Logical Storage Order For Complex Vowels in Tai Tham

From: Ed (ed.trager@gmail.com)
Date: Fri Jan 28 2011 - 10:23:17 CST

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    Hi, Everyone,

    In ISO/IEC JTC1/SC2/WG2 document N3121, "Proposal for encoding the
    Lanna script in the BMP of the UCS", the table of examples on pages
    2-3 of section 5 "Dependent vowel signs" appears to imply (but note
    that the text does not *explicitely* state) that the decompositions
    shown are in fact the logical storage order.

    For most of the examples shown, the logical order makes sense. But
    for combinations containing U+1A6C OA BELOW, it appears that an
    arbitrary choice has been made regarding the logical storage position
    of U+1A6C.

    In the examples in N3121, U+1A6C OA BELOW appears after U+1A6E VOWEL E
    (which makes sense to me) but (for example) before U+1A65 VOWEL I
    --and the latter does not make sense to me.

    As shown in the attached image, I would have expected that
    U+1A65 VOWEL I appear *BEFORE* U+1A6C OA BELOW . My expectation follows
    from the order in which I write the marks: That is, I write Tai Tham
    on paper from left to write, and from top to bottom. So I write vowel
    marks appearing *ABOVE* base consonants before I write vowel marks

    U+1A6C OA BELOW is the most common vowel sign that can result in this
    kind of confusion. However it may not be the only one. There are a
    number of dipthong and tripthong vowels which occur in the various Tai
    languages and these are of course written using various combinations
    of 2 or more Tai Tham vowel signs.

    It appears that N3121 was not the "final" version document used when
    Tai Tham was approved for encoding; but I am not clear what the
    subsequent document(s) were?

    In any case, the examples provided in N3121 seem to me insufficient
    and, as already noted, nowhere does it explicitely state in N3121 that
    the decompositions represent the backing store order.

    Perhaps there is a need for a separate document to clarify what the
    backing store order should be for dipthong and tripthong vowels, inter
    alia, for Tai languages/dialects using Tai Tham script?

    I have discussed these issues with Theppitak Karoonboonyanan and
    others; and we recently sent an email to Martin Hosken but have not
    heard back from him. We are working on creating a Tai Tham Unicode
    font which, when completed, will be included in the Google Font
    Directory initiative. It is important for us to have a clear
    understanding of the expected backing storage order when implementing
    OpenType or other smart font technology (Graphite) features within the
    font(s) that we create.

    - Ed Trager


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