Open Font Format

Digital font technology consists of three components: the digital fonts themselves and the text rasterizer and layout engine. The rasterizer is a piece of software built into the Macintosh, UNIX and Windows operating systems as well as many embedded RTOS and applications. All three components—the font, the rasterizer, and layout engine—are necessary for displaying and printing text on a computer system. It is the interaction between the fonts, the font rasterizer, the layout engine, and the software application in which the fonts are used that determines the appearance of the letterforms in the text.

Historically, scalable digital fonts have come in two basic flavors: PostScript®[i] Type 1 and TrueType™[ii]. Both of them are multi-platform outline font standards for which the technical specifications are openly available. "Outline font" means that they describe letter shapes ("glyphs") by means of points, which when connected by lines and curves, represents the “outline” of each glyph. This representation is resolution independent, meaning that outlines, by their very nature, can be scaled to pretty much any arbitrary size. Depending on the particular program being used and the operating system it's run under, there may be upper and lower limits to the size the font can be scaled to, but few users will ever encounter these limits.

OpenType®[iii] is the unification of the two most powerful and widely used font formats, PostScript and TrueType, into a single font format. Designed to handle languages with complex typographic requirements, such as Arabic, it supports alternate glyphs and contextual ligature substitution – these features finally have support at the system level. Developed by Adobe and Microsoft, OpenType brings the two most popular font technologies together and extends them with new typographic and line layout capabilities. Users no longer need to concern themselves over the flavor of their fonts – OpenType resolves these issues by putting both PostScript and TrueType into a single unified data structure. An OpenType font can contain either or both kinds of data, which can then be rasterized by any system, which supports OpenType. OpenType/TrueType has already been integrated in many operating systems as the de-facto standard.

Overview of Open Font Format

The ISO/IEC 14496-22 "Open Font Format Specification" (OFFS) is based on the OpenType specification, version 1.4 [1].

An outline font must be represented by the dots of the output device, whether it's screen pixels or the dots of a printer. The process of converting the outline to a pattern of dots on the grid of the device is called "rasterization". On low-resolution screens, when the size of the dots are close to the size of the features that make up a glyph there can be inconsistencies in the representation of certain letter features, due to different rounding based on how the outline happens to sit on the grid. A common example of this is shown below when stems of equal width are rendered with different pixel values at a single size. Worse, key features of the glyphs can disappear at small sizes [2].

OpenType fonts have means of dealing with these inconsistencies, called "hinting". This consists of additional instructions encoded in the font to help prevent these problems. Hinting instructions reshape the outline to insure that it is grid-fitted and that the rendered glyph is a faithful reproduction of the original designer intent.

Unicode is the universal standard for character encoding [3, 4], which allows the exchange of documents set in different or multiple languages and scripts. With Unicode, each character in each script used in the world is given a unique identifier. This means that the letter ‘a’ will always have the same character code, no matter what system you are working on. A Unicode compliant font would ensure that a Unicode compliant document would display and print properly, regardless of the system being used. Since OpenType fully supports Unicode and since an OpenType font can contain upwards of 65,000 characters, it is now possible to create a single font, which supports multiple languages and scripts. This is an important step in making global computing, multimedia encoding and publishing feasible.

Many non-Latin scripts, such as Indic or Arabic, require complex layout and typography in order for them to be properly rendered. For example, Arabic characters change their shape depending upon where they are in a word.

OpenType provides font developers with the ability to add line layout information to fonts in the form of OpenType tables. OpenType savvy applications will use these tables to take advantage of advanced line layout features. In the following example, BASE table entries, defined relative to the em height, are used to adjust the vertical position of lines of text composed with glyphs of different scripts and point sizes. The dominant script is Latin, and the single kanji character needs to be moved down in order to align it correctly with the Latin text [5].

Traditional Latin fonts may also contain line layout tables, which allow the automatic insertion of ligatures, swashes and ornaments. OpenType improves the way users create professional looking documents by replacing the cumbersome process of using several expert sets and swash fonts, dramatically facilitating use.

Document portability becomes possible by embedding fonts directly within documents and multimedia presentations. OpenType embedding is lossless, which means that content creators can be assured that users will see the fonts as they were intended, complete with the original hinting, and that the fonts will not be replaced by system fonts or synthetic reproductions of a font. Font embedding is also more efficient than transmitting large image files of stylized text in the form of pre-rendered graphics (e.g. GIFs).

WG 11 previously considered compression applications which have been standardized [6]. Although outside the basic definition of OpenType, related compression is a useful practical application for use with the Open Font Format.

OpenType system includes concepts such as font embedding and compression. The OpenType font specification is enabling for these technologies. OpenType provides capabilities for compressing the font data, either during the production process or when a user chooses to embed a font. This compression, if applied, may significantly reduce the time necessary to transmit a font. An OpenType font can also be subsetted during embedding to contain only those characters, which are used in the document. This subsetting reduces the data size and the time spent in transmitting fonts to MPEG-4 terminals [6], greatly enhancing the speed and bandwidth consumption. Most importantly, with embedded fonts, users are assured of high quality type, no matter where the font is rendered.

Target applications

A wide range of applications that utilize advanced text and graphics capabilities – these include newscast, commercial broadcasting, e-learning, games, interactive TV and Rich Media, multimedia messaging, Digital Cinema sub-titling; document publishing; etc.

References

[3] The Unicode Consortium. The Unicode Standard, Version 4.0 Boston, MA, Addison-Wesley Developers Press, 2003. ISBN 0-321-18578-1;

[4] ISO/IEC 10646:2003 "Information technology - Universal Coded Character Set (UCS)"

[i] PostScript is a registered trademark of Adobe Systems Incorporated.

[ii] TrueType is a trademark of Apple Computer Incorporated.

[iii] OpenType is a registered trademark of Microsoft Corporation.

Title	ISO/IEC 14496-22 "Open Font Format"
Source	Systems
Status	Proposal
Editors	Mike Ksar, Vladimir Levantovsky

Introduction

Overview of Open Font Format

Target applications

References