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


How the Thai shaping engine works

The Uniscribe Thai shaping engine processes text in stages. The stages are:

  1. Analyzing the characters for valid diacritic combinations.
  2. Shaping (substituting) glyphs with OTLS (OpenType Library Services).
  3. Positioning glyphs with OTLS.

The descriptions which follow will help font developers understand the rationale for the Thai feature encoding model, and help application developers better understand how layout clients can divide responsibilities with operating system functions.


Analyzing the characters

The unit that the shaping engine receives for the purpose of shaping is a string of Unicode characters, in a sequence. The contextual analysis engine verifies valid diacritic combinations. For additional information see Other Encoding Issues; 'Handling invalid combining marks' in this document.


Shaping with OTLS

The first step Uniscribe takes in shaping the character string is to map all characters to their nominal form glyphs.

Next, Uniscribe calls OTLS to apply the features. All OTL processing is divided into a set of predefined features (described and illustrated in the Features section of this document). Each feature is applied, one by one, to the appropriate glyphs in the syllable and OTLS processes them. Uniscribe makes as many calls to the OTL Services as there are features. This ensures that the features are executed in the desired order.

The steps of the shaping process are outlined below. Not all of the features listed apply to all Thai script languages.

Shaping features:

  1. Language forms
    1. Apply feature 'ccmp' to preprocess any glyphs that require composition or decomposition.


Positioning glyphs with OTLS

Uniscribe next applies features concerned with positioning, calling functions of OTLS to position glyphs.

Positioning features:

  1. Kerning
    1. Apply feature 'kern' to provide pair kerning between base glyphs requiring adjustment for better typographical quality.

  2. Mark to base
    1. Apply feature 'mark' to position diacritic glyphs to the base glyph.

  3. Mark to mark
    1. Apply feature 'mkmk' to position diacritic glyphs to other diacritic glyphs.



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