From: Ed (firstname.lastname@example.org)
Date: Fri Jan 28 2011 - 10:23:17 CST
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
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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