Re: Generic base characters

From: Christopher Fynn (
Date: Sat Jul 14 2007 - 01:52:18 CDT

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    Re: Generic base characters

    This issue (displaying isolated combining marks or displaying combining marks on
    a generic base other than dotted circle) for use in school books etc. has come
    up a number of times over the years on this list and elsewhere.

    The Microsoft OpenType shaping engine, Uniscribe, seems to automatically insert
    a dotted circle as a base character for isolated combining marks - and this
    behavior is outside of the control of the font developer.

    IMO it would be much better if font developers were responsible for defining
    their own sets of these base glyphs for combining marks - including the base for
    isolated combining marks - and their own lookups for rendering the resulting

    As base glyphs one might want to include the dotted circle; non-breaking space
    or fixed width spaces such as em space or en space; an "x" like glyph in the
    case of Lao,a minus dash for Thai and so on (whatever are the conventions for
    the scripts the font is designed to display).

    Right now if I have a lookup in an OpenType font to place an isolated mark on
    the dotted circle Uniscribe also inserts dotted circle and I end up getting two
    doted circles the second with the combining mark attached - rather than one
    dotted circle with combining mark. Not all OpenType shaping engines exhibhit
    this behaviour.

    The problem here is not with ISO 10646 or Unicode - it is a problem related to
    the current limitations and default behavior of some "smart font" shaping
    systems - and inconsistencies between how different shaping engines

    A possible solution might be to definea specific in OpenType fonts where
    the font designer could list the "generic" base glyphs he/she wants to make
    available for a particular script - he/she would then also be responsible for
    including the positioning or substitution lookups for the resulting
    combinations. If this table were absent the rendering engine could fall back to
    the old behavior (e.g. in the case of Uniscribe automatically inserting a dotted
    circle) - which should cover any backwards compatibility issues.

    - Chris

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