PostScript and TrueType

What is the Difference between PostScript and TrueType?

PostScript Type 1
PostScript Type 1 fonts can also be called outline fonts. They are a special form of the PostScript programming language, themselves building programs of their own, namely the font software. A font in the PostScript language contains an organized collection of procedures to describe the glyph forms. See the PostScript Language Reference Manual for more detailed information. Different font programs contain different amounts of information, gathered together in the so-called ?dictionary?. The dictionary saves the obligatory and optional entries and serves the PostScript interpreter as a data object in order to access all font functions.

PostScript font technology makes it possible to scale fonts, meaning to create the forms of the font in all point sizes. The major advantage in comparison to metal type or bitmap fonts is that the user needs only one font weight to create the forms of all point sizes. In relation to digital typefaces, the disadvantage of outline fonts compared to bitmap fonts is that the rasterizing takes time to compute and that outline fonts in small point sizes do not reach the quality of hand-drawn bitmaps.

All glyphs of Type 1 fonts are described from abstract, mathematical outlines of lines and curves. Points on these outlines describe the curves and are differentiated as on-curve and off-curve points. On-curve points lie on the outline of a glyph and off-curve points are part of on-curve points. They control the form of the curves which describe a glyph.


TrueType
The TrueType technology is made up of two components: fonts in TrueType font format and the TrueType rasterizer. Both are necessary to display and print TrueType fonts on computer systems. TrueType is compatible with both Apple and Windows systems, however, a separate TrueType font file must be created for each operating system.

TrueType is an outline font format which allows the scaling of typefaces. This means that all point sizes of a glyph are made from one and the same outline. The file itself contains different data blocks or tables which are accessed with a table directory. These tables contain information about the glyphs, outlines, bitmaps and other typographic and font specific data.

TrueType has three glyph types: simple, combined and glyphs without contour. Simple glyphs contain compressed outline and hinting data. Combined glyphs, often used for accented glyphs, base their outlines on other glyphs, whose components are again based on other simple or combined glyphs. A combined glyph contains the hinting for each component individually. Glyphs without contour, for example, the space, define the behavior of a text, have however no glyph of their own and hinting cannot be applied to them.

Outlines describe glyph forms mathematically and are constructed of on-curve points and off-curve points. Two on-curve points in a row will be connected by a line; curves are defined by quadratic B-splines. Each spline consists of a number of quadratic Bézier curves which are defined by three off-curve points.

The user generally never sees the outlines of a font. This is because both screens and printers use a bitmap matrix for display. The outlines of a TrueType font are scaled to the desired size or turned to the desired position and then converted into bitmaps. Turning a pixel on or off depends directly on the outline and follows these rules:

1. If the middle of a pixel is within the outline of a glyph, the pixel will be turned on and become part of the glyph.
2. If the outline falls exactly on the middle of a pixel, the pixel will be turned on.



A Question of Format
So which of the two font formats is ?better?? The answer is anything but simple and must be seen from a variety of angles. Both formats, TrueType and Type1, consist of algorithms or instructions with which an outline font can be rasterized in pixels for display. The main differences between the two formats are of technical and philosophical nature, as well as dependent upon various corporate policies.

Technical

Regarded technically, the hinting process of the TrueType technology is superior to that of Type 1 because it allows instructions in three directions, down, up and diagonal. Type 1 only allows upward and downward hinting. In connection with Apple and Microsoft it must also be noted that Type 1 fonts need the ATM while TrueType is an integrated part of both systems.

Philosophical

When TrueType and Type 1 fonts are used in the same situation, meaning same operating system and same end machine, the differences are minimal, usually even imperceptible to most users. The use of a particular format is today simply a matter of user preference.

Corporate

PostScript Type 1 is a development of Adobe Systems and was accepted for some time as the standard for scalable fonts. Adobe kept the format a secret and therefore kept a monopoly on this format. Apple Computer Corporation retaliated by developing its own outline font format, TrueType, which was later purchased and further developed by Microsoft.

TrueType is now a standard part of both Macintosh and Windows (3.1 and higher) operating systems and is responsible for display. Only after the introduction of TrueType did Adobe make its Type 1 font format public and available to other font manufacturers to make Type 1 compatible PostScript clones. To keep ahead in this race, Adobe introduced the ATM to serve as the driver between the operating system and the application. The ATM provides all printers, even non-PostScript, with the rasterized PostScript fonts. TrueType and Type 1 fonts have since established themselves within different niches, Type 1 in professional areas and TrueType in low-cost and home uses.


This Font Technology Feature is an excerpt from:
Ivir, Milo. ’Entwicklung digitaler Schrifttechnologien und ihre Einflußnahme auf die Typografie im Zeitalter des elektronischen Publizierens’ (The Development of Digital Font Technologies and their influence on Typography in the Age of Electronic Publishing). Diploma thesis, Institute for Technology and Print Planning, College of Art, Berlin, 1998.