BossBey File Manager

Editing: ft2faq.html

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

<html>

<head>
  <meta http-equiv="Content-Type"
        content="text/html; charset=US-ASCII">
  <meta name="description"
        content="FreeType FAQ">
  <link rel="stylesheet"
        href="css/freetype.css">
  <title>FreeType&nbsp;2 FAQ</title>
</head>

<tr>
    <td bgcolor="#06425F">
      <h1 class="intro">FREETYPE&nbsp;2 FAQ</h1>
    </td>
  </tr>

<tr>
    <td bgcolor="#669999">
      &nbsp;&nbsp;&nbsp;&nbsp;<a href="#general"
                                 class="index">General</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#builds"
                                             class="index">Compilation &amp;
                                             Configuration</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#autohint"
                                             class="index">The FreeType&nbsp;2
                                             auto-hinter</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#other"
                                              class="index">Other</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#FFFFFF">|</font>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="documentation.html"
                                                   class="index">FreeType&nbsp;2
                                                   documentation</a>
    </td>
  </tr>
</table>

<table align="center"
       width="80%">
  <tr>
    <td>
      <h1 class="section">Table of Contents</h1>

<ul>
        <li><a href="#general">General questions</a>
          <ul>
            <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>
          </ul>
        </li>

<li><a href="#builds">Compilation &amp; Configuration</a>
          <ul>
            <li>
              <a href="#builds-compile">How do I compile the FreeType&nbsp;2
              library?</a>
            </li>
            <li>
              <a href="#builds-config">How do I configure my library
              build?</a>
            </li>
            <li>
              <a href="#builds-differences">Why does FreeType render
              differently on different platforms?</a>
            </li>
          </ul>
        </li>

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

<li>
          <a href="#other">Other questions</a>
          <ul>
            <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>
          </ul>
        </li>
      </ul>

<hr>

<h1 class="section">
        <a name="general">
          General questions
        </a>
      </h1>

<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>

<hr>

<a name="general-uses"></a>
      <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>

<hr>

<a name="general-donts"></a>
      <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>

<hr>

<a name="general-portability"></a>
      <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>
          <p>Everything is written in standard ANSI&nbsp;C.</p>
        </li>
        <li>
          <p>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.</p>
        </li>
        <li>
          <p>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.</p>
        </li>
      </ul>

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

<hr>

<a name="general-freetype1"></a>
      <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>

<hr>

<a name="general-ft1"></a>
      <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>

<hr>

<a name="general-edit"></a>
      <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.org/">FontForge</a>.</p>

<hr>

<h1 class="section">
        <a name="builds">
          Compilation &amp; Configuration
        </a>
      </h1>

<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>

<hr>

<a name="builds-config"></a>
      <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>

<hr>

<a name="builds-differences"></a>
      <h3>
        Why does FreeType render differently on different platforms?
      </h3>

<p>Different distributions compile FreeType with different options. 
      The developer version of a distribution's FreeType package, which is
      needed to compile your program against FreeType, includes the file
      <tt>ftoption.h</tt>.  Compare each platform's copy of
      <tt>ftoption.h</tt> to find the differences.</p>

<hr>

<h1 class="section">
        <a name="autohint">
          The FreeType&nbsp;2 auto-hinter
        </a>
      </h1>

<a name="autohint-work"></a>
      <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.

<hr>

<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.</p>

<p>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>

<hr>

<h1 class="section">
        <a name="other">
          Other questions
        </a>
      </h1>

<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>.</p>

<p>A complete example is given at the end of the <a
      href="tutorial/step3.html">FreeType&nbsp;2 tutorial</a>.

<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>

<hr>

<a name="other-color"></a>
      <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>

<hr>

<a name="other-size"></a>
      <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>

<hr>

<a name="other-bbox"></a>
      <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>

<hr>

<a name="other-antialias"></a>
      <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>

<hr>

<a name="other-opentype"></a>
      <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>

<font size=-3>Last update: 25-Feb-2011</font>

<table width="100%"
       border="0"
       cellspacing="0"
       cellpadding="5">
  <tr>
    <td bgcolor="#669999">
      &nbsp;&nbsp;&nbsp;&nbsp;<a href="#general"
                                class="index">General</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#builds"
                                             class="index">Compilation &amp;
                                             Configuration</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#autohint"
                                             class="index">The FreeType&nbsp;2
                                             auto-hinter</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#other"
                                             class="index">Other</a>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<font color="#FFFFFF">|</font>
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="documentation.html"
                                                   class="index">FreeType&nbsp;
                                                   documentation</a>
    </td>
  </tr>

</body>
</html>

Cancel