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Characters and Glyphs

Characters are conceptual entities that correspond to units of written language. Examples of characters include the letters of the Roman alphabet, the Kanji ideographs used in Japanese, and symbols that indicate mathematical operations. Characters are represented as numbers in a computer’s memory or on disk, and a character encoding defines the mapping between a numerical value and a specific character. For example, the ASCII and Unicode character encodings both assign the value 97 (decimal) to the character “a”. The Cocoa text system uses the Unicode character encoding internally, although it can read and write other encodings on disk.

You can think of a glyph as the rendered image of a character. The words of this sentence are made visible through glyphs. A collection of glyphs that share certain graphic qualities is called a font.

The difference between a character and a glyph isn’t immediately apparent in English since there’s typically a one-to-one mapping between the two. But, in some Indic languages, for example, a single character can map to more than one glyph. And, in many languages, two or more characters may be needed to specify a single glyph.

To take a simple example, the glyph “ö” can be the result of two characters, one representing the base character “o” and the other representing the diacritical mark “¨”. A user of a word processor can press an arrow key one time to move the insertion point from one side of the “ö” glyph to the other; however, the current position in the character stream must be incremented by two to account for the two characters that make up the single glyph.

Thus, the text system must manage two related but different streams of data: the stream of characters (and their attributes) and the stream of glyphs that are derived from these characters. The NSTextStorage object stores the attributed characters, and the NSLayoutManager object stores the derived glyphs. Finding the correspondence between these two streams is the responsibility of the layout manager.

For a given glyph, the layout manager can find the corresponding character or characters in the character stream. Similarly, for a given character, the layout manager can locate the associated glyph or glyphs. For example, when a user selects a range of text, the layout manager must determine which range of characters corresponds to the selection.

When characters are deleted, some glyphs may have to be redrawn. For example, if the user deletes the characters “ee” from the word “feel”, the “f” and “l” can be represented by the “fl” ligature rather than the two glyphs “f” and “l”. The NSLayoutManager object directs a glyph generator object to generate new glyphs as needed. Once the glyphs are regenerated, the text must be laid out and displayed. Working with the NSTextContainer object and other objects of the text system, the layout manager determines where each glyph appears in the NSTextView. Finally, the text view renders the text.

Because an NSLayoutManager object is central to the operation of the text system, it also serves as the repository of information shared by various components of the system. For more information about NSLayoutManager, refer to its reference documentation and to Text Layout Programming Guide for Cocoa.

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