Re: Why people still want to encode precomposed letters

From: John Hudson (
Date: Tue Nov 25 2008 - 16:47:13 CST

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    verdy_p wrote:

    > For example I spoke about the "special" mark-to-mark positioning for Vietnamese, where accents are not placed on
    > their normal anchor (above centered) if they are above a circumflex: "kerning" still does occur, despite of the
    > generic mark-to-mark anchor, it affects the position of the acute or grace accent (above a circumflex), but it does
    > not move to the right or left the possible subsequent mark-to-mark anchor after the acute/grave accent (which
    > remains above-centered, but may be at a lower position than the one normally set by the acute/grave accent alone).

    > In that case, the term "kerning" is still the one to choose for the position adjustment: this is a more complex
    > case: you still need generic mark-to-mark positioning, but you also need kerning for some combinations of accents,
    > to move them slightly away from their normal anchor position, but also to "slide down" the next generic anchor
    > (something that kerning also performs for base-to-base positioning !).

    No, that isn't kerning either. Kerning is an advance width adjustment.
    Since combining marks are zero-width, they do not kern. Kerning affects
    the horizontal positioning of all marks following the kern. Mark
    positioning must not affect the positioning of subsequent bases, ergo
    they do not kern.

    If one wants variant mark-to-mark positioning as for, e.g. Vietnamese
    tone marks beside circumflex, then one defines variant anchors in
    locale-specific lookups (or more likely in the case of Vietnamese, one
    builds precomposed multi-mark glyphs, substitute these in
    locale-specific GSUB lookups, and then position then using the same
    generic anchors as used for similarly positioned marks).

    John Hudson

    Tiro Typeworks
    Gulf Islands, BC
    You can't build a healthy democracy with people
    who believe in little green men from Venus.
                        -- Arthur C. Clark

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