script

In text shaping, a script is a writing system: a set of symbols, rules, and conventions that is used to represent a language or multiple languages.

In general computing lingo, the word "script" can also be used to mean an executable program (usually one written in a human-readable programming language). For the sake of clarity, HarfBuzz documents will always use more specific terminology when referring to this meaning, such as "Python script" or "shell script." In all other instances, "script" refers to a writing system.

For developers using HarfBuzz, it is important to note the distinction between a script and a language. Most scripts are used to write a variety of different languages, and many languages may be written in more than one script.

shaper

In HarfBuzz, a shaper is a handler for a specific script-shaping model. HarfBuzz implements separate shapers for Indic, Arabic, Thai and Lao, Khmer, Myanmar, Tibetan, Hangul, Hebrew, the Universal Shaping Engine (USE), and a default shaper for non-complex scripts.

cluster

In text shaping, a cluster is a sequence of codepoints that must be treated as an indivisible unit. Clusters can include code-point sequences that form a ligature or base-and-mark sequences. Tracking and preserving clusters is important when shaping operations might separate or reorder code points.

HarfBuzz provides three cluster levels that implement different approaches to the problem of preserving clusters during shaping operations.

grapheme

In linguistics, a grapheme is one of the indivisible units that make up a writing system or script. Often, graphemes are individual symbols (letters, numbers, punctuation marks, logograms, etc.) but, depending on the writing system, a particular grapheme might correspond to a sequence of several Unicode code points.

In practice, HarfBuzz and other text-shaping engines are not generally concerned with graphemes. However, it is important for developers using HarfBuzz to recognize that there is a difference between graphemes and shaping clusters (see above). The two concepts may overlap frequently, but there is no guarantee that they will be identical.

syllable

In linguistics, a syllable is an a sequence of sounds that makes up a building block of a particular language. Every language has its own set of rules describing what constitutes a valid syllable.

For text-shaping purposes, the various definitions of "syllable" are important because script-specific shaping operations may be applied at the syllable level. For example, a reordering rule might specify that a vowel mark be reordered to the beginning of the syllable.

Syllables will consist of one or more Unicode code points. The definition of a syllable for a particular writing system might correspond to how HarfBuzz identifies clusters (see above) for the same writing system. However, it is important for developers using HarfBuzz to recognize that there is a difference between syllables and shaping clusters. The two concepts may overlap frequently, but there is no guarantee that they will be identical.