TrueType fonts

A digital font contains much more than just the characters associated with a given alphabet or script. A TrueType font file includes many different kinds of information used by the TrueType rasterizer and the operating system software to ensure that characters display on the computer screen or print out exactly as the font designer intended them to. All of the information in a TrueType font is arranged in a series of tables. For technical information about these tables, you can see our TrueType specification.

In addition to the shapes of each character, a TrueType font includes information about how the characters should be spaced vertically and horizontally within a block of text, character mapping details (governing the variety of characters included in the font and the keystrokes needed to access them), and much more besides. The fonts also include manufacturer's details, such as copyrights, names and licensing permissions. If you have a Windows 95 system, you can download our font properties extension, and find out more about the kind of information contained in your TrueType fonts.

Description of characters

One of the more obvious things TrueType fonts include is the shape of each character. Each and every letterform contained in a TrueType font is stored as an outline, or more accurately, as a mathematical description of the character constructed from a series of points. For this reason, TrueType is known as an outline font format.

Probably the greatest thing about storing characters as outlines is that only one outline per character is needed to produce all the sizes of that character you'll ever need. A single outline can be scaled to an enormous range of different sizes, some of which are illustrated below. This enables the same character to be displayed on monitors of different resolutions, and to be printed out at a large number of different sizes.To scale a character outline is a simple mathematical operation, as indeed are other transformations such as rotation and reflections.

The user never actually sees the outlines stored in an outline font, because before a character can be displayed on the screen or the printer, a bitmap has to be produced, by the TrueType rasterizer. This is because screen displays and printers both use dot patterns to represent images (sufficiently magnify any screen image or print-out, and you'll notice the pixel pattern). The character outlines contained in the TrueType font are scaled to the requested size, and are converted into bitmaps by turning on the pixels encompassed by the outline. This process is known as scan conversion or rasterization.

Character sets and mapping

TrueType fonts also contain character maps - information concerning the types and quantity of characters included in the font, and details as to how these characters are accessed from the keyboard. See the MSDN and GlobalDev sites for reference information on character sets and codepages.

TrueType fonts on the PC and the Mac

Although TrueType fonts can be used on both the Macintosh and Windows platforms, slight differences in the way each operating system handles the fonts lead vendors to produce separate versions of the font for each platform. Some vendors will provide you with both Mac and Windows format TrueType files, while others may treat them as different products. Contact individual vendors for specific details.

This oddity arises because of the different file system used on the two platforms. Information can be included in the font to determine whether the font can be used on both kinds of system, or one or the other.

On a Macintosh, the TrueType font file is sometimes referred to as an SFNT and, under Windows as a .TTF. The information contained in the fonts is the same, and making the necessary adjustments to allow the font to run on both platforms is a relatively straightforward task.