FreeType Glyph Conventions

1. Font files, format and information

A font is a collection of various character images that can be used to display or print text. The images in a single font share some common properties, including look, style, serifs, etc. Typographically speaking, one has to distinguish between a font family and its multiple font faces, which usually differ in style though come from the same template.

The single term font is nearly always used in ambiguous ways to refer to either a given family or given face, depending on the context. For example, most users of word-processors use "font" to describe a font family (e.g. "Courier", "Palatino", etc.); however most of these families are implemented through several data files depending on the file format: For TrueType, this is usually one per face (i.e. arial.ttf for "Arial Regular", ariali.ttf for "Arial Italic", etc.). The file is also called a "font" but really contains a font face.

A digital font is thus a data file that may contain one or more font faces. For each of these, it contains character images, character metrics, as well as other kind of information important to the layout of text and the processing of specific character encodings. In some awkward formats, like Adobe's Type 1, a single font face is described through several files (i.e. one contains the character images, another one the character metrics). We will ignore this implementation issue in most parts of this document and consider digital fonts as single files, though FreeType 2.0 is able to support multiple-files fonts correctly.

As a convenience, a font file containing more than one face is called a font collection. This case is rather rare but can be seen in many Asian fonts, which contain images for two or more representation forms of a given scripts (usually for horizontal and vertical layout.

2. Character images and mappings

The character images are called glyphs. A single character can have several distinct images, i.e. several glyphs, depending on script, usage or context. Several characters can also take a single glyph (good examples are Roman ligatures like "fi" and "fl" which can be represented by a single glyph). The relationships between characters and glyphs can be very complex, but won't be discussed in this document. Moreover, some formats use more or less awkward schemes to store and access glyphs. For the sake of clarity, we only retain the following notions when working with FreeType:

• A font file contains a set of glyphs; each one can be stored as a bitmap, a vector representation or any other scheme (most scalable formats use a combination of mathematical representation and control data/programs). These glyphs can be stored in any order in the font file, and is typically accessed through a simple glyph index.

• The font file contains one or more tables, called a character map (or charmap in short), which is used to convert character codes for a given encoding (e.g. ASCII, Unicode, DBCS, Big5, etc..) into glyph indices relative to the font file. A single font face may contain several charmaps. For example, most TrueType fonts contain an Apple-specific charmap as well as a Unicode charmap, which makes them usable on both Mac and Windows platforms.

3. Character and font metrics

Each glyph image is associated with various metrics which are used to describe how must be placed and managed when rendering text. These are described in more details in section III, they relate to glyph placement, cursor advances as well as text layout. They are extremely important to compute the flow of text when rendering a string of text.

Each scalable format also contains some global metrics, expressed in notional units, to describe some properties of all glyphs in the same face. Examples for global metrics are the maximum glyph bounding box, the ascender, descender and text height for the font.

Though these metrics also exist for non-scalable formats, they only apply for a set of given character dimensions and resolutions, and are usually expressed in pixels then.