1 <chapter id="what-is-harfbuzz"> 2 <title>What is Harfbuzz?</title> 3 <para> 4 Harfbuzz is a <emphasis>text shaping engine</emphasis>. It solves 5 the problem of selecting and positioning glyphs from a font given a 6 Unicode string. 7 </para> 8 <section id="why-do-i-need-it"> 9 <title>Why do I need it?</title> 10 <para> 11 Text shaping is an integral part of preparing text for display. It 12 is a fairly low level operation; Harfbuzz is used directly by 13 graphic rendering libraries such as Pango, and the layout engines 14 in Firefox, LibreOffice and Chromium. Unless you are 15 <emphasis>writing</emphasis> one of these layout engines yourself, 16 you will probably not need to use Harfbuzz - normally higher level 17 libraries will turn text into glyphs for you. 18 </para> 19 <para> 20 However, if you <emphasis>are</emphasis> writing a layout engine 21 or graphics library yourself, you will need to perform text 22 shaping, and this is where Harfbuzz can help you. Here are some 23 reasons why you need it: 24 </para> 25 <itemizedlist> 26 <listitem> 27 <para> 28 OpenType fonts contain a set of glyphs, indexed by glyph ID. 29 The glyph ID within the font does not necessarily relate to a 30 Unicode codepoint. For instance, some fonts have the letter 31 "a" as glyph ID 1. To pull the right glyph out of 32 the font in order to display it, you need to consult a table 33 within the font (the "cmap" table) which maps 34 Unicode codepoints to glyph IDs. Text shaping turns codepoints 35 into glyph IDs. 36 </para> 37 </listitem> 38 <listitem> 39 <para> 40 Many OpenType fonts contain ligatures: combinations of 41 characters which are rendered together. For instance, it's 42 common for the <literal>fi</literal> combination to appear in 43 print as the single ligature "". Whether you should 44 render text as <literal>fi</literal> or "" does not 45 depend on the input text, but on the capabilities of the font 46 and the level of ligature application you wish to perform. 47 Text shaping involves querying the font's ligature tables and 48 determining what substitutions should be made. 49 </para> 50 </listitem> 51 <listitem> 52 <para> 53 While ligatures like "" are typographic 54 refinements, some languages <emphasis>require</emphasis> such 55 substitutions to be made in order to display text correctly. 56 In Tamil, when the letter "TTA" () letter is 57 followed by "U" (), the combination should appear 58 as the single glyph "". The sequence of Unicode 59 characters "" needs to be rendered as a single 60 glyph from the font - text shaping chooses the correct glyph 61 from the sequence of characters provided. 62 </para> 63 </listitem> 64 <listitem> 65 <para> 66 Similarly, each Arabic character has four different variants: 67 within a font, there will be glyphs for the initial, medial, 68 final, and isolated forms of each letter. Unicode only encodes 69 one codepoint per character, and so a Unicode string will not 70 tell you which glyph to use. Text shaping chooses the correct 71 form of the letter and returns the correct glyph from the font 72 that you need to render. 73 </para> 74 </listitem> 75 <listitem> 76 <para> 77 Other languages have marks and accents which need to be 78 rendered in certain positions around a base character. For 79 instance, the Moldovan language has the Cyrillic letter 80 "zhe" () with a breve accent, like so: . Some 81 fonts will contain this character as an individual glyph, 82 whereas other fonts will not contain a zhe-with-breve glyph 83 but expect the rendering engine to form the character by 84 overlaying the two glyphs and . Where you should draw the 85 combining breve depends on the height of the preceding glyph. 86 Again, for Arabic, the correct positioning of vowel marks 87 depends on the height of the character on which you are 88 placing the mark. Text shaping tells you whether you have a 89 precomposed glyph within your font or if you need to compose a 90 glyph yourself out of combining marks, and if so, where to 91 position those marks. 92 </para> 93 </listitem> 94 </itemizedlist> 95 <para> 96 If this is something that you need to do, then you need a text 97 shaping engine: you could use Uniscribe if you are using Windows; 98 you could use CoreText on OS X; or you could use Harfbuzz. In the 99 rest of this manual, we are going to assume that you are the 100 implementor of a text layout engine. 101 </para> 102 </section> 103 <section id="why-is-it-called-harfbuzz"> 104 <title>Why is it called Harfbuzz?</title> 105 <para> 106 Harfbuzz began its life as text shaping code within the FreeType 107 project, (and you will see references to the FreeType authors 108 within the source code copyright declarations) but was then 109 abstracted out to its own project. This project is maintained by 110 Behdad Esfahbod, and named Harfbuzz. Originally, it was a shaping 111 engine for OpenType fonts - "Harfbuzz" is the Persian 112 for "open type". 113 </para> 114 </section> 115 </chapter>
