.OTF

Introduction

OpenType is a font format developed by Microsoft and Adobe in the late 1990s and standardised as the Open Font Format, ISO/IEC 14496-22. It extends the TrueType container with Adobe’s PostScript outlines, in a table called CFF, and with layout tables that hold typographic features such as ligatures, small capitals, kerning and the shaping rules complex scripts need. Variable fonts, which hold a whole range of weights or widths in one file, are also OpenType.

A font with PostScript outlines usually has the extension .otf; an OpenType font with TrueType outlines usually keeps .ttf.

Structure

An OpenType font uses the same table directory as TrueType: an offset table, a 16-byte record per table, then the tables, all big-endian. A font with PostScript outlines starts with the version tag OTTO and stores its glyphs in a CFF or CFF2 table instead of glyf and loca. The layout features live in GSUB (glyph substitution), GPOS (glyph positioning) and GDEF, and variable fonts add tables such as fvar for their axes.

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