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  <title>FreeType Glyph Conventions</title>
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<h1 align=center>
  FreeType Glyph Conventions
</h1>

<h2 align=center>
  Version&nbsp;2.1
</h2>

<h3 align=center>
  Copyright&nbsp;1998-2000 David Turner (<a
  href="mailto:david@freetype.org">david@freetype.org</a>)<br>
  Copyright&nbsp;2000 The FreeType Development Team (<a
  href="mailto:devel@freetype.org">devel@freetype.org</a>)
</h3>

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      <a href="glyphs-6.html">Previous</a>
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      <a href="index.html">Contents</a>
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      &nbsp;
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    <h2>
      VII. FreeType bitmaps
    </h2>
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<p>The purpose of this section is to present the way FreeType manages
    bitmaps and pixmaps, and how they relate to the concepts previously
    defined.  The relationships between vectorial and pixel coordinates is
    explained.</p>

<a name="section-1">
    <h3>
      1. Vectorial versus pixel coordinates
    </h3>

<p>This sub-section explains the differences between vectorial and pixel
    coordinates.  To make things clear, brackets will be used to describe
    pixel coordinates, e.g. [3,5], while parentheses will be used for
    vectorial ones, e.g. (-2,3.5).</p>

<p>In the pixel case, as we use the <em>Y&nbsp;upwards</em> convention;
    the coordinate [0,0] always refers to the <em>lower left pixel</em> of a
    bitmap, while coordinate [width-1, rows-1] to its <em>upper right
    pixel</em>.</p>

<p>In the vectorial case, point coordinates are expressed in floating
    units, like (1.25, -2.3). Such a position doesn't refer to a given
    pixel, but simply to an immaterial point in the 2D plane.</p>

<p>The pixels themselves are indeed <em>square boxes</em> of the 2D
    plane, whose centers lie in half pixel coordinates.  For example, the
    lower left pixel of a bitmap is delimited by the square (0,0)-(1,1), its
    center being at location (0.5,0.5).</p>

<p>This introduces some differences when computing distances.  For
    example, the <em>length</em> in pixels of the line [0,0]-[10,0] is 11.
    However, the vectorial distance between (0,0)-(10,0) covers exactly
    10&nbsp;pixel centers, hence its length is&nbsp;10.</p>

<a name="section-2">
    <h3>
      2. FreeType bitmap and pixmap descriptor
    </h3>

<p>A bitmap or pixmap is described through a single structure, called
    <tt>FT_Bitmap</tt>, defined in the file
    <tt>&lt;freetype/ftimage.h&gt;</tt>.  It is a simple descriptor whose
    fields are:</p>

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    <caption>
      <b><tt>FT_Bitmap</tt></b>
    </caption>

<tr>
      <td valign=top>
        <tt>rows</tt>
      </td>
      <td valign=top>
        the number of rows, i.e. lines, in the bitmap
      </td>
    </tr>

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      <td valign=top>
        <tt>width</tt>
      </td>
      <td valign=top>
        the number of horizontal pixels in the bitmap
      </td>
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<tr>
      <td valign=top>
        <tt>pitch</tt>
      </td>
      <td valign=top>
        its absolute value is the number of bytes per bitmap line; it can
        be either positive or negative depending on the bitmap's vertical
        orientation
      </td>
    </tr>

<tr>
      <td valign=top>
        <tt>buffer</tt>
      </td>
      <td valign=top>
        a typeless pointer to the bitmap pixel bufer
      </td>
    </tr>

<tr>
      <td valign=top>
        <tt>pixel_mode</tt>
      </td>
      <td valign=top>
        an enumeration used to describe the pixel format of the bitmap;
        examples are <tt>ft_pixel_mode_mono</tt> for 1-bit monochrome
        bitmaps and <tt>ft_pixel_mode_grays</tt> for 8-bit anti-aliased
        "gray" values
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    </tr>

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      <td valign=top>
        <tt>num_grays</tt>
      </td>
      <td valign=top>
        this is only used for "gray" pixel modes, it gives the number of
        gray levels used to describe the anti-aliased gray levels --
        256&nbsp;by default with FreeType&nbsp;2
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<p>Note that the sign of the <tt>pitch</tt> fields determines whether
    the rows in the pixel buffer are stored in ascending or descending
    order.</p>

<p>Remember that FreeType uses the <em>Y&nbsp;upwards</em> convention in
    the 2D plane, which means that a coordinate of (0,0) always refer to the
    <em>lower-left corner</em> of a bitmap.</p>

<p>If the pitch is positive, the rows are stored in decreasing vertical
    position; the first bytes of the pixel buffer are part of the
    <em>upper</em> bitmap row.</p>

<p>On the opposite, if the pitch is negative, the first bytes of the
    pixel buffer are part of the <em>lower</em> bitmap row.</p>

<p>In all cases, one can see the pitch as the byte increment needed to
    skip to the <em>next lower scanline</em> in a given bitmap buffer.</p>

<p>The "positive pitch" convention is very often used, though
    some systems might need the other.</p>

<a name="section-3">
    <h3>
      3. Converting outlines into bitmaps and pixmaps
    </h3>

<p>Generating a bitmap or pixmap image from a vectorial image is easy
    with FreeType.  However, one must understand a few points regarding the
    positioning of the outline in the 2D plane before converting it to a
    bitmap:</p>

<ul>
      <li>
        <p>The glyph loader and hinter always places the outline in the 2D
        plane so that (0,0) matches its character origin.  This means that
        the glyph's outline, and corresponding bounding box, can be placed
        anywhere in the 2D plane (see the graphics in section&nbsp;III).</p>
      </li>
      <li>
        <p>The target bitmap's area is mapped to the 2D plane, with its
        lower left corner at (0,0).  This means that a bitmap or pixmap of
        dimensions [<tt>w,h</tt>] will be mapped to a 2D rectangle window
        delimited by (0,0)-(<tt>w,h</tt>).</p>
      </li>
      <li>
        <p>When scan-converting the outline, everything that falls within
        the bitmap window is rendered, the rest is ignored.</p>
      </li>

<p>A common mistake made by many developers when they begin using
      FreeType is believing that a loaded outline can be directly rendered
      in a bitmap of adequate dimensions.  The following images illustrate
      why this is a problem.</p>

<ul>
        <li>
          The first image shows a loaded outline in the 2D plane.
        </li>
        <li>
          The second one shows the target window for a bitmap of arbitrary
          dimensions [w,h].
        </li>
        <li>
          The third one shows the juxtaposition of the outline and window in
          the 2D plane.
        </li>
        <li>
          The last image shows what will really be rendered in the bitmap.
        </li>
      </ul>

<p>Indeed, in nearly all cases, the loaded or transformed outline must
    be translated before it is rendered into a target bitmap, in order to
    adjust its position relative to the target window.</p>

<p>For example, the correct way of creating a <em>standalone</em> glyph
    bitmap is as follows</p>

<ul>
      <li>
        <p>Compute the size of the glyph bitmap.  It can be computed
        directly from the glyph metrics, or by computing its bounding box
        (this is useful when a transformation has been applied to the
        outline after the load, as the glyph metrics are not valid
        anymore).</p>
      </li>
      <li>
        <p>Create the bitmap with the computed dimensions.  Don't forget to
        fill the pixel buffer with the background color.</p>
      </li>
      <li>
        <p>Translate the outline so that its lower left corner matches
        (0,0).  Don't forget that in order to preserve hinting, one should
        use integer, i.e. rounded distances (of course, this isn't required
        if preserving hinting information doesn't matter, like with rotated
        text).  Usually, this means translating with a vector
        <tt>(-ROUND(xMin), -ROUND(yMin))</tt>.</p>
      </li>
      <li>
        <p>Call the rendering function (it can be
        <tt>FT_Outline_Render()</tt> for example).</p>
      </li>
    </ul>

<p>In the case where one wants to write glyph images directly into a
    large bitmap, the outlines must be translated so that their vectorial
    position correspond to the current text cursor/character origin.</p>

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