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Font formats overview


Raster, Vector and TrueType Fonts

Windows-based applications can use three different kinds of font technologies to display and print text: raster, vector, and TrueType. The differences between these fonts reflect the way that the glyph for each character or symbol is stored in the respective font-resource file. In raster fonts, a glyph is a bitmap that Windows uses to draw a single character or symbol in the font. In vector fonts, a glyph is a collection of line endpoints that define the line segments Windows uses to draw a character or symbol in the font. In TrueType fonts, a glyph is a collection of line and curve commands as well as a collection of hints. Windows uses the line and curve commands to define the outline of the bitmap for a character or symbol in the TrueType font. Windows uses the hints to adjust the length of the lines and shapes of the curves used to draw the character or symbol. These hints and the respective adjustments are based on the amount of scaling used to reduce or increase the size of the bitmap. 

Because the bitmaps for each glyph in a raster font are designed for a specific resolution of device, raster fonts are generally considered to be device dependent. Vector fonts, on the other hand, are not device dependent, because each glyph is stored as a collection of scalable lines. However, vector fonts are generally drawn more slowly than raster or TrueType fonts. TrueType fonts provide both relatively fast drawing speed and true device independence. By using the hints associated with a glyph, a developer can scale the characters from a TrueType font up or down and still maintain their original shape. 

As previously mentioned, the glyphs for a font are stored in a font-resource file. A font-resource file is actually a Windows library that contains only data - there is no code. For raster and vector fonts, this data is divided into two parts: a header describing the font's metrics and the glyph data. A font-resource file for a raster or vector font is identified by the .FON filename extension. For TrueType fonts, there are two files for each font: the first file contains a relatively short header and the second contains the actual font data. The first file is identified by a .FOT extension and the second is identified by a .TTF extension. 




The OpenType font format is an extension of the TrueType font format, adding support for PostScript font data. The OpenType font format was developed jointly by Microsoft and Adobe. OpenType fonts and the operating system services which support OpenType fonts provide users with a simple way to install and use fonts, whether the fonts contain TrueType outlines or CFF (PostScript) outlines.

The OpenType font format addresses the following goals:

  • broader multi-platform support

  • better support for international character sets

  • better protection for font data

  • smaller file sizes to make font distribution more efficient

  • broader support for advanced typographic control


OpenType fonts are also referred to as TrueType Open v.2.0 fonts, because they use the TrueType 'sfnt' font file format. PostScript data included in OpenType fonts may be directly rasterized or converted to the TrueType outline format for rendering, depending on which rasterizers have been installed in the host operating system. But the user model is the same: OpenType fonts just work. Users will not need to be aware of the type of outline data in OpenType fonts. And font creators can use whichever outline format they feel provides the best set of features for their work, without worrying about limiting a font's usability.

OpenType fonts can include the OpenType Layout tables, which allow font creators to design better international and high-end typographic fonts. The OpenType Layout tables contain information on glyph substitution, glyph positioning, justification, and baseline positioning, enabling text-processing applications to improve text layout.

As with TrueType fonts, OpenType fonts allow the handling of large glyph sets using Unicode encoding. Such encoding allows broad international support, as well as support for typographic glyph variants.

Additionally, OpenType fonts may contain digital signatures, which allows operating systems and browsing applications to identify the source and integrity of font files, including embedded font files obtained in web documents, before using them. Also, font developers can encode embedding restrictions in OpenType fonts, and these restrictions cannot be altered in a font signed by the developer.



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