INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
ORGANISATION INTERNATIONALE DE NORMALISATION
ISO/IEC JTC 1/SC 29/WG 11
CODING OF MOVING PICTURES AND AUDIO

ISO/IEC JTC 1/SC 29/WG 11N10069

July 2008, Hannover, Germany

Title

ISO/IEC 14496-22 "Open Font Format"

Source

Systems

Status

Approved

Editor

Vladimir Levantovsky

Introduction

Digital font technology consists of two components: font rendering engine and the digital fonts themselves. Both components—the font and the rasterizer—are necessary for displaying and printing text on a computer system. It is the interaction between the fonts, the font rasterizer 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.

OpenType®[iii] [1] 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 and scripts with complex typographic requirements, such as Arabic and Indic scripts, it supports alternate glyphs and contextual substitutions – 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 are 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.

The ISO/IEC 14496-22 "Open Font Format Specification" (OFFS) was based on the OpenType specification version 1.4 contributed to ISO by Adobe Systems Inc. and Microsoft Corp. The first edition of the standard was published in 2007 and is now available from ISO/IEC website [2]. The specification is also freely available for download from ITTF website using the following link:

http://standards.iso.org/ittf/PubliclyAvailableStandards/c043466_ISO_IEC_14496-22_2007(E).zip.

The SC29/WG11 has also completed the work on conformance requirements, test sequences and reference software implementation for ISO/IEC 14496-22 – the additional information can be found in the following ISO documents:

At the time of writing this white paper, SC29/WG11 was in the process of developing the second edition of the standard, extending OFF technology to support latest version of the Unicode Standard, improving its functionality and adding support for new features [3].

Overview of Open Font Format

The OFFS addresses the following goals:

High quality glyph scaling

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 there aren't enough dots making up the 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 [4].


OFF 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 Conformance and Enhanced Typography

Unicode is a universal standard for character encoding [5, 6], 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 OFF fully supports Unicode and since an OFF 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.

OFF provides font developers with the ability to add line layout information to fonts in the form of advanced layout tables. OFF 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 [7].


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

Font Embedding

Document portability becomes possible by embedding fonts directly within documents and multimedia presentations. Font 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).

Open Font Format provides capabilities for compressing the font data [8], 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 OFF 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, 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

[1]      OpenType specification

          URL: http://www.microsoft.com/typography/otspec/default.htm

[2]      ISO/IEC 14496-22:2007 "Open Font Format" specification.

URL: http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=43466

[3]      ISO/IEC 14496-22/FCD "Open Font Format" specification (second edition).

URL: http://www.chiariglione.org/mpeg/working_documents/mpeg-04/part22/off.zip

[4]      Beat Stamm “The raster tragedy at low resolution” 1998.

          URL: http://www.microsoft.com/typography/tools/trtalr.htm

[5]      The Unicode Consortium. The Unicode Standard, Version 5.0 Boston, MA, Addison-Wesley Developers Press, 2006. ISBN 0-321-48091-0;

Unicode 5.1.0 (http://www.unicode.org/versions/Unicode5.1.0/)

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

          URL: http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=39921

[7]      John Hudson (for Microsoft Typography) “An Introduction to OpenType”

          URL: http://www.microsoft.com/typography/developers/opentype/

[8]      ISO/IEC 14496-18 "Font compression and streaming"

          URL: http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=40151


[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.