harfbuzz/docs/usermanual-ch01.xml

<sect1 id="what-is-harfbuzz">
  <title>What is Harfbuzz?</title>
  <para>
    Harfbuzz is a <emphasis>text shaping engine</emphasis>. It solves
    the problem of selecting and positioning glyphs from a font given a
    Unicode string.
  </para>
  <sect2 id="why-do-i-need-it">
    <title>Why do I need it?</title>
    <para>
      Text shaping is an integral part of preparing text for display. It
      is a fairly low level operation; Harfbuzz is used directly by
      graphic rendering libraries such as Pango, and the layout engines
      in Firefox, LibreOffice and Chromium. Unless you are
      <emphasis>writing</emphasis> one of these layout engines yourself,
      you will probably not need to use Harfbuzz - normally higher level
      libraries will turn text into glyphs for you.
    </para>
    <para>
      However, if you <emphasis>are</emphasis> writing a layout engine
      or graphics library yourself, you will need to perform text
      shaping, and this is where Harfbuzz can help you. Here are some
      reasons why you need it:
    </para>
    <itemizedlist>
      <listitem>
        <para>
          OpenType fonts contain a set of glyphs, indexed by glyph ID.
          The glyph ID within the font does not necessarily relate to a
          Unicode codepoint. For instance, some fonts have the letter
          &quot;a&quot; as glyph ID 1. To pull the right glyph out of
          the font in order to display it, you need to consult a table
          within the font (the &quot;cmap&quot; table) which maps
          Unicode codepoints to glyph IDs. Text shaping turns codepoints
          into glyph IDs.
        </para>
      </listitem>
      <listitem>
        <para>
          Many OpenType fonts contain ligatures: combinations of
          characters which are rendered together. For instance, it's
          common for the <literal>fi</literal> combination to appear in
          print as the single ligature &quot;fi&quot;. Whether you should
          render text as <literal>fi</literal> or &quot;fi&quot; does not
          depend on the input text, but on the capabilities of the font
          and the level of ligature application you wish to perform.
          Text shaping involves querying the font's ligature tables and
          determining what substitutions should be made.
        </para>
      </listitem>
      <listitem>
        <para>
          While ligatures like &quot;fi&quot; are typographic
          refinements, some languages <emphasis>require</emphasis> such
          substitutions to be made in order to display text correctly.
          In Tamil, when the letter &quot;TTA&quot; (ட) letter is
          followed by &quot;U&quot; (உ), the combination should appear
          as the single glyph &quot;டு&quot;. The sequence of Unicode
          characters &quot;டஉ&quot; needs to be rendered as a single
          glyph from the font - text shaping chooses the correct glyph
          from the sequence of characters provided.
        </para>
      </listitem>
      <listitem>
        <para>
          Similarly, each Arabic character has four different variants:
          within a font, there will be glyphs for the initial, medial,
          final, and isolated forms of each letter. Unicode only encodes
          one codepoint per character, and so a Unicode string will not
          tell you which glyph to use. Text shaping chooses the correct
          form of the letter and returns the correct glyph from the font
          that you need to render.
        </para>
      </listitem>
      <listitem>
        <para>
          Other languages have marks and accents which need to be
          rendered in certain positions around a base character. For
          instance, the Moldovan language has the Cyrillic letter
          &quot;zhe&quot; (ж) with a breve accent, like so: ӂ. Some
          fonts will contain this character as an individual glyph,
          whereas other fonts will not contain a zhe-with-breve glyph
          but expect the rendering engine to form the character by
          overlaying the two glyphs ж and ˘. Where you should draw the
          combining breve depends on the height of the preceding glyph.
          Again, for Arabic, the correct positioning of vowel marks
          depends on the height of the character on which you are
          placing the mark. Text shaping tells you whether you have a
          precomposed glyph within your font or if you need to compose a
          glyph yourself out of combining marks, and if so, where to
          position those marks.
        </para>
      </listitem>
    </itemizedlist>
    <para>
      If this is something that you need to do, then you need a text
      shaping engine: you could use Uniscribe if you are using Windows;
      you could use CoreText on OS X; or you could use Harfbuzz. In the
      rest of this manual, we are going to assume that you are the
      implementor of a text layout engine.
    </para>
  </sect2>
  <sect2 id="why-is-it-called-harfbuzz">
    <title>Why is it called Harfbuzz?</title>
    <para>
      Harfbuzz began its life as text shaping code within the FreeType
      project, (and you will see references to the FreeType authors
      within the source code copyright declarations) but was then
      abstracted out to its own project. This project is maintained by
      Behdad Esfahbod, and named Harfbuzz. Originally, it was a shaping
      engine for OpenType fonts - &quot;Harfbuzz&quot; is the Persian
      for &quot;open type&quot;.
    </para>
  </sect2>
</sect1>