TrueType Open at a Glance
TrueType Open addresses complex typographical issues that especially affect people using text-processing applications in multi-lingual and non-Latin environments.
TrueType Open fonts may contain alternative forms of characters and mechanisms for accessing them. For example, in Arabic, the shape of a character often varies with the character's position in a word. As shown here, the Ha character will take any of four shapes, depending on whether it stands alone or whether it falls at the beginning, middle, or end of a word. TrueType Open helps a text-processing application determine which variant to substitute when composing text.
Figure 1a. Isolated, initial, medial, and final forms of the Arabic character Ha
Similarly, TrueType Open helps an application use the correct forms of characters when text is positioned vertically instead of horizontally, such as with Kanji. For example, Kanji uses alternative forms of parentheses when positioned vertically.
Figure 1b. Alternative forms of parentheses used when positioning Kanji vertically
The TrueType Open font format also supports the composition and decomposition of ligatures. For example, English, French, and other languages based on Latin can substitute a single ligature, such as "fi", for its component glyphs—in this case, "f" and "i." Conversely, the individual "f" and "i" glyphs could replace the ligature, possibly to give a text-processing application more flexibility when spacing glyphs to fill a line of justified text.
Figure 1c. Two Latin glyphs and their associated ligature
Figure 1d. Three Arabic glyphs and their associated ligature
Glyph substitution is just one way TrueType Open extends font capabilities. Using precise X and Y coordinates for positioning glyphs, TrueType Open fonts also can identify points for attaching one glyph to another to create cursive text and glyphs that need diacritical or other special marks.
TrueType Open fonts also may contain baseline information that specifies how to position glyphs horizontally or vertically. Because baselines may vary from one script (set of characters) to another, this information is especially useful for aligning text that mixes glyphs from scripts for different languages.
Figure 1e. A line of text, baselines adjusted, mixing Latin and Arabic scripts