SSP default ignorable characters, was: Variation selectors and vowel marks

From: Peter Kirk (peterkirk@qaya.org)
Date: Sun Apr 25 2004 - 19:30:26 EDT

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    On 25/04/2004 15:12, Ernest Cline wrote:

    >
    >
    > ...
    >
    >If HVS's are ever adopted, they clearly would belong on the
    >SSP in a new one row Hebrew Variant Selectors block chosen
    >from the 233 rows of available default ignorable characters.
    >
    >

    Thank you for reminding me about these default ignorable characters in
    the SSP. See TUS p.111,
    http://www.unicode.org/versions/Unicode4.0.0/ch05.pdf:

    > To allow a greater degree of compatibility across versions of the
    > standard, the ranges U+2060..U+206F, U+FFF0..U+FFFB, and
    > U+E0000..U+E0FFF are reserved for format and control characters
    > (General Category = Cf). Unassigned code points in these ranges should
    > be ignored in processing and display. For more information, see
    > Section 5.20, Default Ignorable Code Points.

    At the moment I see little need to define special variation selectors
    for this purpose as the BMP variation selectors, U+FE00-U+FE0F, seem
    adequate to me, and are presumably also default ignorable. But it is
    good to know that this block of unassigned default ignorable characters
    exists.

    I wonder if it would be possible to set aside part of the SSP block of
    default ignorable characters as a private use area? These can then be
    used for private use combining marks (which would have to have combining
    class zero, but consistent ordering of PUA marks should be the PUA
    user's responsibility), which would simply be ignored by fonts which
    don't support them which is the best fallback for combining marks. Or
    they could be used as private use variation selectors, dare I suggest
    it? I would suggest perhaps a block of 256 characters, which would allow
    those who choose to do so to use this for some kind of invisible
    annotation. Obviously these are not really plain text issues; but then
    the point of a private use area is to allow people to do things which
    are not standardised. I suspect that allowing this kind of thing will be
    a good way of getting many people off the backs of the UTC! 

    -- 
Peter Kirk
peter@qaya.org (personal)
peterkirk@qaya.org (work)
http://www.qaya.org/

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