The OpenType font format is an extension of the TrueType font format, adding support for PostScript font data. The OpenType font format was developed jointly by Microsoft and Adobe. OpenType fonts and the operating system services which support OpenType fonts provide users with a simple way to install and use fonts, whether the fonts contain TrueType outlines or CFF (PostScript) outlines.
The OpenType font format addresses the following goals:
- broader multi-platform support
- better support for international character sets
- better protection for font data
- smaller file sizes to make font distribution more efficient
- broader support for advanced typographic control
OpenType fonts are also referred to as TrueType Open v.2.0 fonts, because they use the TrueType 'sfnt' font file format. PostScript data included in OpenType fonts may be directly rasterized or converted to the TrueType outline format for rendering, depending on which rasterizers have been installed in the host operating system. But the user model is the same: OpenType fonts just work. Users will not need to be aware of the type of outline data in OpenType fonts. And font creators can use whichever outline format they feel provides the best set of features for their work, without worrying about limiting a font’s usability.
OpenType fonts can include the OpenType Layout tables, which allow font creators to design better international and high-end typographic fonts. The OpenType Layout tables contain information on glyph substitution, glyph positioning, justification, and baseline positioning, enabling text-processing applications to improve text layout.
As with TrueType fonts, OpenType fonts allow the handling of large glyph sets using Unicode encoding. Such encoding allows broad international support, as well as support for typographic glyph variants.
Additionally, OpenType fonts may contain digital signatures, which allows operating systems and browsing applications to identify the source and integrity of font files, including embedded font files obtained in web documents, before using them. Also, font developers can encode embedding restrictions in OpenType fonts, and these restrictions cannot be altered in a font signed by the developer.
The following documents may be consulted for further information:
- Adobe Technical Note #5176: “The Compact Font Format Specification.”
- Adobe Technical Note #5177: “Type 2 Charstring Format.”
- TrueType 1.0 Font Files, Technical Specification. Microsoft.
- OpenType Layout Font Specification. Microsoft.
- Adobe Type 1 Font Format.
- Addison Wesley, 1991; ISBN 0-201-57044-0.
- Technical Note #5015: “The Type 1 Font Format Supplement.”
- This document contains all updates to the Type 1 format.
- Adobe Technical Note #5087: “Multiple Master Font Programs for the Macintosh.”
- Adobe Technical Note #5088 “Font Naming Issues.”
- This document discusses general font name issues.