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Docs: Usermanual- What is HarfBuzz; add intro to shaping concepts.

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Nathan Willis 2018-09-28 16:36:38 -05:00 committed by Khaled Hosny
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docs/usermanual-what-is-harfbuzz.xml
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@ -15,6 +15,57 @@
platforms, and it supports all of the standard font formats in use
today.
</para>
<section id="what-is-text-shaping">
<title>What is text shaping?</title>
<para>
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 a document.
</para>
<para>
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.
</para>
<para>
Modern software systems generally only deal with strings in the
Unicode encoding scheme (although legacy systems and documents may
involve other encodings).
</para>
<para>
There are several font formats that a program might
encounter, each of which has a set of standard text-shaping
rules.
</para>
<para>The dominant format is <ulink
url="http://www.microsoft.com/typography/otspec/">OpenType</ulink>. The
OpenType specification defines a series of <ulink url="https://github.com/n8willis/opentype-shaping-documents">shaping models</ulink> for
various scripts (including Indic, Arabic, Hangul, Hebrew, Khmer,
Myanmar, Thai and Lao, Tibetan, and a Universal Shaping Engine
designed to cover other scripts). These shaping models depend on
the font including certain features in its <literal>GSUB</literal>
and <literal>GPOS</literal> tables.
</para>
<para>
Alternatively, OpenType fonts can include shaping features for
the <ulink url="https://graphite.sil.org/">Graphite</ulink> shaping model.
</para>
<para>
TrueType fonts can also include OpenType shaping
features. Alternatively, TrueType fonts can also include <ulink url="https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html">Apple
Advanced Typography</ulink> (AAT) tables to implement shaping
support. AAT fonts are generally only found on macOS systems.
</para>
<para>
Text strings will usually be tagged with a script and language
tag that provide the context for text shaping. <ulink
url="https://docs.microsoft.com/en-us/typography/opentype/spec/scripttags">Script</ulink>
and <ulink
url="https://docs.microsoft.com/en-us/typography/opentype/spec/languagetags">language</ulink>
tags are defined by OpenType.
</para>
</section>
<section id="why-do-i-need-a-shaping-engine">
<title>Why do I need a shaping engine?</title>
<para>