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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="what-is-harfbuzz"></a>What is HarfBuzz?</h2></div></div></div>
<div class="toc"><dl class="toc">
-<dt><span class="section"><a href="what-is-harfbuzz.html#why-do-i-need-it">Why do I need it?</a></span></dt>
+<dt><span class="section"><a href="what-is-harfbuzz.html#what-is-text-shaping">What is text shaping?</a></span></dt>
+<dt><span class="section"><a href="why-do-i-need-a-shaping-engine.html">Why do I need a shaping engine?</a></span></dt>
+<dt><span class="section"><a href="ch01s03.html">What does HarfBuzz do?</a></span></dt>
+<dt><span class="section"><a href="what-harfbuzz-doesnt-do.html">What HarfBuzz doesn't do</a></span></dt>
<dt><span class="section"><a href="why-is-it-called-harfbuzz.html">Why is it called HarfBuzz?</a></span></dt>
</dl></div>
<p>
- HarfBuzz is a <span class="emphasis"><em>text shaping engine</em></span>. It solves
- the problem of selecting and positioning glyphs from a font given a
- Unicode string.
+ HarfBuzz is a <span class="emphasis"><em>text-shaping engine</em></span>. If you
+ give HarfBuzz a font and a string containing a sequence of Unicode
+ codepoints, HarfBuzz selects and positions the corresponding
+ glyphs from the font, applying all of the necessary layout rules
+ and font features. HarfBuzz then returns the string to you in the
+ form that is correctly arranged for the language and writing
+ system.
+ </p>
+<p>
+ HarfBuzz can properly shape all of the world's major writing
+ systems. It runs on all major operating systems and software
+ platforms and it supports the major font formats in use
+ today.
</p>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
-<a name="why-do-i-need-it"></a>Why do I need it?</h2></div></div></div>
+<a name="what-is-text-shaping"></a>What is text shaping?</h2></div></div></div>
+<p>
+ Text shaping is the process of translating a string of character
+ codes (such as Unicode codepoints) into a properly arranged
+ sequence of glyphs that can be rendered onto a screen or into
+ final output form for inclusion in a document.
+ </p>
+<p>
+ The shaping process is dependent on the input string, the active
+ font, the script (or writing system) that the string is in, and
+ the language that the string is in.
+ </p>
+<p>
+ Modern software systems generally only deal with strings in the
+ Unicode encoding scheme (although legacy systems and documents may
+ involve other encodings).
+ </p>
+<p>
+ There are several font formats that a program might
+ encounter, each of which has a set of standard text-shaping
+ rules.
+ </p>
+<p>The dominant format is <a class="ulink" href="http://www.microsoft.com/typography/otspec/" target="_top">OpenType</a>. The
+ OpenType specification defines a series of <a class="ulink" href="https://github.com/n8willis/opentype-shaping-documents" target="_top">shaping models</a> for
+ various scripts from around the world. These shaping models depend on
+ the font incorporating certain features as
+ <span class="emphasis"><em>lookups</em></span> in its <code class="literal">GSUB</code>
+ and <code class="literal">GPOS</code> tables.
+ </p>
<p>
- 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
- <span class="emphasis"><em>writing</em></span> 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.
+ Alternatively, OpenType fonts can include shaping features for
+ the <a class="ulink" href="https://graphite.sil.org/" target="_top">Graphite</a> shaping model.
</p>
<p>
- However, if you <span class="emphasis"><em>are</em></span> 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:
+ TrueType fonts can also include OpenType shaping
+ features. Alternatively, TrueType fonts can also include <a class="ulink" href="https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html" target="_top">Apple
+ Advanced Typography</a> (AAT) tables to implement shaping
+ support. AAT fonts are generally only found on macOS and iOS systems.
</p>
-<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
-<li class="listitem"><p>
- 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
- "a" 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 "cmap" table) which maps
- Unicode codepoints to glyph IDs. Text shaping turns codepoints
- into glyph IDs.
- </p></li>
-<li class="listitem"><p>
- Many OpenType fonts contain ligatures: combinations of
- characters which are rendered together. For instance, it's
- common for the <code class="literal">fi</code> combination to appear in
- print as the single ligature "fi". Whether you should
- render text as <code class="literal">fi</code> or "fi" 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.
- </p></li>
-<li class="listitem"><p>
- While ligatures like "fi" are typographic
- refinements, some languages <span class="emphasis"><em>require</em></span> such
- substitutions to be made in order to display text correctly.
- In Tamil, when the letter "TTA" (ட) letter is
- followed by "U" (உ), the combination should appear
- as the single glyph "டு". The sequence of Unicode
- characters "டஉ" needs to be rendered as a single
- glyph from the font - text shaping chooses the correct glyph
- from the sequence of characters provided.
- </p></li>
-<li class="listitem"><p>
- 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.
- </p></li>
-<li class="listitem"><p>
- 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
- "zhe" (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.
- </p></li>
-</ul></div>
<p>
- 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.
+ Text strings will usually be tagged with a script and language
+ tag that provide the context needed to perform text shaping
+ correctly. The necessary <a class="ulink" href="https://docs.microsoft.com/en-us/typography/opentype/spec/scripttags" target="_top">script</a>
+ and <a class="ulink" href="https://docs.microsoft.com/en-us/typography/opentype/spec/languagetags" target="_top">language</a>
+ tags are defined by OpenType.
</p>
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