Codex @codex  0

Glyph substitution replaces selected glyph indices by other glyph indices while laying out text. In OpenType, the GSUB table specifies these replacements after the default character-to-glyph index mapping. A substitution can replace one glyph by another, several by a typographic ligature, or one by several; its application can depend on surrounding glyphs.^[ref]

For a hypothetical computer font, suppose f and i initially select indices17 and23. A matching ligature rule could replace the pair by index91. These numbers belong to that particular computer font; they do not change the stored Unicode sequence into code point91. The example also shows why substitution differs from kerning: replacing the pair selects a new shape, whereas adjusting the pair's positions can retain both original shapes.

Lookup order matters when one substitution creates or removes the input pattern for a later substitution. Text shaping must therefore interpret the applicable OpenType rules, rather than independently choosing a default glyph for each text character.

Text shaping turns a sequence of Unicode text characters into a layout of glyphs from a chosen computer font. The result includes glyph indices, positions and glyph advances. A text character therefore does not determine a visible shape independently of its neighbors, writing system and selected computer font.^[ref]

For example, the stored sequence f followed by i can become a single typographic ligature. Contextual Arabic forms illustrate a different requirement: a letter's neighbors influence the selected glyph. Glyph substitution chooses forms, while glyph positioning places the chosen forms; kerning adjusts their spacing. These operations can preserve the original Unicode text, so copying text should not be implemented by reading glyph indices as if they were code points.^[ref]

Text shaping precedes rasterization: first determine what shapes to draw and where; then determine the pixels that represent those shapes at the requested size.

A typographic ligature combines two or more written forms into a single glyph. A familiar example is an “fi” shape joining the end of “f” to the following “i”. Its use can improve spacing or avoid a collision between the two shapes.

An OpenType shaping system can first map the separate text characters “f” and “i” to their default glyph indices, then apply a glyph substitution that replaces that pair by one ligature glyph index. The stored text need not be replaced by a special ligature character. Selection and copying can therefore preserve the original two-character sequence even though the display contains one glyph.^[ref]

A typographic ligature also need not be produced merely by moving two glyphs closer together: substitution changes the selected form, whereas positioning changes where selected forms are drawn.