Complex text layout

Complex text layout (abbreviated CTL) or complex text rendering refers to 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 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 U+03C2 ς GREEK SMALL LETTER FINAL SIGMA and U+03C3 σ GREEK 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", but for typesetting purposes they are distinct and CTL is not required to choose the correct form.

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