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Typography concerns the selection and arrangement of written forms. In digital systems, the visible result depends on several distinct operations: interpreting text characters, selecting glyphs from a computer font, positioning them, and converting their geometry into pixels through rasterization.
This distinction matters when a document looks wrong. Missing glyphs, incorrect glyph substitutions and poorly rendered pixels can produce different failures even when the underlying Unicode text is unchanged.[ref]
OpenType Layout Overview by Microsoft. Characters versus glyphs and OpenType Layout terminology: abstract characters, visual glyphs, substitution and positioning