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

How the Mongolian shaping engine works

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

  1. Analyzing the characters for contextual shape.
  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 Mongolian 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 determines the correct contextual form the character should take based on the character before and after it. The contextual shape maps to an OTL feature for that form (isol, init, medi and fina).

Additionally, during the analysis process, the engine also verifies valid diacritic combinations. For additional information see Other Encoding Issues; 'Handling invalid combining marks' in this document.

Shaping with OTLS

The first step the shaping engine takes in shaping the character string is to map all characters to their nominal form glyphs (e.g. the glyph for U+1820). Then, the shaping engine applies contextual shape features to the glyph string.

Next, Uniscribe calls OTLS to apply the features. All OTL processing is divided into a set of predefined features (described and illustrated in the Feature 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 Mongolian script languages.

Shaping features:

  1. Language forms
    1. Apply feature 'ccmp' to preprocess any glyphs that require composition or decomposition.
    2. Apply feature 'isol' to get the isolated form of characters.
    3. Apply feature 'fina' to get final form glyphs.
    4. Apply feature 'medi' to get medial form glyphs.
    5. Apply feature 'init' to get initial form glyphs.
    6. Apply feature 'rlig' to compose any mandatory ligatures, like 'lam alef'.

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.

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