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| FreeType Glyph Conventions |
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| Version 2.1 |
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| Copyright 1998-2000 David Turner (<a |
| href="mailto:david@freetype.org">david@freetype.org</a>)<br> |
| Copyright 2000 The FreeType Development Team (<a |
| href="mailto:devel@freetype.org">devel@freetype.org</a>) |
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| <a href="glyphs-2.html">Next</a> |
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| <h2> |
| I. Basic typographic concepts |
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| |
| <a name="section-1"> |
| <h3> |
| 1. Font files, format and information |
| </h3> |
| |
| <p>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 <em>font family</em> and its |
| multiple <em>font faces</em>, which usually differ in style though come |
| from the same template.</p> |
| |
| For example, "Palatino Regular" and "Palatino Italic" are two distinct |
| <em>faces</em> from the same famous <em>family</em>, called "Palatino" |
| itself.</p> |
| |
| <p>The single term <em>font</em> 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. |
| <tt>arial.ttf</tt> for "Arial Regular", <tt>ariali.ttf</tt> for "Arial |
| Italic", etc.). The file is also called a "font" but really contains a |
| font face.</p> |
| |
| <p>A <em>digital font</em> is thus a data file that may contain <em>one |
| or more font faces</em>. 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.</p> |
| |
| <p>As a convenience, a font file containing more than one face is called |
| a <em>font collection</em>. 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.</p> |
| |
| |
| <a name="section-2"> |
| <h3> |
| 2. Character images and mappings |
| </h3> |
| |
| <p>The character images are called <em>glyphs</em>. 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:</p> |
| |
| <ul> |
| <li> |
| <p>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.</p> |
| </li> |
| <li> |
| <p>The font file contains one or more tables, called a <em>character |
| map</em> (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.</p> |
| </li> |
| </ul> |
| |
| |
| <a name="section-3"> |
| <h3> |
| 3. Character and font metrics |
| </h3> |
| |
| <p>Each glyph image is associated to various metrics which are used to |
| describe how must be placed and managed when rendering text. Though |
| they 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.</p> |
| |
| <p>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.</p> |
| |
| <p>Though these metrics also exist for non-scalable formats, they only |
| apply for a set of given character dimensions and resolutions, and |
| they are usually expressed in pixels then.</p> |
| |
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