</li>

<li>We use a boolean named <code>use_kerning</code>, which
 is set to the result of the
 macro <code>FT_HAS_KERNING</code>. It is certainly
 faster not to call <code>FT_Get_Kerning</code> when we
 know that the font face does not contain kerning
 information.</li>

<li>We move the position of the pen before a new
 glyph is drawn.</li>

<li>We initialize the variable <code>previous</code> with
 the value&nbsp;0, which always corresponds to the
 &lsquo;missing glyph&rsquo; (also
 called <code>.notdef</code> in the PostScript world).
 There is never any kerning distance associated with this
 glyph.</li>

<li>We do not check the error code returned by
 <code>FT_Get_Kerning</code>. This is because the
 function always sets the content of <code>delta</code>
 to (0,0) if an error occurs.</li>
 </ul>

<h4>b. Centering</h4>

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.

<pre>
FT_GlyphSlot slot = face-&gt;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 &lt; 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 &amp;&amp; previous &amp;&amp; glyph_index )
 {
 FT_Vector delta;

FT_Get_Kerning( face, previous, glyph_index,
                    FT_KERNING_DEFAULT, &amp;delta );

pen_x += delta.x &gt;&gt; 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-&gt;glyph, &amp;glyphs[num_glyphs] );
 if ( error )
 continue; /* ignore errors, jump to next glyph */

/* increment pen position */
 pen_x += slot->advance.x &gt;&gt; 6;

/* record current glyph index */
 previous = glyph_index;

/* increment number of glyphs */
 num_glyphs++;
}</pre>

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 <code>pen_x</code> contains the total
 advance for the string of text. We can now compute the
 bounding box of the text string with a simple
 function.

<pre>
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 &lt; num_glyphs; n++ )
 {
 FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels,
 &amp;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 &lt; bbox.xMin )
      bbox.xMin = glyph_bbox.xMin;

if ( glyph_bbox.yMin &lt; bbox.yMin )
      bbox.yMin = glyph_bbox.yMin;

if ( glyph_bbox.xMax &gt; bbox.xMax )
      bbox.xMax = glyph_bbox.xMax;

if ( glyph_bbox.yMax &gt; bbox.yMax )
      bbox.yMax = glyph_bbox.yMax;
  }

/* check that we really grew the string bbox */
 if ( bbox.xMin &gt; bbox.xMax )
 {
 bbox.xMin = 0;
 bbox.yMin = 0;
 bbox.xMax = 0;
 bbox.yMax = 0;
 }

/* return string bbox */
 *abbox = bbox;
}</pre>

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,
 <code>FT_Outline_Get_BBox</code> 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.

<pre>
/* 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 &lt; 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( &amp;image, FT_RENDER_MODE_NORMAL,
                              &amp;pen, 0 );
  if ( !error )
  {
    FT_BitmapGlyph  bit = (FT_BitmapGlyph)image;

my_draw_bitmap( bit-&gt;bitmap,
                    bit-&gt;left,
                    my_target_height - bit-&gt;top );

FT_Done_Glyph( image );
 }
}</pre>

Some remarks.

<ul>
 <li>The pen position is expressed in the Cartesian space
 (i.e., y&nbsp;upwards).</li>

<li>We call <code>FT_Glyph_To_Bitmap</code> with
 the <code>destroy</code> parameter set to&nbsp;0
 (false), in order to avoid destroying the original glyph
 image. The new glyph bitmap is accessed through
 <code>image</code> after the call and is typecast to
 <code>FT_BitmapGlyph</code>.</li>

<li>We use translation when
 calling <code>FT_Glyph_To_Bitmap</code>. This ensures
 that the <code>left</code> and <code>top</code> fields
 of the bitmap glyph object are already set to the
 correct pixel coordinates in the Cartesian space.</li>

<li>Of course, we still need to convert pixel coordinates
 from Cartesian to device space before rendering, hence
 the <code>my_target_height - bitmap->top</code> in the
 call to <code>my_draw_bitmap</code>.</li>
 </ul>

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.

<h3 id="section-5">5. Advanced Text Rendering:
 Transformation and Centering and Kerning</h3>

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.

<h4>a. Packing and Translating Glyphs</h4>

We start by packing the information related to a single
 glyph image into a single structure instead of parallel
 arrays.

<pre>
typedef struct TGlyph_
{
 FT_UInt index; /* glyph index */
 FT_Vector pos; /* glyph origin on the baseline */
 FT_Glyph image; /* glyph image */

} TGlyph, *PGlyph;</pre>

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.

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 &lt; num_chars; n++ )
{
  glyph-&gt;index = FT_Get_Char_Index( face, text[n] );

if ( use_kerning &amp;&amp; previous &amp;&amp; glyph->index )
  {
    FT_Vector  delta;

FT_Get_Kerning( face, previous, glyph-&gt;index,
                    FT_KERNING_MODE_DEFAULT, &amp;delta );

pen_x += delta.x &gt;&gt; 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-&gt;glyph, &amp;glyph-&gt;image );
  if ( error ) continue;

/* translate the glyph image now */
 FT_Glyph_Transform( glyph-&gt;image, 0, &amp;glyph-&gt;pos );

pen_x   += slot->advance.x &gt;&gt; 6;
  previous = glyph->index;

/* increment number of glyphs */
 glyph++;
}

/* count number of glyphs loaded */
num_glyphs = glyph - glyphs;</pre>

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.

<pre>
void compute_string_bbox( FT_BBox *abbox )
{
 FT_BBox bbox;

bbox.xMin = bbox.yMin =  32000;
  bbox.xMax = bbox.yMax = -32000;

for ( n = 0; n &lt; num_glyphs; n++ )
  {
    FT_BBox  glyph_bbox;

FT_Glyph_Get_CBox( glyphs[n], ft_glyph_bbox_pixels,
                       &amp;glyph_bbox );

if (glyph_bbox.xMin &lt; bbox.xMin)
      bbox.xMin = glyph_bbox.xMin;

if (glyph_bbox.yMin &lt; bbox.yMin)
      bbox.yMin = glyph_bbox.yMin;

if (glyph_bbox.xMax &gt; bbox.xMax)
      bbox.xMax = glyph_bbox.xMax;

if (glyph_bbox.yMax &gt; 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;
}</pre>

With the above modifications,
 the <code>compute_string_bbox</code> function can now
 compute the bounding box of a transformed glyph string,
 which allows further code simplications.

<pre>
FT_BBox bbox;
FT_Matrix matrix;
FT_Vector delta;

... load glyph sequence ...
... set up `matrix' and `delta' ...

/* transform glyphs */
for ( n = 0; n &lt; num_glyphs; n++ )
 FT_Glyph_Transform( glyphs[n].image, &amp;matrix, &amp;delta );

/* compute bounding box of transformed glyphs */
compute_string_bbox( &amp;bbox );</pre>

<h4>b. Rendering a Transformed Glyph Sequence</h4>

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.

<pre>
FT_Vector start;
FT_Matrix matrix;

FT_Glyph   image;
FT_Vector  pen;
FT_BBox    bbox;

/* get bbox of original glyph sequence */
compute_string_bbox( &amp;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 &lt; num_glyphs; n++ )
{
 /* create a copy of the original glyph */
 error = FT_Glyph_Copy( glyphs[n].image, &amp;image );
 if ( error ) continue;

/* transform copy (this will also translate it to the */
 /* correct position */
 FT_Glyph_Transform( image, &amp;matrix, &amp;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, &amp;bbox );
 if ( bbox.xMax &lt;= 0 || bbox.xMin &gt;= my_target_width ||
 bbox.yMax &lt;= 0 || bbox.yMin &gt;= my_target_height )
 continue;

/* convert glyph image to bitmap (destroy the glyph copy!) */
 error = FT_Glyph_To_Bitmap(
 &amp;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-&gt;bitmap,
                    bit-&gt;left,
                    my_target_height - bit-&gt;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 );
 }
}</pre>

There are a few changes compared to the original version
 of this code.

<ul>
 <li>We keep the original glyph images untouched; instead,
 we transform a copy.</li>

<li>We perform clipping computations in order to avoid
 rendering and drawing glyphs that are not within our
 target surface.</li>

<li>We always destroy the copy when calling
 <code>FT_Glyph_To_Bitmap</code> 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 <code>FT_Done_Glyph</code> is only called
 within the compound statement).</li>

<li>The translation of the glyph sequence to the start pen
 position is integrated into the call
 to <code>FT_Glyph_Transform</code> instead
 of <code>FT_Glyph_To_Bitmap</code>.</li>
 </ul>

It is possible to call this function several times to
 render the string with different angles, or even change
 the way <code>start</code> is computed in order to move it
 to different place.

This code is the basis of the FreeType&nbsp;2
 demonstration program
 named <a href="http://git.savannah.gnu.org/cgit/freetype/freetype2-demos.git/tree/src/ftstring.c"><code>ftstring.c</code></a>.
 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
 &lsquo;FreeType&rsquo;. We would store three identical
 glyph images in our table for the letter &lsquo;e&rsquo;,
 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 <a href="http://git.savannah.gnu.org/cgit/freetype/freetype2-demos.git/tree/src/ftview.c"><code>ftview.c</code></a>.
 In general, &lsquo;ftview&rsquo; 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 <a href="http://git.savannah.gnu.org/cgit/freetype/freetype2-demos.git/tree/src/ftdiff.c"><code>ftdiff.c</code></a>,
 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 &lsquo;light&rsquo;
 hinting mode) &ndash; all code in this tutorial assumes
 integer coordinates.

<h3 id="section-6">6. Accessing Metrics in Design Font
 Units, and Scaling Them</h3>

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 <a href="../reference/ft2-base_interface.html#FT_Size"><code>FT_Size</code></a>
 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.

<ul>
 <li>&lsquo;True&rsquo; WYSIWYG text layout.</li>
 <li>Accessing font content for conversion or analysis
 purposes.</li>
 </ul>

<h4>a. Scaling Distances to Device Space</h4>

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.

<pre class="example">
 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</pre>

Here, the value <code>EM_size</code> 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 <code>face-&gt;units_per_EM</code>. You should check
 that a font face contains scalable glyph images by using
 the <code>FT_IS_SCALABLE</code> macro, which returns true
 if appropriate.

When you call the
 function <a href="../reference/ft2-base_interface.html#FT_Set_Pixel_Sizes"><code>FT_Set_Pixel_Sizes</code></a>,
 you are specifying integer values
 of <code>pixel_size_x</code> and
 <code>pixel_size_y</code> FreeType shall use. The library
 will immediately compute the values
 of <code>x_scale</code> and
 <code>y_scale</code>.

When you call the
 function <a href="../reference/ft2-base_interface.html#FT_Set_Char_Size"><code>FT_Set_Char_Size</code></a>,
 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 <code>face-&gt;size-&gt;metrics</code> structure.

<dl>
 <dt>x_ppem</dt>
 <dd>The field name stands for &lsquo;x&nbsp;pixels per
 EM&rsquo;; this is the horizontal size rounded to
 integer pixels of the EM square, which also is
 the horizontal character pixel size,
 called <code>pixel_size_x</code> in the above
 example.</dd>

<dt>y_ppem</dt>
 <dd>The field name stands for &lsquo;y&nbsp;pixels per
 EM&rsquo;; this is the vertical size rounded to integer
 pixels of the EM square, which also is the vertical
 character pixel size,
 called <code>pixel_size_y</code> in the above
 example.</dd>

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

<dt>y_scale</dt>
 <dd>This is a 16.16 fixed-point scale to directly scale
 vertical distances from design space to 1/64th of device
 pixels.</dd>
 </dl>

You can scale a distance expressed in font units to 26.6
 pixel format directly with the help of
 the <a href="../reference/ft2-computations.html#FT_MulFix"><code>FT_MulFix</code></a>
 function.

<pre>
/* convert design distances to 1/64th of pixels */
pixels_x = FT_MulFix( design_x, face-&gt;size-&gt;metrics.x_scale );
pixels_y = FT_MulFix( design_y, face-&gt;size-&gt;metrics.y_scale );</pre>

Alternatively, you can also scale the value directly by
 using doubles.

<pre>
FT_Size_Metrics* metrics = &amp;face-&gt;size-&gt;metrics; /* shortcut */
double pixels_x, pixels_y;
double x_scale, y_scale;

/* compute floating point scale factors */
x_scale = face-&gt;size-&gt;metrics.x_scale / 65536.0;
y_scale = face-&gt;size-&gt;metrics.y_scale / 65536.0;

/* convert design distances to floating point pixels */
pixels_x = design_x * x_scale;
pixels_y = design_y * y_scale;</pre>

<h4>b. Accessing Design Metrics (Glyph &amp; Global)</h4>

You can access glyph metrics in font units simply by
 specifying the <code>FT_LOAD_NO_SCALE</code> bit flag
 in <code>FT_Load_Glyph</code>
 or <code>FT_Load_Char</code>. The metrics returned
 in <code>face-&gt;glyph-&gt;metrics</code> will all be in
 font units.

You can access unscaled kerning data using the
 <code>FT_KERNING_MODE_UNSCALED</code> mode.

Finally, a few global metrics are available directly in
 font units as fields of the <code>FT_Face</code> handle,
 as described in <a href="#section-3">section&nbsp;3</a> of
 this part.

<h3 id="conclusion">Conclusion</h3>

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 &amp;
 caching); this is sufficient knowledge to build a pretty
 decent text service on top of FreeType.

The demo programs in the &lsquo;ft2demos&rsquo; bundle
 (especially &lsquo;ftview&rsquo;) 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.
 </div>

<div class="updated">
 Last update: 2-May-2017
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