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  <h1><a href="../../../index.html">FreeType</a> Glyph
    Conventions&nbsp;/&nbsp;V</h1>
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        <div id="text-processing">
          <h2>V. Text Processing</h2>

          <p>This section demonstrate algorithms which use the
            concepts previously defined to render text, whatever
            layout you use.  It assumes <em>simple</em> text handling
            suitable for scripts like Latin or Cyrillic, using a
            one-to-one relationship between input character codes and
            output glyphs indices.  Scripts like Arabic or Khmer,
            which need a &lsquo;shaping engine&rsquo; to do the
            character code to glyph index conversion, are beyond the
            scope (and should be done by proper layout engines
            like <a href="http://www.pango.org/">Pango</a>
            anyway).</p>


          <h3 id="section-1">1. Writing simple text strings</h3>

          <p>In this first example, we will generate a simple string
            of text in the Latin script, i.e. with a horizontal
            left-to-right layout.  Using exclusively pixel metrics,
            the process looks like:

            <ol class="algorithm">
              <li>
                Convert the character string into a series of glyph
                indices.
              </li>
              <li>
                Place the pen to the cursor position.
              </li>
              <li>
                Get or load the glyph image.
              </li>
              <li>
                Translate the glyph so that its &lsquo;origin&rsquo;
                matches the pen position.
              </li>
              <li>
                Render the glyph to the target device.
              </li>
              <li>
                Increment the pen position by the glyph's advance
                width (in pixels).
              </li>
              <li>
                Start over at step&nbsp;3 for each of the remaining
                glyphs.
              </li>
              <li>
                When all glyphs are done, set the text cursor to the
                new pen position.
              </li>
            </ol>

          <p>Note that kerning isn't part of this algorithm.</p>


          <h3 id="section-2">2. Pseudo-subpixel positioning</h3>

          <p>It is somewhat useful to use subpixel positioning when
          rendering text.  This is crucial, for example, to provide
          semi-WYSIWYG text layouts.  Text rendering is very similar
          to the algorithm described in subsection&nbsp;1, with the
          following few differences:</p>

          <ul>
            <li>
              <p>The pen position is expressed in fractional
                pixels.</p>
            </li>
            <li>
              <p>Because translating a hinted outline by a non-integer
                distance will ruin its grid-fitting, the position of
                the glyph origin must be rounded before rendering the
                character image.</p>
            </li>
            <li>
              <p>The advance width is expressed in fractional pixels,
                and isn't necessarily an integer.</p>
            </li>
          </ul>

          <p>Here an improved version of the algorithm:</p>

          <ol class="algorithm">
            <li>
              Convert the character string into a series of glyph
              indices.
            </li>
            <li>
              Place the pen to the cursor position.  This can be a
              non-integer point.
            </li>
            <li>
              Get or load the glyph image.
            </li>
            <li>
              Translate the glyph so that its &lsquo;origin&rsquo;
              matches the rounded pen position.
            </li>
            <li>
              Render the glyph to the target device.
            </li>
            <li>
              Increment the pen position by the glyph's advance width
              in fractional pixels.
            </li>
            <li>
              Start over at step&nbsp;3 for each of the remaining
              glyphs.
            </li>
            <li>
              When all glyphs are done, set the text cursor to the new
              pen position.
            </li>
          </ol>

          <p>Note that with fractional pixel positioning, the space
            between two given letters isn't fixed, but determined by
            the accumulation of previous rounding errors in glyph
            positioning.  For auto-hinted glyphs, this problem can be
            alleviated by using the <tt>lsb_delta</tt>
            and <tt>rsb_delta</tt> values (see the documentation of
            the <a href="../reference/ft2-base_interface.html#FT_GlyphSlotRec">FT_GlyphSlotRec</a>
            structure for more details).</p>

          <p>TODO: Real subpixel positioning with glyph shifting
            before hinting.</p>


          <h3 id="section-3">3. Simple kerning</h3>

          <p>Adding kerning to the basic text rendering algorithm is
            easy: When a kerning pair is found, simply add the scaled
            kerning distance to the pen position before step&nbsp;4.
            Of course, the distance should be rounded in the case of
            algorithm&nbsp;1, though it doesn't need to for
            algorithm&nbsp;2.  This gives us:</p>

          <p>Algorithm&nbsp;1 with kerning:</p>

          <ol class="algorithm">
            <li>
              Convert the character string into a series of glyph
              indices.
            </li>
            <li>
              Place the pen to the cursor position.
            </li>
            <li>
              Get or load the glyph image.
            </li>
            <li>
              Add the rounded scaled kerning distance, if any, to the
              pen position.
            </li>
            <li>
              Translate the glyph so that its &lsquo;origin&rsquo;
              matches the pen position.
            </li>
            <li>
              Render the glyph to the target device.
            </li>
            <li>
              Increment the pen position by the glyph's advance width
              in pixels.
            </li>
            <li>
              Start over at step&nbsp;3 for each of the remaining
              glyphs.
            </li>
          </ol>

          <p>Algorithm&nbsp;2 with kerning:</p>

          <ol class="algorithm">
            <li>
              Convert the character string into a series of glyph
              indices.
            </li>
            <li>
              Place the pen to the cursor position.
            </li>
            <li>
              Get or load the glyph image.
            </li>
            <li>
              Add the scaled unrounded kerning distance, if any, to
              the pen position.
            </li>
            <li>
              Translate the glyph so that its &lsquo;origin&rsquo;
              matches the rounded pen position.
            </li>
            <li>
              Render the glyph to the target device.
            </li>
            <li>
              Increment the pen position by the glyph's advance
              width in fractional pixels.
            </li>
            <li>
              Start over at step&nbsp;3 for each of the remaining
              glyphs.
            </li>
          </ol>


          <h3 id="section-4">4. Right-to-left layout</h3>

          <p>The process of laying out right-to-left scripts like
            (modern) Hebrew text is very similar.  The only difference
            is that the pen position must be decremented before the
            glyph rendering (remember: the advance width is always
            positive, even for Hebrew glyphs).</p>

          <p>Right-to-left algorithm&nbsp;1:</p>

          <ol class="algorithm">
            <li>
              Convert the character string into a series of glyph
              indices.
            </li>
            <li>
              Place the pen to the cursor position.
            </li>
            <li>
              Get or load the glyph image.
            </li>
            <li>
              Decrement the pen position by the glyph's advance
              width in pixels.
            </li>
            <li>
              Translate the glyph so that its &lsquo;origin&rsquo;
              matches the pen position.
            </li>
            <li>
              Render the glyph to the target device.
            </li>
            <li>
              Start over at step&nbsp;3 for each of the remaining
              glyphs.
            </li>
          </ol>

          <p>The changes to algorithm&nbsp;2, as well as the inclusion
            of kerning are left as an exercise to the reader.</p>


          <h3 id="section-5">5. Vertical layouts</h3>

          <p>Laying out vertical text uses exactly the same processes,
            with the following significant differences:</p>

          <ul>
            <li>
              <p>The baseline is vertical, and the vertical metrics
                must be used instead of the horizontal one.</p>
            </li>
            <li>
              <p>The left bearing is usually negative, but this
                doesn't change the fact that the glyph origin must be
                located on the baseline.</p>
            </li>
            <li>
              <p>The advance height is always positive, so the pen
                position must be decremented if one wants to write top
                to bottom (assuming the <i>Y</i>&nbsp;axis is oriented
                upwards).</p>
            </li>
          </ul>

          <p>Here the algorithm:</p>

          <ol class="algorithm">
            <li>
              Convert the character string into a series of glyph
              indices.
            </li>
            <li>
              Place the pen to the cursor position.
            </li>
            <li>
              Get or load the glyph image.
            </li>
            <li>
              Translate the glyph so that its &lsquo;origin&rsquo;
              matches the pen position.
            </li>
            <li>
              Render the glyph to the target device.
            </li>
            <li>
              Decrement the vertical pen position by the glyph's
              advance height in pixels.
            </li>
            <li>
              Start over at step&nbsp;3 for each of the remaining
              glyphs.
            </li>
            <li>
              When all glyphs are done, set the text cursor to the new
              pen position.
            </li>
          </ol>
        </div>

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        <div class="updated">
          <p>Last update: 1-Jul-2013</p>
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