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.
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.
The main characteristics of CTL complexity are:
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 and, 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.
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:
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.
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.