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