Complex text layout (CTL) or complex text rendering is the typesetting of writing systems in which the shape or positioning of a grapheme depends on its relation to other graphemes. The term is used in the field of software internationalization, where each grapheme is a character.
The Devanagariddhrya-ligature, as displayed in the font, which should be invoked by the layout engine to render the sequence द + ् + ध + ् + र + ् + य = द्ध्र्य.The word العربيةal-arabiyyah, "the Arabic [language]" in Arabic, in successive stages of rendering. The first line shows the letters in left-to-right order and unjoined, as they might appear in an application without complex text layout. In the second line, bidirectional display has been applied, and in the third the glyph-shaping mechanism has rendered the letters according to context.
Scripts which require CTL for proper display may be known as complex scripts. Examples include the Arabic alphabet and scripts of the Brahmic family, such as Devanagari, Khmer script or the Thai alphabet. Many scripts do not require CTL. For instance, the Latin alphabet or Chinese characters can be typeset by simply displaying each character one after another in straight rows or columns. However, even these scripts have alternate forms or optional features (such as cursive writing) which require CTL to produce on computers.
Bi-directional text, where characters may be written from either right-to-left or left-to-right direction.
Context-sensitive shaping and ligatures, where a character may change its shape, dependent on its location and/or the surrounding characters. For example, a character in Arabic script can have as many as four different shape-forms, depending on context.
Ordering, where the displayed order of the characters is not the same as the logical order. For example, in Devanagari, which is written from left to right, the grapheme for "short i" appears to the left of ("before") the consonant that it follows: in किki, the ि-i should render on the left, its bow reaching until above the कk- to the right.
Not all occurrences of these characteristics require CTL. For example, the Greek alphabet has context-sensitive shaping of the letter sigma, which appears as ς at the end of a word and σ elsewhere. However, these two forms are normally stored as different characters; for instance, Unicode has both U+03C2ςGREEK SMALL LETTER FINAL SIGMA and U+03C3σGREEK SMALL LETTER SIGMA, and does not treat them as equivalent. For collation and comparison purposes, software should consider the string "δῖος Ἀχιλλεύς" equivalent to "δῖοσ Ἀχιλλεύσ",[1] but for typesetting purposes they are distinct and CTL is not required to choose the correct form.
ImplementationsEdit
Most text-rendering software that is capable of CTL will include information about specific scripts, and so will be able to render them correctly without font files needing to supply instructions on how to lay out characters. Such software is usually provided in a library; examples include:
Pango, a cross-platform library which nowadays incorporates HarfBuzz
However, such software is unable to properly render any script for which it lacks instructions, which can include many minority scripts. The alternative approach is to include the rendering instructions in the font file itself. Rendering software still needs to be capable of reading and following the instructions, but this is relatively simple.
The OpenType format is primarily intended for systems using the first approach (layout knowledge in the renderer, not the font), but it has a few features that assist with CTL, such as contextual ligatures. AAT and Graphite instructions can be embedded in OpenType font files.
^"FAQ - Greek Language & Script". Unicode Consortium. 2012-12-03. Retrieved 2013-09-13. It is easier to simply equate the two sigma codes for operations which are concerned with word content, for example.
External linksEdit
Examples of complex rendering — SIL international's examples of complex writing systems around the world
Complex Text Layout — The Open Group's Desktop Technologies
D-Type Unicode Text Module — Portable software library for complex text
BidiRenderer — An application that illustrates the shaping and layout of complex text in bidirectional paragraphs using FriBidi, FreeType, and HarfBuzz
Tehreer-Android — A library that gives full control over text related technologies such as bidirectional algorithm, open type shaping, text typesetting and text rendering
Tehreer-Cocoa — Standalone font/text engine for iOS
October 20, 2023
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For assistance with enabling complex text layout on your computer see Help Multilingual support This article includes a list of references related reading or external links but its sources remain unclear because it lacks inline citations Please help to improve this article by introducing more precise citations July 2013 Learn how and when to remove this template message Complex text layout CTL or complex text rendering is the typesetting of writing systems in which the shape or positioning of a grapheme depends on its relation to other graphemes The term is used in the field of software internationalization where each grapheme is a character The Devanagari ddhrya ligature as displayed in the JanaSanskritSans font which should be invoked by the layout engine to render the sequence द ध र य द ध र य The word العربية al arabiyyah the Arabic language in Arabic in successive stages of rendering The first line shows the letters in left to right order and unjoined as they might appear in an application without complex text layout In the second line bidirectional display has been applied and in the third the glyph shaping mechanism has rendered the letters according to context Scripts which require CTL for proper display may be known as complex scripts Examples include the Arabic alphabet and scripts of the Brahmic family such as Devanagari Khmer script or the Thai alphabet Many scripts do not require CTL For instance the Latin alphabet or Chinese characters can be typeset by simply displaying each character one after another in straight rows or columns However even these scripts have alternate forms or optional features such as cursive writing which require CTL to produce on computers Contents 1 Characteristics requiring CTL 2 Implementations 3 See also 4 References 5 External linksCharacteristics requiring CTL EditThe main characteristics of CTL complexity are Bi directional text where characters may be written from either right to left or left to right direction Context sensitive shaping and ligatures where a character may change its shape dependent on its location and or the surrounding characters For example a character in Arabic script can have as many as four different shape forms depending on context Ordering where the displayed order of the characters is not the same as the logical order For example in Devanagari which is written from left to right the grapheme for short i appears to the left of before the consonant that it follows in क ki the i should render on the left its bow reaching until above the क k to the right Not all occurrences of these characteristics require CTL For example the Greek alphabet has context sensitive shaping of the letter sigma which appears as s at the end of a word and s elsewhere However these two forms are normally stored as different characters for instance Unicode has both U 03C2 s GREEK SMALL LETTER FINAL SIGMA and U 03C3 s GREEK SMALL LETTER SIGMA and does not treat them as equivalent For collation and comparison purposes software should consider the string dῖos Ἀxilleys equivalent to dῖos Ἀxilleys 1 but for typesetting purposes they are distinct and CTL is not required to choose the correct form Implementations EditMost text rendering software that is capable of CTL will include information about specific scripts and so will be able to render them correctly without font files needing to supply instructions on how to lay out characters Such software is usually provided in a library examples include Core Text for macOS Uniscribe with Universal Shaping Engine and DirectWrite for Microsoft Windows HarfBuzz a cross platform library Pango a cross platform library which nowadays incorporates HarfBuzzHowever such software is unable to properly render any script for which it lacks instructions which can include many minority scripts The alternative approach is to include the rendering instructions in the font file itself Rendering software still needs to be capable of reading and following the instructions but this is relatively simple Examples of this latter approach include Apple Advanced Typography AAT and Graphite Both of these names encompass both the instruction format and the software supporting it AAT is included on Apple operating systems while Graphite is available for Microsoft Windows and Linux based systems The OpenType format is primarily intended for systems using the first approach layout knowledge in the renderer not the font but it has a few features that assist with CTL such as contextual ligatures AAT and Graphite instructions can be embedded in OpenType font files See also EditTypography Unicode Writing systems which require complex text layout Arabic alphabet Most of the Brahmic family of scripts N Ko script Tengwar diacritics and numbers References Edit FAQ Greek Language amp Script Unicode Consortium 2012 12 03 Retrieved 2013 09 13 It is easier to simply equate the two sigma codes for operations which are concerned with word content for example External links EditExamples of complex rendering SIL international s examples of complex writing systems around the world Complex Text Layout The Open Group s Desktop Technologies Supporting Indic Scripts in Mozilla also other CTL scripts Project SILA Graphite and Mozilla integration project CTL Architecture in Solaris Solaris Globalization Whitepapers Complex Scripts Microsoft Global Development and Computing Portal Theppitak s Homepage information about Thai language processing HarfBuzz s page at Freedesktop org D Type Unicode Text Module Portable software library for complex text BidiRenderer An application that illustrates the shaping and layout of complex text in bidirectional paragraphs using FriBidi FreeType and HarfBuzz Tehreer Android A library that gives full control over text related technologies such as bidirectional algorithm open type shaping text typesetting and text rendering Tehreer Cocoa Standalone font text engine for iOS Retrieved from https en wikipedia org w index php title Complex text layout amp oldid 1168211785, wikipedia, wiki, book, books, library,
