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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="why-do-i-need-a-shaping-engine"></a>Why do I need a shaping engine?</h2></div></div></div>
<p>
      Text shaping is an integral part of preparing text for
      display. Before a Unicode sequence can be rendered, the
      codepoints in the sequence must be mapped to the corresponding
      glyphs provided in the font, and those glyphs must be positioned
      correctly relative to each other. For many of the scripts
      supported in Unicode, these steps involve script-specific layout
      rules, including complex joining, reordering, and positioning
      behavior. Implementing these rules is the job of the shaping engine.
    </p>
<p>
      Text shaping is a fairly low-level operation. HarfBuzz is
      used directly by text-handling libraries like <a class="ulink" href="https://www.pango.org/" target="_top">Pango</a>, as well as by the layout
      engines in Firefox, LibreOffice, and Chromium. Unless you are
      <span class="emphasis"><em>writing</em></span> one of these layout engines
      yourself, you will probably not need to use HarfBuzz: normally,
      a layout engine, toolkit, or other library will turn text into
      glyphs for you.
    </p>
<p>
      However, if you <span class="emphasis"><em>are</em></span> writing a layout engine
      or graphics library yourself, then you will need to perform text
      shaping, and this is where HarfBuzz can help you.
    </p>
<p>
      Here are some specific scenarios where a text-shaping engine
      like HarfBuzz helps you:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>
          OpenType fonts contain a set of glyphs (that is, shapes
	  to represent the letters, numbers, punctuation marks, and
	  all other symbols), which are indexed by a <code class="literal">glyph ID</code>.
	</p>
<p>
          A particular glyph ID within the font does not necessarily
	  correlate to a predictable Unicode codepoint. For instance,
	  some fonts have the letter "a" as glyph ID 1, but
	  many others do not. In order to retrieve the right glyph
	  from the font to display "a", you need to consult
	  the table inside the font (the <code class="literal">cmap</code>
	  table) that maps Unicode codepoints to glyph IDs. In other
	  words, <span class="emphasis"><em>text shaping turns codepoints into glyph
	  IDs</em></span>.
        </p>
</li>
<li class="listitem">
<p>
          Many OpenType fonts contain ligatures: combinations of
          characters that are rendered as a single unit. For instance,
	  it is common for the "f, i" letter
	  sequence to appear in print as the single ligature glyph
	  "ﬁ".
	</p>
<p>
	  Whether you should render an "f, i" sequence
	  as <code class="literal">fi</code> or as "ﬁ" does not
          depend on the input text. Instead, it depends on the whether
	  or not the font includes an "ﬁ" glyph and on the
	  level of ligature application you wish to perform. The font
	  and the amount of ligature application used are under your
	  control. In other words, <span class="emphasis"><em>text shaping involves
	  querying the font's ligature tables and determining what
	  substitutions should be made</em></span>. 
        </p>
</li>
<li class="listitem">
<p>
          While ligatures like "ﬁ" are optional typographic
          refinements, some languages <span class="emphasis"><em>require</em></span> certain
          substitutions to be made in order to display text correctly.
        </p>
<p>
	  For example, in Tamil, when the letter "TTA" (ட)
	  letter is followed by the vowel sign "U" (ு), the pair
	  must be replaced by the single glyph "டு". The
	  sequence of Unicode characters "ட,ு" needs to be
	  substituted with a single "டு" glyph from the
	  font.
	</p>
<p>
	  But "டு" does not have a Unicode codepoint. To
	  find this glyph, you need to consult the table inside 
	  the font (the <code class="literal">GSUB</code> table) that contains
	  substitution information. In other words, <span class="emphasis"><em>text shaping 
	  chooses the correct glyph for a sequence of characters
	  provided</em></span>.
        </p>
</li>
<li class="listitem">
<p>
          Similarly, each Arabic character has four different variants
	  corresponding to the different positions it might appear in
	  within a sequence. Inside a font, there will be separate
	  glyphs for the initial, medial, final, and isolated forms of
	  each letter, each at a different glyph ID.
	</p>
<p>
	  Unicode only assigns one codepoint per character, so a
	  Unicode string will not tell you which glyph variant to use
	  for each character. To decide, you need to analyze the whole
	  string and determine the appropriate glyph for each character
	  based on its position. In other words, <span class="emphasis"><em>text
	  shaping chooses the correct form of the letter by its
	  position and returns the correct glyph from the font</em></span>.
        </p>
</li>
<li class="listitem">
<p>
          Other languages involve marks and accents that need to be
          rendered in specific positions relative a base character. For
          instance, the Moldovan language includes the Cyrillic letter
          "zhe" (ж) with a breve accent, like so: "ӂ".
	</p>
<p>
	  Some fonts will provide this character as a single
	  zhe-with-breve glyph, but other fonts will not and, instead,
	  will expect the rendering engine to form the character by 
          superimposing the separate "ж" and "˘"
	  glyphs.
	</p>
<p>
	  But exactly where you should draw the breve depends on the
	  height and width of the preceding zhe glyph. To find the
	  right position, you need to consult the table inside
	  the font (the <code class="literal">GPOS</code> table) that contains
	  positioning information.
          In other words, <span class="emphasis"><em>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.</em></span>
        </p>
</li>
</ul></div>
<p>
      If tasks like these are something that you need to do, then you
      need a text shaping engine. You could use Uniscribe if you are
      writing Windows software; you could use CoreText on macOS; or
      you could use HarfBuzz.
    </p>
<div class="note"><p>
	In the rest of this manual, the text will assume that the reader
	is that implementor of a text-layout engine.
      </p></div>
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