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Shaping a text run involves transforming the input sequence of Unicode codepoints with some combination of operations that is specified in the shaping model for the script.
The specific conditions that trigger a given operation for a text run varies from script to script, as do the order that the operations are performed in and which codepoints are affected. However, the same general set of shaping operations is common to all of the script shaping models.
A reordering operation moves a glyph from its original ("logical") position in the sequence to some other ("visual") position.
The shaping model for a given script might involve more than one reordering step.
A joining operation replaces a glyph with an alternate form that is designed to connect with one or more of the adjacent glyphs in the sequence.
A contextual substitution operation replaces either a single glyph or a subsequence of several glyphs with an alternate glyph. This substitution is performed when the original glyph or subsequence of glyphs occurs in a specified position with respect to the surrounding sequence. For example, one substitution might be performed only when the target glyph is the first glyph in the sequence, while another substitution is performed only when a different target glyph occurs immediately after a particular string pattern.
The shaping model for a given script might involve multiple contextual-substitution operations, each applying to different target glyphs and patterns, and which are performed in separate steps.
A contextual positioning operation moves the horizontal and/or vertical position of a glyph. This positioning move is performed when the glyph occurs in a specified position with respect to the surrounding sequence.
Many contextual positioning operations are used to place mark glyphs (such as diacritics, vowel signs, and tone markers) with respect to base glyphs. However, some scripts may use contextual positioning operations to correctly place base glyphs as well, such as when the script uses stacking characters.