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  <h1>The FreeType&nbsp;2 FAQ</h1>
</h1>

<hr>

<ul>
    <li><a href="#general">General</a>
      <ul><p>
        <li>
          <a href="#general-what">What is FreeType&nbsp;2?</a>
        </li>
        <li>
          <a href="#general-uses">What can I do with FreeType&nbsp;2?</a>
        </li>
        <li>
          <a href="#general-donts">What can I not do with
          FreeType&nbsp;2?</a>
        </li>
        <li>
          <a href="#general-portability">How portable is
          FreeType&nbsp;2?</a>
        </li>
        <li>
          <a href="#general-freetype1">What are the differences between
          FreeType&nbsp;1.x and FreeType&nbsp;2?</a>
        </li>
        <li>
          <a href="#general-ft1">Is FreeType&nbsp;2 backwards compatible
          to FreeType&nbsp;1.x?</a>
        </li>
        <li>
          <a href="#general-edit">Can I use FreeType&nbsp;2 to edit fonts
          or create new ones?</a>
        </li>
      </p></ul>
    </li>

<li><a href="#builds">Compilation &amp; Configuration</a>
      <ul><p>
        <li>
          <a href="#builds-compile">How do I compile the FreeType&nbsp;2
          library?</a>
        </li>
        <li>
          <a href="#builds-problems">I've tried to compile FreeType&nbsp;2,
          but I'm having problems.  What can I do?</a>
        </li>
        <li>
          <a href="#builds-config">How do I configure my library build?</a>
        </li>
      </p></ul>
    </li>

<li>
      <a href="#autohint">The FreeType&nbsp;2 auto-hinter</a>
      <ul><p>
        <li>
          <a href="#autohint-work">How does the auto-hinter work?
        </li>
        <li>
          <a href="#autohint-other-scripts">Why doesn't the auto-hinter work
          well with my script?</a>
        </li>
      </p></ul>
    </li>

<li>
      <a href="#other">Other questions</a>
      <ul><p>
        <li>
          <a href="#other-depth">Can I use FreeType to draw text on a pixmap
          with arbitrary depth?</a>
        </li>
        <li>
          <a href="#other-color">How can I set the colour of text rendered
          by FreeType?</a>
        </li>
        <li>
          <a href="#other-size">I set the pixel size to 8&times;8, but the
          resulting glyphs are larger (or smaller) than that.  Why?</a>
        </li>
        <li>
          <a href="#other-bbox">How can I compute the bounding box of a text
          string without loading its glyphs before?</a>
        </li>
        <li>
          <a href="#other-antialias">Which anti-aliasing algorithm is
          used by FreeType&nbsp;2?</a>
        </li>
        <li>
          <a href="#other-opentype">When will FreeType&nbsp;2 support
          OpenType?</a>
        </li>
      </p></ul>
    </li>
  </ul>

<p></p>

<table width="100%">
    <tr bgcolor="#CCCCEE">
      <td>
        <a name="general"></a>
        <h2 align=center>General questions &amp; answers</h2>
      </td>
    </tr>
    <tr>
      <td>

<a name="general-what"></a>
        <h3>
          What is FreeType&nbsp;2?
        </h3>

<p>It is a software library that can be used by all kinds of
        applications to access the contents of font files.  Most notably, it
        supports the following &lsquo;features&rsquo;:</p>

<ul>
          <li><p>
            It provides a uniform interface to access font files.  It
            supports both bitmap and scalable formats, including TrueType,
            OpenType, Type1, CID, CFF, Windows FON/FNT, X11 PCF, and others.
          </p></li>

<li><p>
            It supports high-speed anti-aliased glyph bitmap generation with
            256 gray levels.
          </p></li>

<li><p>
            It is extremely modular, each font format being supported by a
            specific module.  A build of the library can be tailored to
            support only the formats you need, thus reducing code size (a
            minimal anti-aliasing build of FreeType can be less than 30KB)
          </p></li>
        </ul>

<h3>
          What can I do with FreeType&nbsp;2?
        </h3>

<p>FreeType&nbsp;2 is already used in many products.  For example,
        it serves as a font rendering engine</p>

<ul>
          <li>
            in graphics subsystem and libraries to display text
          </li>
          <li>
            in text layout and pagination services to measure and eventually
            render text
          </li>
          <li>
            in font inspection and conversion tools
          </li>
        </ul>

<p>Generally speaking, the library allows you to access and manage
        the contents of font files in a very easy way.</p>

<h3>
          What can I not do with FreeType&nbsp;2?
        </h3>

<p>FreeType&nbsp;2 doesn't try to perform a number of sophisticated
        things, because it focuses on being an excellent <em>font
        service</em>.</p>

<p>This means that the following features are not supported directly
        by the library, even though they can be more or less implemented on
        top of it, or by using it:</p>

<ul>
          <li>
            <p><b>rendering glyphs to arbitrary surfaces</b><br>
            FreeType&nbsp;2 doesn't try to be a graphics library and thus
            only supports two pixel formats when rendering glyphs:
            monochrome 1-bit bitmaps, or 8-bit gray-level pixmaps.</p>

<p>If you need to draw glyphs to other kinds of surfaces (for
            example, a 24-bit RGB pixmap), you need to use your favorite
            graphics library to do just that.</p>

<p><em>Note however that in the case of rendering scalable glyph
            outlines to anti-aliased pixmaps, an application can also
            provide its own rendering callback in order to draw or compose
            directly the anti-aliased glyph on any target surface.</em></p>
          </li>

<li>
            <p><b>glyph caching</b><br>
            Each time you request a glyph image from a font, FreeType&nbsp;2
            does it by parsing the relevant portion of the font file or font
            stream and interpreting it according to its font format.  This
            can be very slow for certain formats, including scalable ones
            like TrueType or Type&nbsp;1.</p>

<p>Any decent text-rendering sub-system must thus be capable of
            caching glyph data in order to reach appropriate rendering
            speed.</p>

<p><em>Note that we provide a caching sub-system with
            FreeType&nbsp;2 since version 2.0.1 which has become quite
            stable at the time of this writing (version 2.2.1).  However, it
            might not suit your needs.</em></p>
          </li>

<li>
            <p><b>text layout</b><br>
            The library doesn't support text layout operations.
            Sophisticated features like glyph substitution, positioning
            (kerning), justification, bi-directional ordering, etc.m are not
            part of a <em>font service</em> in itself.  They must be handled
            one level higher.</p>
          </li>
        </ul>

<h3>
          How portable is FreeType&nbsp;2?
        </h3>

<p>The FreeType 2 source code is <em>extremely</em> portable for the
        following reasons:</p>

<ul>
          <li>
            Everything is written in standard ANSI&nbsp;C.
          </li>
          <li>
            We are very pedantic to avoid any kinds of compiler warnings.
            The current source code has been compiled with many compilers
            without producing a single warning.
          </li>
          <li>
            The library doesn't use any static writable data at all, making
            it an ideal choice on various embedded systems (e.g., it can be
            run from ROM directly).  It is completely thread-safe too.
          </li>
        </ul>

<p>We have made great efforts to ensure that the library is
        efficient, compact, and customizable.</p>

<h3>
          What are the differences between FreeType&nbsp;1.x and
          FreeType&nbsp;2?
        </h3>

<p>The biggest differences are as follows.</p>

<ul>
          <li>
            <p>FreeType&nbsp;1 only supports the TrueType format, while
            FreeType&nbsp;2 supports a lot more.</p>
          </li>

<li>
            <p>The FreeType&nbsp;2 API is simpler as well as more powerful than the
            FreeType&nbsp;1 API.</p>
          </li>

<li>
            <p>FreeType&nbsp;1 includes an extension to support OpenType
            text layout processing.  This support hasn't become part of
            FreeType&nbsp;2; a much improved version is now part of the <a
            href="http:///www.pango.org">Pango<a/> library.</p>
          </li>
        </ul>

<h3>
          Is FreeType&nbsp;2 backwards compatible with FreeType&nbsp;1.x?
        </h3>

<p>Short answer: No.  However, transition from 1.x to&nbsp;2 should
        be rather straightforward.</p>

<p>The FreeType&nbsp;2 API is a lot simpler than the one in&nbsp;1.x
        while being much more powerful.  We thus encourage you to adapt your
        source code to it as this should not involve much work.</p>

<h3>
          Can I use FreeType&nbsp;2 to edit fonts or create new ones?
        </h3>

<p>No.  The library has been specifically designed to <em>read</em>
        font files with small code size and very low memory usage.</p>

<p>A good, freely available font editor is <a
        href="http://fontforge.sf.net">FontForge</a>.</p>
      </td>
    </tr>
  </table>

<p></p>

<table width="100%">
    <tr bgcolor="#CCCCEE">
      <td>
        <a name="builds"></a>
        <h2 align=center>Compilation &amp; Configuration</h2>
      </td>
    </tr>
    <tr>
      <td>

<a name="builds-compile"></a>
        <h3>
          How do I compile the FreeType&nbsp;2 library?
        </h3>

<p>The library can be compiled in various ways, and detailed
        documentation is available in documentation directory of the
        FreeType&nbsp;2 source tree.</p>

<p>For compilation on the command line, GNU make is necessary; other
        build tools won't work.  The source bundle also comes with project
        files for some graphical IDEs like Visual C; note, however, that
        those files are sometimes not up to date since it is contributed
        code not used by the core developers.</p>

<h3>
          How do I configure my build of the library?
        </h3>

<p>This is fully described in the file <tt>CUSTOMIZATION</tt> in
        FreeType's documentation directory.  Basically, you have to edit the
        header file <tt>ftoption.h</tt> for compile-time options and to
        select the modules with the file <tt>modules.cfg</tt>.  Finally, it
        is possible to replace the standard system interface (dealing with
        memory allocation and stream I/O) with a custom one.</p>
      </td>
    </tr>
  </table>

<p></p>

<table width="100%">
    <tr bgcolor="#CCCCEE">
      <td>
        <a name="autohint"></a>
        <h2 align=center>The FreeType&nbsp;2 auto-hinter</h2>
      </td>
    </tr>
    <tr>
      <td>

<h3>
          How does the auto-hinter work?
        </h3>

<p><em>Please note that the name of auto-hinter module is
        <b>autofit</b>, which is a reimplementation of the old autohint
        module.</em></p>

<p>A rather complete description of the hinting algorithm (which is
        slightly out of date regarding the internal structures) can be found
        in the TUG-boat article <a
        href="http://www.tug.org/TUGboat/Articles/tb24-3/lemberg.pdf">Real-Time
        Grid Fitting of Typographic Outlines</a>.</p>

<p>The auto-hinter performs grid-fitting on scalable font formats
        that use B&eacute;zier outlines as their primary glyph image format
        (this means nearly all scalable font formats today).  If a given
        font driver doesn't provide its own hinter, the auto-hinter is used
        by default.  If a format-specific hinter is provided, it is still
        possible to use the auto-hinter using the
        <tt>FT_LOAD_FORCE_AUTOHINT</tt> bit flag when calling
        <tt>FT_Load_Glyph()</tt>.</p>

<p>Currently, the auto-hinter doesn't use external hints to do its
        job, as it automatically computes global metrics (when it
        &lsquo;opens&rsquo; a font for the first time) and glyph
        &lsquo;hints&rsquo; from their outline.

<a name="autohint-other-scripts"></a>
        <h3>
          Why doesn't the auto-hinter work well with my script?
        </h3>

<p>The auto-hinter was first designed to manage and hint Latin-based
        fonts, as they consist of most of the fonts available today.  It now
        supports Asian fonts, but not other complex scripts like Arabic.

Hinting various scripts isn't really more difficult than Latin, just
        different, with a set of different constraints, which must be
        hard-coded into the autofit module.  Volunteers welcome!</p>
      </td>
    </tr>
  </table>

<p></p>

<table width="100%">
    <tr bgcolor="#CCCCEE">
      <td>
        <a name="other"></a>
        <h2 align=center>Other questions</h2>
      </td>
    </tr>
    <tr>
      <td>

<a name="other-depth"></a>
        <h3>
          Can I use FreeType to draw text on a pixmap with arbitrary depth?
        </h3>

<p>Not directly, as FreeType is a font library, not a
        general-purpose graphics library or text rendering service.
        However, note that the anti-aliased renderer allows you to convert a
        vectorial glyph outline into a list of &lsquo;spans&rsquo; (i.e.,
        horizontal pixel segments with the same coverage) that can be
        rendered through user-provided callbacks.</p>

<p>By providing the appropriate span callback, you can render
        anti-aliased text to any kind of surface.  You can also use any
        colour, fill pattern or fill image if you want to.  This process is
        called <em>direct rendering</em>.

<font color="blue">
        <pre>
    FT_Raster_Params  params;
    FT_Outline        outline;

... load vectorial glyph in &lsquo;outline&rsquo; ...

params.flags      = FT_RASTER_FLAG_AA | FT_RASTER_FLAG_DIRECT;
    params.gray_spans = (FT_Raster_Span_Func)your_own_span_function;
    params.user       = your_own_data_pointer;

error = FT_Outline_Render( library, &amp;outline, &amp;params );</pre>
        </font>

<p>Note that direct rendering is <em>not</em> available with
        monochrome output, as the current renderer uses a two-pass algorithm
        to generate glyphs with correct drop-out control.</p>

<h3>
          How can I set the colour of text rendered by FreeType?
        </h3>

<p>Basically, you can't do that, because FreeType is simply a font
        library.  In general, you need to use your favorite graphics library
        to draw the FreeType glyphs with the appropriate colour.</p>

<p>Note that for anti-aliased glyphs, you can &lsquo;set the
        colour&rsquo; by using <em>direct rendering</em> as described in <a
        href="#other-depth">this answer</a>.</p>

<h3>
          I set the pixel size to 8&times;8, but the resulting glyphs are
          larger (or smaller) than that.  Why?
        </h3>

<p>A lot of people have difficulties to understand this topic, because
        they think of glyphs as fixed-width or fixed-height
        &lsquo;cells&rsquo;, like those of fonts used in terminals/consoles.
        This assumption is not valid with most &lsquo;modern&rsquo; font
        formats, even for bitmapped-based ones like <b>PCF</b> or
        <b>BDF</b>.</p>

<p>Be aware that the <em>character size</em> that is set either
        through <tt>FT_Set_Char_Size()</tt> or <tt>FT_Set_Pixel_Sizes()</tt>
        isn't directly related to the dimension of the generated glyph
        bitmaps!</p>

<p>Rather, the character size is indeed the size of <em>an abstract
        square</em>, called the <em>EM</em>, used by typographers to design
        fonts.  Scaling two distinct fonts to the same character size, be it
        expressed in points or pixels, generally results in bitmaps with
        <em>distinct dimensions</em>!</p>

<p>Note that historically, the EM corresponded to the width of a
        capital &lsquo;M&rsquo; in Latin typefaces.  However, later
        improvements in typography led to designs that greatly deviate from
        this rule.  Today, it is not possible to connect the EM size to a
        specific font &lsquo;feature&rsquo; in a reliable way.</p>

<h3>
          How can I compute the bounding box of a text string without
          loading its glyphs before?
        </h3>

<p>This is not possible in general.  Reason is that hinting distorts
        the glyph shape for optimal rasterization, and this process
        sometimes creates outlines which have considerably different
        metrics.  The TrueType format provides the (optional)
        &lsquo;hdmx&rsquo; table which contains device horizontal metrics
        for selected pixel sizes, but even here the vertical metrics are
        missing.</p>

<p>It is probably best to use both a glyph and a metrics cache to
        avoid recomputation.</p>

<h3>
          Which anti-aliasing algorithm is used by FreeType&nbsp;2?
        </h3>

<p>The algorithm has been specifically designed for FreeType.  It is
        based on ideas that were originally found in the implementation of
        the <a href="http://www.levien.com/libart">libArt</a> graphics
        library to compute the <em>exact pixel coverage</em> of a vector
        image with no sub-sampling and filtering.</p>

<p>However, these two implementations are radically distinct and use
        vastly different models.  The FreeType&nbsp;2 renderer is optimized
        specifically for rendering small complex shapes, like glyphs, at
        very high speed while using very few memory.  On the other hand,
        libArt has been designed for general shape and polygon processing,
        especially large ones.</p>

<p>The FreeType&nbsp;2 anti-aliasing renderer is indeed
        <em>faster</em> than the monochrome renderer for small character
        sizes (typically &lt;20&nbsp;pixels).  The reason is that the
        monochrome renderer must perform two passes on the outline in order
        to perform drop-out control according to the TrueType
        specification.</p>

<h3>
          When will FreeType&nbsp;2 support OpenType?
        </h3>

<p>Well, the engine already reads OpenType/CFF files perfectly.
        What it doesn't do is handling &lsquo;OpenType Layout&rsquo;
        tables.</p>

<p>FreeType&nbsp;1 comes with a set of extensions that are used to
        load and manage OpenType Layout tables.  It even has a demonstration
        program named <tt>ftstrtto</tt> to show its capabilities.  However,
        this code is no longer maintained, and we strongly advise to not use
        it.</p>

<p>For FreeType&nbsp;2, we have decided that the layout operations
        provided through these tables are better placed in a specific
        text-layout library like <a
        href="http://www.pango.org">Pango</a>.</p>
      </td>
    </tr>
  </table>

<a href="../../index2.html">Back to FreeType homepage</a><p>

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</table>

<p><font size=-3>Last update: 28-Jul-2006</font></p>

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