OpenType

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.

History

OpenType is intended by Microsoft and Adobe to be the successor to the TrueType font format developed by Apple Computer and licensed by Microsoft, and the Type 1 ("PostScript") font format developed by Adobe.

Microsoft tried to license Apple's advanced typography technology, "GX Typography," in the early 1990s, and upon being refused turned to develop its own technology dubbed "TrueType Open" in 1994. Adobe joined Microsoft in 1996, adding support for the type of outlines used in its PostScript Type 1 fonts, and the name OpenType was then used for the combined technologies.

Adobe and Microsoft continued to develop and refine OpenType over the next decade. Then, in late 2005, OpenType began the process of becoming an open standard under the International Standards Organization (ISO), under the MPEG group which had previously adopted OpenType by reference. The new standard is essentially OpenType 1.4, with appropriate language changes for ISO, and is to be called the "Open Font Format." The initial adoption of the new standard is expected to be complete in late 2006.

Description

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 format).

OpenType has several distinctive features:

the font encoding is based on Unicode and can support any script (or multiple scripts at once)
OpenType fonts can have up to 65,536 glyphs
fonts can have advanced typographic features that allow proper typographic treatment of complex scripts and advanced typographic effects for simpler scripts, such as the Latin script used in writing English.
font files are intended to be cross-platform, and can be used without modification on Mac OS, Windows and some Unix systems
if no additional glyphs or extensive typographic features are added, OpenType CFF fonts can be considerably smaller than their Type 1 counterparts

Comparison to other formats

Compared with Apple Computer's "GX Typography" now called Apple Advanced Typography, or AAT, OpenType offers less flexibility in typographic options, but superior language-related options and support.

OpenType has been much more successful than GX/AAT. There are many more fonts and supporting applications, despite GX/AAT being an older technology. The single-platform nature of GX/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 much easier to develop for than GX was. First, the simple declarative substitutions and positioning of OpenType are much simpler to understand and code for than GX's more 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 relatively easily add support. 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 is supporting those languages.

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.

OpenType support

OpenType support may be divided into several categories: virtually all applications and most operating systems work with OpenType fonts just as well as other, older formats. What is of particular interest is: extended language support through Unicode, support for "complex" writing scripts such as Arabic and the Indic languages, and advanced typographic support for Latin script languages such as English.

As of early 2005, extended language support via Unicode for both OpenType and TrueType is present in most Windows applications (including Microsoft Office, Publisher and most Adobe applications), and many Mac OS applications, especially Apple's own such as TextEdit and Keynote.

OpenType support for complex written scripts has so far mainly appeared in Microsoft applications such as Office and Publisher. Adobe InDesign provides extensive OpenType capability in Japanese but does not directly support Middle Eastern or Indic scripts - though a separate version of InDesign is available that supports Middle Eastern scripts such as Arabic and Hebrew.

Advanced typographic support for Latin script languages has so far mainly appeared in Adobe applications such as Adobe InDesign, Adobe Photoshop and Adobe Illustrator. QuarkXPress 6.5 and below (Quark, Inc.) are not Unicode compliant. Hence text which contains anything other than WinANSI/MacRoman characters will not display correctly in an OpenType font (nor in other Unicode font formats, for that matter). Corel's CorelDRAW does not support OpenType typographic features, either. However, Quark offers support similar to Adobe's in QuarkXPress 7, which shipped in May 2006. Additionally, AAT-supporting applications running on Mac OS X 10.4 and later, including TextEdit and Keynote, get considerable OpenType support.

Apple's support for OpenType in Mac OS 10.4 includes most advanced typographic features necessary for latin‐script languages, such as small caps, oldstyle figures, and various sorts of ligatures. It does not yet support contextual alternates, positional forms, nor glyph reordering as handled by Microsoft's Uniscribe library on Windows. Thus, Mac OS 10.4 does not offer support for Arabic or Indic scripts via OpenType (though such scripts are supported by existing AAT fonts).

In free software environments like Linux, OpenType support is provided by the FreeType project, included in free implementations of the X Window System such as Xorg.

SING gaiji solution

In 2005, Adobe shipped a new technology in their Creative Suite applications bundle that offers a solution for the gaiji problem. Ideographic writing scripts such as Chinese and Japanese do not have fixed collections of characters. They use thousands of glyphs commonly and tens of thousands less commonly. Not all glyphs ever invented and used in far eastern literature have even been catalogued. A typical font might contain 8,000 to 15,000 of the most commonly used glyphs. From time to time, though, an author needs a glyph not present in the font of choice. Such missing characters are known in Japan as gaiji, and they often disrupt work.

Another aspect of the gaiji problem is that of variant glyphs for certain characters. Often certain characters have been written differently over periods of time. It is not unusual for place names or personal family names to use a historical form of a character. Thus it is possible for an end user using standard fonts to be left unable to spell correctly either their own name or the name of the place where they live.

Several ways to deal with gaiji have been devised. Solutions that treat them as characters usually assign arbitrary Unicode values to them in the PUA (private use area). Such characters cannot be used outside the environment in which the association of the private Unicode to the glyph shape is known. Documents based on them are not portable. Other installations treat gaiji as graphics. This can be cumbersome because text layout and composition cannot apply to graphics. They cannot be searched for. Often their rendering looks different from surrounding characters because the machinery for rendering graphics usually is different from the machinery for rendering glyphs from fonts.

The SING technology that made its debut with Adobe's Creative Suite 2 allows for the creation of glyphs, each packaged as a stand-alone font, after a fashion. Such a packaged glyph is called a glyphlet. The format, which Adobe has made public, is based on OpenType. The package consists of the glyph outline in TrueType or CFF (PostScript style outlines) form; standard OpenType tables declaring the glyph's metrics and behavior in composition; and metadata, extra information included for identifying the glyphlet, its ownership, and perhaps pronunciation or linguistic categorization.

The SING (Smart INdependent Glyphlets) specification states that glyphlets are to travel with the document they are used in. That way documents are portable, leaving no danger of characters in the document that cannot be displayed. Because glyphlets are essentially OpenType fonts, standard font machinery can render them. The SING specification also describes an XML format that includes all the data necessary for reconstituting the glyphlet in binary form. A typical glyphlet might require 1 to 2 kilobytes to represent.

OpenType is a registered trademark of Microsoft Corporation.

Features supported by Mac OS X

See also Datafork TrueType.

As of Mac OS X v10.4 (Tiger), Apple has started including support for OpenType features in the operating system itself, relieving application developers of the task. However there are many caveats:

If a font is detected to have OpenType tables, any AAT tables present are ignored.
Only the following GDEF/GPOS/GSUB features (tags) are confirmed as supported:
- liga: Common ligatures
- dlig: Discretionary ligatures
- sups: Superior/superscript position
- tnum: Tabular numerals
- pnum: Proportional numerals
- onum: Old style numerals
- lnum: Lining numerals
- sinf: Scientific inferiors
- zero: Slashed zero
- ital: Italics
- trad: Traditional CJK forms
- jp78: JIS 1978 CJK forms
- jp83: JIS 1983 CJK forms
- expt: Expert CJK forms
- nlck: NLC Kanji forms
- ruby: Ruby notation forms
- pwid: Proportional CJK widths
- fwid: Full CJK widths
- hwid: Half CJK widths
- twid: Third CJK widths
- qwid: Quarter CJK widths
- palt: Alternate proportional CJK widths
- halt: Alternate half CJK widths
- vert or vrt2: Optimised kana vertical alternates
- this list is incomplete
These features are known not to be supported:
- mark: Mark‐to‐base positioning
- mkmk: Mark‐to‐mark positioning
- mset: Mark positioning via substitution
- nukt: Nukta form
- akhn: Akhand ligature
- rphf: Reph form
- blwf: Below-base form
- half: Half-form (pre-base form)
- pstf: Post-base form
- vatu: Vattu variants
- pres: Pre-base substitution
- blws: Below-base substitution
- abvs: Above-base substitution
- psts: Post-base substitution
- haln: Halant form substitution
- blwm: Below-base mark positioning
- abvm: Above-base mark positioning
- dist: Distances

External links

Typesetting Font formats

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