Windows Glyph Handling (was: 28th IUC paper - Tamil Unicode New)

From: Richard Wordingham (
Date: Mon Aug 22 2005 - 21:11:18 CDT

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    John Hudson wrote:

    > In the OpenType model re-ordering is specifically NOT handled at the glyph
    > level, but at the character level. This is why it only works with standard
    > Unicode characters, and not codepoints in what I've come to regard as the
    > 'Pretty Useless Area'.

    Do you mean 'OpenType' or something like 'Uniscribe/OpenType' or
    'Microsoft'? For example, the OpenType Layout page ( ) says:

    'A text-processing client follows a standard process to convert the string
    of characters entered by a user into positioned glyphs. To produce text with
    OpenType Layout fonts:

    '1. Using the cmap table in the font, the client converts the character
    codes into a string of glyph indices.

    '2. Using information in the GSUB table, the client modifies the resulting
    string, substituting positional or vertical glyphs, ligatures, or other
    alternatives as appropriate.


    There's no mention of re-ordering by Uniscribe (or equivalent) or of
    breaking into runs by script - Step 0!. You have to read higher level
    documentation to realise that the characters may be re-ordered (or even
    changed!) before the font tables can affect them. (The separation between
    Steps 0 and 1 is not as clean as I imply.)


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