Microsoft Typography | Developer information | Specifications | OpenType font development
Thaana OpenType Specification | Terms | Shaping | Features | Other | Appendix


Features for the Thaana script

The features listed below have been defined to create the basic forms for the languages that are supported on Thaana systems. Regardless of the model an application chooses for supporting layout of complex scripts, Uniscribe requires a fixed order for executing features within a run of text to consistently obtain the proper basic form. This is achieved by calling features one-by-one in the standard order listed below.

The order of the lookups within each feature is also very important. For more information on lookups and defining features in OpenType fonts, see Encoding feature information in the OpenType font development section.

The standard order for applying Thaana features encoded in OpenType fonts:

Feature Feature function Layout operation Required
Positioning features:
kern Pair kerning GPOS
mark Mark to base positioning GPOS X
       
[GSUB = glyph substitution, GPOS = glyph positioning]


Descriptions and examples of above features


Kerning

Feature Tag: "kern"

The 'kern' feature is used to adjust amount of space between glyphs, generally to provide optically consistent spacing between glyphs. Although a well-designed typeface has consistent inter-glyph spacing overall, some glyph combinations require adjustment for improved legibility. Besides standard adjustment in either horizontal or vertical direction, this feature can supply size-dependent kerning data via device tables, "cross-stream" kerning in the Y text direction, and adjustment of glyph placement independent of the advance adjustment. Note that this feature would not be used in monospaced fonts.

The font stores a set of adjustments for pairs of glyphs (GPOS lookup type 2 or 8). These may be stored as one or more tables matching left and right classes, and/or as individual pairs. If both forms are used, the classes should be listed last, so as to provide a means to replace any non-ideal values that may result from the class tables. Additional adjustments may be provided for larger sets of glyphs (e.g., triplets, quadruplets, etc.) to overwrite the results of pair kerns in particular combinations. These should precede the pairs.

Creating OTL kern feature with Microsoft VOLT


Mark to base positioning

Feature Tag: "mark"

The 'mark' feature positions mark glyphs in relation to a base glyph, or a ligature glyph. This feature may be implemented as a MarkToBase Attachment lookup (GPOS LookupType = 4) or a MarkToLigature Attachment lookup (GPOS LookupType = 5).

Positioning mark to base using Microsoft VOLT



this page was last updated 25 February 2002
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Thaana OpenType Specification | Terms | Shaping | Features | Other | Appendix
Microsoft Typography | Developer information | Specifications | OpenType font development