Re: Variation selectors and vowel marks

From: Peter Kirk (
Date: Sat Apr 24 2004 - 05:51:38 EDT

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    On 24/04/2004 01:00, Asmus Freytag wrote:

    > The rationale for making a variation selector ineligible to apply to
    > combining marks comes from normalization. In normalization all
    > combining character sequences are put in their canonical order, based
    > on their combining class. If we want to allow accents to be placed on
    > base characters to which a variation selector has been applied, then
    > the variation selector needs to come in between the base character and
    > the accent. Unless we make it a combining character, the VS would
    > interrupt the combining character sequence (separating the accent from
    > the base character). But if it is a a combining character, it takes
    > part in the reordering. Therefore, it needs to have combining class 0,
    > so it stays with the base character. And that's the reason it cannot
    > be used to apply to characters whose combining class is not 0.
    > So you see, the rule is not based on the linguistic nature of the
    > combining character, but on its combining class.

    But if the VS has class 0, it stays wherever it is put because it does
    not participate in reordering. If it is put after a particular combining
    character, it will stay there. So this argument doesn't really hold.
    Yes, problems do arise if there is more than one combining character
    between the base character and the VS and they are not in canonical
    order. But this is a marginal case which can be avoided by ensuring that
    canonical order is always used.

    So, provided that the need to encode a variant of a combining character
    can be demonstrated, I can see no good reason not to allow a VS to be
    used for this, with appropriate notes about ordering of combining
    character sequences.

    An alternative of course would be to define a special VS with the same
    combining class as the character it applies to, so that the two will
    always remain together. Thus there would potentially be the need for a
    considerable set of VSs. But I don't think this is really necessary.

    Peter Kirk (personal) (work)

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