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Thaana OpenType Specification | Terms | Shaping | Features | Other | Appendix


Other encoding issues


Handling invalid combining marks

Combining marks and signs that appear in text not in conjunction with a valid consonant base are considered invalid. Uniscribe displays these marks using the fallback rendering mechanism defined in the Unicode Standard (section 5.12, 'Rendering Non-Spacing Marks' of the Unicode Standard 3.1), i.e. positioned on a dotted circle.

For the fallback mechanism to work properly, a Thaana OTL font should contain a glyph for the dotted circle (U+25CC). In case this glyph is missing from the font, the invalid signs will be displayed on the missing glyph shape (white box).

In addition to the 'dotted circle' other Unicode code points that are recommended for inclusion in any Thaana font are; LTR (left to right mark; U+200E), and RTL (right to left mark; U+200F).

If an invalid combination is found, like two 'aba filis' on the same base character, the diacritic that causes the invalid state is placed on a dotted circle to indicate to the user the invalid combination. The shaping engine for non-OpenType fonts will cause invalid mark combinations to overstrike. This is the problem that inserting the dotted circle for the invalid base solves. It should also be noted that the dotted circle is not inserted into the application's backing store. This is a run-time insertion into the glyph array that is returned from the ScriptShape function.

The invalid diacritic logic for Thaana is based on the rule that only one fili can be placed on a base glyph.



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