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TrueType vs. Type 1
Part II
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“Apple adopted Adobe’s PostScript page description language (PDL) for its Apple LaserWriter printer in 1985.”
“PostScript font specification for Type 1 fonts, which included hinting, was not publicly available.”
“Apple developed their own scaleable font technology, first code-named Royal, and later introduced as TrueType.”
“Adobe’s released the long-protected specifications for the PostScript Type 1 font format in March 1990.”
“Apple’s forthcoming ‘Rhapsody’ operating system uses Display PostScript as the basis of its entire imaging model, which also gives it native support for PostScript fonts.”
“The primary advantage of TrueType over PS1 fonts is the fact that TrueType allows better hinting.”
“PostScript uses ‘dumb’ fonts and a ‘smart’ interpreter, while TrueType uses relatively smarter fonts and a dumber interpreter.”
“Apple long ago agreed to increase its system-level support for PostScript, but has not aggressively pursued doing so.”
“PostScript’s pair of Files are often smaller than TrueType’s single file.”
“Many PostScript Level 2 printers (and all PostScript 3 printers) have the TrueType rasterizer in ROM, built in.”
“Many of the theoretical advantages of TrueType are not actually realized in most commercially available TrueType fonts.”
“As of this writing there is no font editing software available at the retail level with native TrueType support, although this will change in late 1997.”
“Although most major vendors have TrueType fonts, only a few (such as Bitstream) offer their entire libraries in both formats.”
“Having two fonts with identical names can confuse the operating system, with unpredictable results.” |
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Commonalities
TrueType (TT) and PostScript Type 1 (PS1) are both multi-platform outline font standards for which the technical specifications are openly available. “Multi-platform” means that both font types are usable on multiple sorts of computer systems. “Outline font” means that they describe letter shapes (“glyphs”) by means of points, which in turn define lines and curves.
 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.
An outline font must be represented by the dots of the output device, whether it’s screen pixels or the dots of a laser, ink-jet or wire-pin printer. The process of converting the outline to a pattern of dots on the grid of the device is called “rasterization.”
When there aren’t enough dots making up the glyph (such as at small sizes or low resolutions), there can be inconsistencies in the representation of certain letter features, at a single size, due to different rounding based on how the outline happens to sit on the grid. A common form of this is that the widths of the letter stems can vary when they shouldn’t. Worse, key features of the glyphs can disappear at small sizes.
However, PostScript Type 1 and TrueType fonts both have a means of dealing with these inconsistencies, called “hinting.” This consists of additional information encoded in the font to help prevent these problems.
Brief History
PostScript predates TrueType by about six years. First, we had many different formats for digital fonts, none of which were standardized. Then Apple adopted Adobe’s PostScript page description language (PDL) for its Apple LaserWriter printer in 1985. This, combined with the introduction of desktop publishing software, sparked a revolution in page layout technology.
Soon the PostScript language was adopted for use in higher-end imagesetting devices, and became the native operating mode and language of many graphics programs as well. PostScript’s dominance seemed assured. Adobe was in complete control of the PostScript technology at this point. Although the command structure of the PostScript language was publicly available, and it was possible for someone to build a PostScript interpreter to compete with Adobe’s rasterizing software, it wouldn’t be able to interpret the hints. This was because the PostScript font specification for Type 1 fonts, which included hinting, was not publicly available. Adobe had only released the specifications for Type 3 fonts, which had some minor advantages, but did not image well on low resolution devices.
It rapidly became obvious to the major system software creators (Apple, Microsoft, and later IBM) that it was important to have scaleable font technology supported at the level of the operating system itself. But neither Apple nor Microsoft wanted to have a key piece of their system software technology controlled by an outside company. So Apple developed their own scaleable font technology, first code-named Royal, and later introduced as TrueType.
Apple traded the technology with Microsoft in exchange for the latter’s TrueImage PostScript clone technology (which was buggy at the time, and never got used by Apple, although it has surfaced in various later incarnations). The TrueType specifications would be made public, and TrueType would be built into the next versions of the Mac and Windows operating systems.
Adobe’s pre-emptive response started with the release of the long-protected specifications for the PostScript Type 1 font format in March 1990. This was followed by introduction of Adobe Type Manager (ATM) software in mid-1990. ATM scales Type 1 (but not Type 3) PostScript fonts for screen display and imaging on both PostScript and non-PostScript printers.
In early 1991, TrueType for the Mac became available, followed by the Windows 3.1 implementation. Now, with either TrueType or ATM, Mac users (and later Windows and OS/2 users) could actually see on-screen at any size what the font output would look like.
So now there are two widely used outline font specifications, one (TrueType) built into the operating systems used by over 95% of computers world-wide, and the other (PostScript Type 1) both well-established and supported directly by most high-end output devices.
But as time goes on, the practical differences begin to blur. Support for TrueType is built in to many implementations of PostScript Level 2, and is standard in PostScript 3. Similarly, the ATM rasterizing technology is slated to be incorporated into Windows NT 5.0, side-by-side with TrueType. Apple’s forthcoming “Rhapsody” operating system uses Display PostScript as the basis of its entire imaging model, which also gives it native support for PostScript fonts. Under the new OpenType format (discussed later), either TrueType or PostScript outlines can be put into a TrueType-style wrapper. Under the new OpenType format (discussed later), either TrueType or PostScript outlines can be put into a TrueType-style wrapper.
Technical Differences
The first difference between TrueType and PostScript fonts is their use of different sorts of mathematics to describe their curves. Conversions between the two formats are typically imperfect: although mathematically speaking the quadratic B-splines of TrueType are a subset of the cubic Bézier curves of PostScript, there are usually small rounding errors no matter which direction one converts fonts; however, the errors are greater in going from PostScript to TrueType than vice versa. More importantly, hinting information does not directly translate in either direction between the two formats.
Some articles have said that TrueType fonts require more points than PostScript, or that they take longer to rasterize because the math is more complicated. In fact, the math is simpler (quadratics are simpler than cubics), and a few shapes take fewer points in TrueType than in PostScript (a circle takes twelve points in PostScript vs. eight in TrueType). However, some PostScript rasterizers are unsurprisingly going to be faster at dealing with PostScript, and it’s true that most fonts will end up using more points in TrueType, even if the mathematical description of the curves is simpler.
The primary advantage of TrueType over PS1 fonts is the fact that TrueType allows better hinting. PostScript Type 1 fonts can hint vertical and horizontal features, overshoots, stem snaps, equal counters, and shallow curves (“flex”). Several of these can have a threshold pixel size at which they activate.
TrueType hints can do all that PostScript can, and almost anything else, as defined by the very flexible instructions. This includes controlling diagonals, moving specified points on the glyph outlines at specific arbitrary sizes to improve legibility. This ability to move points at a specific point size allows the font production staff to hand-tune the bitmap pattern produced by the outline at any specified size.
This is really symptomatic of a larger philosophical difference. PostScript uses “dumb” fonts and a “smart” interpreter, while TrueType uses relatively smarter fonts and a dumber interpreter. This means that PostScript hints tell the rasterizer what features ought to be controlled, and the rasterizer interprets these using its own “intelligence” to decide how to do it. Therefore, if Adobe upgrades the PostScript interpreter, the hinting can be improved.
Contrariwise, TrueType puts very specific instructions (some TT aficionados don’t like to call them hints) into the font to control how it will appear. Thus the font producer has the potential for very fine control over what happens when the font is rasterized under different conditions. However, it requires serious effort on the part of a font developer to actually take advantage of this greater hinting potential.
Another factor is the fact that TrueType rasterizing is built into several operating systems. Both the Mac OS and all the current incarnations of Windows support TrueType directly. These operating systems will rasterize TrueType fonts for the screen, and handle the sending of them to the printer, whether as bitmaps or as fonts in some format the printer can understand.
On the other hand, PostScript Type 1 font support is built into only one significant operating system: OS/2. This OS will do for PS1 fonts what the others do for TrueType. However, recent agreements between Microsoft and Adobe will include the ATM rasterizer for PostScript Type 1 as a part of Windows NT 5.0 and higher.
Scaling PostScript fonts on current versions of the Mac or Windows essentially requires the Adobe Type Manager (ATM) software, which handles the rasterizing to the screen, and rasterizes or converts the fonts for non-PostScript printers. Technically, ATM is not required to use PostScript fonts on PostScript printers, but ATM is required to display the font accurately on screen at arbitrary sizes.
Although ATM is commercial software, it comes with virtually every Adobe application, and with every commercial Adobe typeface package. At this point, it’s still an add-on, rather than an integral part of current Windows and Mac operating systems. Apple long ago agreed to increase its system-level support for PostScript, but has not aggressively pursued doing so.
A smaller, but consistent, advantage has to do with the physical storage of the fonts. TrueType fonts have all the data in a single file (although Windows 3.1 will create a “FOT” file for installed TT fonts, as a pointer to the location of the font data).
However, PostScript fonts require two separate files: one contains the character outlines, and the other contains metrics data (character widths and kern pairs). On the Macintosh, the current system software (System 7.5.5 and earlier) requires PS1 fonts to have not only the outline font, but also a bit-mapped screen font in at least one size, which contains the metrics data. For Windows systems using PostScript, a “PFB” file contains the outlines, while a “PFM” file carries the metrics. OS/2 uses an “OFM” file for the same purpose.
The system-independent “AFM” metrics file can be converted to a Windows PFM file upon installation by ATM, or can be used by a font editing program along with the outline to create a screen font for the Mac that includes any kerning pairs in the original.
On the other hand, PostScript’s pair of Files are often smaller than TrueType’s single file. The size difference ranges from only a 5% savings for an average font, to as much as a doubling of size for TrueType fonts that actually have extensive “hinting” instructions.
Also, most high-end output devices use PostScript as their internal page description language. PostScript fonts can be sent directly to these devices. It used to be the case that TrueType fonts were either downloaded as bitmaps or required that the TrueType rasterizer be downloaded as a PostScript program, which slowed printing a bit.
More recently, many PostScript Level 2 printers (and all PostScript 3 printers) have the TrueType rasterizer in ROM, built in. However, with some Windows printer drivers the user must change the printer driver settings in software to take advantage of this feature (downloading TrueType as “Type 42,” which is basically a PostScript wrapper around the TrueType data).
Further Practical Differences
Many of the theoretical advantages of TrueType are not actually realized in most commercially available TrueType fonts. PostScript backers point to a number of problems that still make PostScript fonts a better solution for many users. Besides the above-mentioned issue of the language of the output device, there are four other practical issues that even the score for PostScript:
First, at present many of the commercially available TrueType fonts one sees at the corner software store are of poor quality, coming in “zillion-fonts-for-a-buck” collections. Many of these fonts were originally shareware or public domain PostScript fonts, and have been converted to TrueType using some basic automatic utility. The outlines and hinting are no better than they were in the PostScript versions, and will suffer slightly in almost any automatic conversion. Usually in the case of extremely cheap collections, they weren’t the best quality PostScript fonts even before conversion to TrueType.
Of course, TrueType backers point out that often these fonts were available before; it’s simply the availability of a universal font scaling technology that makes discount fonts for the masses practical, and of course they are more likely to be released in the most widely available format.
Second, as of this writing there is no font editing software available at the retail level with native TrueType support, although this will change in late 1997 with the release of FontLab 3.0. All prior retail font editing programs use only PostScript-style bézier curves as their native operating mode. Exporting TrueType fonts from these programs entails conversion, and some minor loss of accuracy. The other tools required to do native editing of TrueType fonts, and to take advantage of the theoretical abilities of TrueType hinting, cost thousands of dollars.
Regardless of the specific tools used, achieving first-class hinting in TrueType currently requires intensive manual coding on a glyph-by-glyph basis. This requires substantial time and expertise on the part of the person doing the hinting.
As a result, high-quality TrueType fonts are currently only available from a handful of very large vendors, and only a minority of even those fonts really exploit the potential of TrueType hinting.
Third, TrueType’s hinting advantage only matters when hinting matters: when outputting to low-resolution devices, or for screen display. The increasing, widespread use of 600 dpi and better laser printers makes this less critical for print work. On the other hand, the increasing importance of screen displays for multimedia production and the Internet makes hinting more important.
Fourth, PostScript has some advantages simply from being the longer-established standard, especially for serious graphic arts work. Service bureaus are standardized on, and have large investments in, PostScript fonts. Most of the fonts which have “expert sets” of old style figures, extra ligatures, true small capitals and the like are in PostScript Type 1 format.
Although most major vendors have TrueType fonts, only a few (such as Bitstream) offer their entire libraries in both formats. Most, like Linotype and Monotype, have only a minority of their fonts in TrueType format. Given the current state of the tools, it would take a concerted effort of many years to convert all the major vendors’ font libraries to TrueType.
A final issue that is often raised is the story that some PostScript devices, particularly imagesetters, have problems either with TrueType fonts in general, or especially with mixing TrueType and PostScript on the same page or the same line. This is mostly a historical issue. More recent TrueType operating system implementations, and newer PostScript raster image processing software, have resolved what few problems there were early on.
According to Dov Isaacs, then Adobe’s Manager of Quality Assurance, Printing & Systems Division, “regardless of whether you are on a Mac or a PC running Windows 3.1 or above, you can mix TrueType and Type 1 with the caveat that you should never have both TrueType and Type 1 fonts with the same exact names on the same system.” Having two fonts with identical names can confuse the operating system, with unpredictable results.
Also, if using Windows, one may find that PostScript versions of the Windows TrueType system fonts get substituted by the output device: Times New Roman becomes Times-Roman, and Arial becomes Helvetica. Further, although the basic spacing of the substituted fonts is identical, their kerning pairs are not. This can cause text to reflow if one switches between two different-but-almost-the-same fonts on the computer doing the typesetting, if the program supports kerning pairs (graphics and DTP programs, and some better word processors).
Getting the same font on the actual output can be guaranteed by changing printer settings in the printer control panel, to ensure the TrueType system fonts get used. Hackers can also try editing the WIN.INI file on the computer that is doing the printing (whether to device or file). Delete the relevant lines in the font substitution section, so that the TrueType font used on-screen is also sent to the output device, rather than a printer font being substituted. On Windows NT or Win95, Registry settings control the same behavior. Alternatively, get a scalable version of the font used in the printer, and use it instead of the system fonts.
One actual, but rare, source of problems is not inherent in TrueType, but a result of the fact that rasterizing TrueType can require a bit more RAM in the raster image processor (RIP) than rasterizing PostScript—primarily in older PostScript rasterizers when the TrueType rasterizing program must be downloaded. If the RIP has barely enough RAM, it’s possible that this could push it over the edge. Service bureaus are notoriously conservative about this sort of thing (understandably, since any delays or problems can cost them money); your best bet is to consult with them, and if they warn of potential problems, test something complex with a mix of fonts for future reference.
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