DIGITAL FONTS FORMATS
PostScript, TrueType, OpenType

author: Olivera Stojadinović
October 2006

translation into English: Danijela Tomazović
November 2009

Digital fonts are placed within computers in the shape of program files. These fonts' formats, as well as their characteristics and applying abilities depend on the programming language they are written in.

The most common formats in Windows OS, Mac OS, Linux OS and other operating systems are PostScript and TrueType, and since several years ago, the new OpenType fonts have been more and more used.

PostScript

PostScript is the programming language used for page description in a so called desktop publishing. John Warnock and Chuck Geschke founded Adobe in 1982 and created PostScript, using the concept of one earlier graphic language from 1976. The first usage of this language has been initiated by Steve Jobs from Apple Company in 1985, for the purpose of rasterization (RIP) in Apple Laser Writer Printers. After this, PostScript becomes the universal programming language used in graphics software. It is based on vector representation of the objects, using straight lines and so called cubic Bézier curves. The shape is determined based on coordinates of the points that are on the outline and coordinates of Bézier control points (BCP).

Unlike the first fonts which were defined using dots – bitmaps, for each size individually – Postscript is the first format using the geometric outline which can be enlarged or reduced as needed, as well as undergo all other geometrical transformations. These shapes, through rasterization, turned into an image with desired resolution on a printer.

In order for the fonts in small resolution to keep their characteristics, hinting has been added. Besides the geometric shapes of letters, this is also within font information. These fonts are known as PostScript Type 1 (unlike Type 3 which did not include hints) and are still used today. Type 2, envisioned as a compact font, later became the base for OpenType. These three fonts were developed in parallel and based on the same idea.

In Windows systems, PostScript 1 fonts consist out of two files of the same name with PFB and PFM extensions. The first file contains geometric description of the letter including the hints, and the second contains metric data, including kerning pairs (p airs of characters with adjusted space between them ). These data are coded. Additionally there are help files with AFM and INF extension from which metric data and information about the font can be read. Based on AFM and INF files, it is possible to generate PFM. In order to use and rasterize these fonts on desktop, until lately, additional software Adobe Type Manager was required. The newer Windows systems have built in support for Type 1 fonts.

In Mac operating systems, Type 1 fonts also consist out of two files. The font characteristics are within PostScript file which is represented by the laser printer icon, while the metric data together with bitmap (in at least one size) used for screen view, are placed within a file shaped like a little chauffeur. If more fonts (usually 4) are placed within the family, they have a common metric file. Only after ATM emerged, it was possible to use the fonts without bitmaps.

PostScript fonts cannot be exchanged between Mac and Windows systems. Also, they are one-byte long, that is, have 256 characters which are listed in code lines. Not only that each language has to have its codepage (while the first 128 characters are the same in all standard code pages), but Windows and Mac also use different codepages, which additionally complicates the communication between them, even in the case when the same, compatible softwares are used .

TrueType

TrueType is the next standard program which has geometric drawings as its base. It was developed in Apple, as a competition to Adobe Type 1 fonts, and the main reason were high prices Adobe was charging for Type 1 format usage. TrueType emerged in 1991 as a part of Mac OS System 7, together with a few basic fonts (Times Roman, Helvetica, Courier). Apple soon licensed TrueType to Microsoft, which implemented it in 3.1 system version right away. Same time, Microsoft ordered a set of fonts from Monotype company, similar to the ones already available on Macintosh ( Times New Roman, Arial, Courier New ), which are still the standard fonts on Windows systems.

The outlines of the letters consist out of straight lines and quadratic Bézier control curves, which are simpler than the cubic ones used by PostScript, but it takes more points to define a shape. The difference is also in the fact that there is only one file marked as TTF. The biggest advancement is made in the field of hinting, which developed into the instructions system with by far better control ability in small sizes and low resolutions.

Another important improvement that came with TrueType format is the emergence of two-byte fonts, that is, implementation of Unicode. Although there are TrueType fonts with one code page only, Microsoft system fonts implemented support for Unicode, starting with Windows 95, in which they included code pages used in Europe, while Windows NT is completely based on Unicode. Also, Microsoft Word becomes a Unicode program.

Although both vector formats are based on Bézier curves, the conversion from one into another deforms the letters a bit, and it is not possible to automatically transform the hint information.

Open Type

OpenType format is envisioned as the common successor to PostScript and TrueType formats. It was originally initiated by Microsoft which tried to get the GX typographic technology license from Apple. Using this technology, “the smart fonts”, which enabled easy manipulation with different typographic features within the same font, were developed. Unfortunately, this advanced technology was developed for Macintosh only, so the software companies were not interested in development of software which will not have Windows or other existing operating systems version as well. Since Apple did not accept the offer, Microsoft partnered with Adobe, so in the 1996 they announced the joint project OpenType, which was supposed to include all the benefits of the existing solutions.

These benefits are: multilingual set of letters with full Unicode support (65 353 code points), together with additional typographic variations such as small caps, ligatures, fractions, alternative characters, all within the same font. Besides, the font has additional features enabling easy manipulation with these characters. Another new function is class kerning which significantly decreases the kern-pairs tables.

One of the most important benefits of this format is placing TrueType and PostScript formats in the single shell which behaves as TrueType in the operating system and the applications, while the inside structure of the font is optionally PostScript (with OTF extension) or TrueType (with TTF extension), as preferred. The compatibility with all the existing systems was achieved by this, and both technologies became equally accessible to all the users. Thus, this also removed the always present barrier that existed in document formats exchange between Mac and PC systems.

Another important decision is that the coding mechanism was made open, which gave possibilities to all companies to publish fonts in OpenType format. FontLab, the modern software for font design, includes all the tools needed for this job.

OpenType fonts can be used in newer operating systems (Windows 2000 and XP, Mac OS X), which support Unicode. Also necessary are Unicode programs. The most common package, Microsoft Office, is developed for a wide user base. Adobe developed graphic software Creative Suite (CS), which uses all the benefits of OpenType fonts. Macromedia and other developers of software for web pages design developed software using a certain number of universal Unicode fonts which are, as per agreement, included in all systems. Other software developers included Unicode support in their software as well, so the choice is becoming wider every day.

Which format to choose ?

OpenType is definitely the best choice for the ones with systems meeting the above requirements, and that is in short, Unicode support. Whether to choose its PS or TT version? Each of these systems brings its own hinting system, so this is something to consider.

From the fonts designer standing point, PostScript fonts have a more basic outline than TrueType, because they include only the nodes in places where the line changes : at extremes, at places of curve inflection and on corners. This is the logic followed by the drawing software as well, so when turning into the letters, the curve points remain in the same places. Drivers for professional printers and imagesetters are written in PostScript programming language, so the fonts are printed without conversion . The hinting is relatively simple and with appropriate adjustments, it can be done automatically. The font display is always good except on screen in small sizes and resolutions, where TrueType is better. In case of high quality monitors with a high resolution, with optical corrections made using standard (antialiasing) and Clear Type technologies, the hinting benefits are not as important.

Older systems and software which do not support Unicode are limited to one-byte fonts which can use only the first 256 character spaces. The TrueType format fonts have the quality only in case of professionally hinted originals acquired from the producer. “The clones” which were derived from automatic conversion from PostScript to TrueType do not have satisfying quality, and should be avoided. This is why it is safer to opt in for Type 1 format.

Regarding printing, it is a paradox that TrueType fonts sometimes “go through” PostScript drivers easier, because it converts them into PS format automatically, while the same driver can reject PostScript fonts which have some error in generating.

If a designer generates the font on her own, it is easier to choose PostScript format, because the results in this case will be better if the parameters are adjusted properly. In the newer systems, FontLab software should be used for generating (regardless of the software used for drawing the letters) because this software has all the data needed for font adjustments available, which ensures for the new software to recognize it properly.

Literature:

Wikipedia: OpenType
Wikipedia: PostScript
Wikipedia: TrueType

Microsoft Typography: What Is TrueType
Microsoft Typography: TrueType History

Adobe: Introduction to OpenType
Microsoft Typography: Open Type Specification