Informations about digital font formats

TrueType

Type 1

OpenType CFF

OpenType TTF

Ikarus

TrueType is an outline font standard originally developed by Apple Computer in the late 1980s as a competitor to Adobe's Type 1 fonts used in PostScript. The primary strength of TrueType is that it offers font developers a high degree of control over precisely how their fonts are displayed, right down to particular pixels, at various font heights.

TrueType history: Apple

On the Macintosh, fonts were drawn from hand-tuned font files that specified individual pixel locations for a font at a particular size. If the user wanted to see a font at another size, the Font Manager found the closest match and applied a basic scaling algorithm. When scaled to large sizes the effect was comical– since these fonts were bitmapped, they would scale the way a JPEG image does, becoming blocky.

In contrast, printer fonts for the popular Apple LaserWriter were based on PostScript Type 1 (and 3) outlines, resulting in excellent output at any size. Although Adobe provided the Adobe Type Manager software to use the same fonts on-screen, the software was fairly expensive. Nevertheless it became a de facto standard for anyone involved in desktop publishing, to the point where Apple wanted to have a similar system built-in. Making matters difficult was the fact that Type 1 fonts were encrypted, and Adobe made considerable amount of their income from licensing the format to interested parties. They were not about to simply allow Apple to include the software for free.

Instead Sampo Kaasila at Apple decided to write an entirely new format, which he worked on under the name Bass (as in Bass-o-matic from the Saturday Night Live sketch) and later Royal. The system developed and was eventually released as TrueType with the launch of Mac OS System 7 in May 1991. The fonts, four-weight families of Times Roman, Helvetica, Courier, and a handful of others, replaced the older bitmap fonts that previous Macintosh System versions had used.

One huge drawback of the TrueType system is that it could not use Type 1 fonts on-screen -- not surprising given its genesis. However this meant that the system was in fact not used by the very people it was intended to help, the DTP users. They had already invested considerable money in commercial Type 1 fonts, which they were not interested in replacing, and therefore had to continue using Type Manager. Adding to the problem was that there were very few fonts available in TrueType format, so even if one wanted to start fresh there was no real way to do so.

As part of Apple's new tactic of distancing itself from Adobe, Apple licensed TrueType to Microsoft, in exchange for a license for TrueImage, a Microsoft-developed PostScript-compatible printer driver that Apple planned to use in their laser printers. This driver was never actually included in any Apple products.

When TrueType was released, John Warnock of Adobe gave an impassioned speech in which he claimed Apple and Microsoft were selling snake oil, and then instantly released the Type 1 format as a published standard for anyone to use. This put even more pressure on TrueType. Apple eventually renewed agreements with Adobe for the use of PostScript in its printers; it is speculated that Apple's tactics resulted in lower royalty payments to Adobe as part of its new licensing agreements.

Apple extended TrueType with the launch of TrueType GX in 1994, as part of QuickDraw GX. This offered powerful extensions in two main areas. First was font morphing, for example allowing fonts to be smoothly adjusted from light to bold or from narrow to extended — competition for Adobe's "multiple master" technology. Second was substitution, where particular sequences of characters can be coded to flip to different designs in certain circumstances, useful for example to offer ligatures for 'fi', 'ffi', 'ct', etc. while maintaining the backing store of characters necessary for spell-checkers and text searching. However, the lack of user-friendly tools for making TrueType GX fonts meant there were no more than a handful of GX fonts. Much of the technology in TrueType GX, including morphing and substitution, lives on as AAT (Apple Advanced Typography) in OS X. Few font developers outside Apple attempt to make AAT fonts.

TrueType history: Microsoft

By around 1991 Microsoft had built TrueType into the Windows operating system. In partnership with their contractors Monotype, Microsoft spent much effort creating a set of high quality TrueType fonts that were compatible with the main fonts being bundled with PostScript equipment at the time. This included the fonts that are standard with Windows to this day: Times New Roman (compatible with Times Roman), Arial (compatible with Helvetica) and Courier New (compatible with Courier). By "compatible" one should understand this to mean two things: first, that the fonts are similar to look at (Arial is by no means identical to Helvetica); and second, very importantly, the fonts have the same character widths so can be used to typeset the same documents without reflowing the text. (The disjunction of the names, particularly between Arial and Helvetica, led some to believe there was a general problem of having to determine an "equivalent" Apple or PostScript font whenever a particular Windows font was called for, or vice versa. In fact, a given font is almost always available on both major platforms.) Microsoft and Monotype technicians used TrueType's hinting technology to make these fonts possibly the best made up to that time in terms of legibility at small sizes on screen.

Microsoft introduced a major rewrite in Windows 95, and then added anti-aliasing technology to smooth the edges of fonts in all applications. More recently ClearType, included with Windows XP, significantly improves readability on TFT based displays.

TrueType in Mac and Windows today

Today's Mac OS X and Windows XP still support TrueType fonts fully, but they also have native support for Adobe's Type 1 format and the OpenType format of Microsoft and Adobe. However most of the system fonts included in standard installations of OS X and Windows are in the TrueType format: system fonts have to be highly legible under a wide variety of conditions, making TrueType the natural format choice.

TrueType on Linux and other platforms

The FreeType project of David Turner attempts to create an independent implementation of the TrueType standard (as well as other font standards). FreeType is included in Linux distributions.

There are potential patent infringements in FreeType 1 because parts of the TrueType hinting virtual machine were patented by Apple, a fact not mentioned in the TrueType standards. (Patent holders who contribute to standards not published by a major standards body such as ISO are not required to disclose the scope of their patents.) More recent FreeType releases include an automatic hinter that analyzes glyph shapes and corrects them, thus avoiding the patented technology. (http://freetype.sourceforge.net/patents.html)

FreeType 2 is a font service and doesn't provide APIs to perform higher-level features, like text layout or graphics processing (e.g., colored text rendering, "hollowing", etc.). However, it greatly simplifies these tasks by providing a simple, easy to use and uniform interface to access the content of font files.

FreeType 2 is released under two free-source licenses: BSD-like FreeType License and the GPL. It can thus be used by any kind of projects, be they proprietary or not.

Technical note: outlines

The outlines of the characters (or glyphs) in TrueType fonts are made of straight line segments and quadratic Bzier curves. These curves are slightly more efficient to process than the cubic Bzier curves prevalent in the PostScript-centered world of graphic design and used in Type 1 fonts. A TrueType glyph outline specification typically takes up less space than a Type 1 outline.

Technical note: hinting language

TrueType systems include a virtual machine that executes programs inside the font, processing the "hints" of the glyphs. These distort the control points which define the outline, with the intention that the rasterizer produces fewer undesirable features on the glyph. Each glyph's hinting program takes account of the size (in pixels) that the glyph is being displayed at, as well as other less important factors of the display environment.

Although incapable of receiving input and producing output as normally understood in programming, the TrueType hinting language does offer the other prerequisites of programming languages: conditional branching (IF statements), looping an arbitrary number of times (FOR- and WHILE-type statements), variables (although these are simply numbered slots in an area of memory reserved by the font), and encapsulation of code into functions. Special instructions called "delta hints" are the lowest level control, moving a control point at just one pixel size.

Good TrueType glyph programming techniques are to do as much as possible using variables defined just once in the whole font (e.g., stem widths, cap height, x-height). This means avoiding delta instructions as much as possible. This helps the font developer to make major changes (e.g., the point at which the entire font's main stems jump from 1 to 2 pixels wide) most of the way through development.

Making a very well-hinted TrueType font is still a significant amount of work, despite the increased user-friendliness of programs for adding hints to fonts compared with the early 1990s. Many TrueType fonts therefore have only rudimentary hints, or have hinting automatically applied by the font editor, with variable end results.