OpenType is a scalable computer font format initially developed by Microsoft, later joined by Adobe Systems. OpenType was first announced in 1996, with significant number of OpenType fonts starting to ship in 2000-2001. Adobe completed conversion of its entire font library to OpenType around the end of 2002. As of early 2005, there are around 10,000 fonts available in OpenType format, with Adobe's library making up under a third of the total.
OpenType uses the general "sfnt" structure of a TrueType font, but it adds several smartfont options which enhance the font's typographical abilities. An OpenType font can include either TrueType outlines or PostScript-style outlines (the latter stored in the compact CFF/Type 2 font format).
OpenType has several distinctive features:
OpenType Layout tags are 4-byte character strings that identify the scripts, language systems, features and baselines in a OpenType Layout font. Microsoft's Layout tag registry establishes conventions for naming and using these tags.
Comparison to other formats
Compared with Apple Computer's "GX Typography" - now called Apple Advanced Typography (AAT) - OpenType offers less flexibility in typographic options, but superior language-related options and support.
OpenType has been much more successful than AAT. There are many more fonts and supporting applications, despite AAT being an older technology. The single-platform nature of AAT and the lack of support from any major software vendor other than Apple itself are both likely factors in this.
From a font developer's perspective, OpenType is, for many common situations, easier to develop for than AAT. First, the simple declarative substitutions and positioning of OpenType are more readily understood than AAT's more complex (but powerful) state tables. Second, Adobe's strategy of licensing at no charge the source code developed for its own font development allowed third-party font editing applications such as FontLab and FontMaster to add support with relative ease. Although Adobe's text-driven coding support is not as visual as Microsoft's separate tool, VOLT (Visual OpenType Layout Tool), the integration with the tools being used to make the fonts has been well received.
Another difference is that an OpenType support framework (such as Microsoft's Uniscribe) needs to provide a fair bit of knowledge about special language processing issues to handle (for example) Arabic. With AAT, the font developer of an AAT font has to encapsulate all that expertise in the font. This means that AAT can handle any arbitrary language, but that it requires more work and expertise from the font developers. On the other hand, OpenType fonts are easier to make, but can only support certain complicated languages if the application or operating system knows how to handle them.
Previous to supporting OpenType, Adobe promoted multiple master fonts for high-end typography. Multiple master fonts lacked the controls for alternate glyphs and languages provided by OpenType, but provided more control over glyph shape.