Rationale for IPC of Newa Dependent Vowels

I have doubts about the Indic_Positional_Category (InPC) values proposed
for four new dependent vowels being added in Unicode 10.0.0.

On examining the vowel chart (p1265
of http://www.unicode.org/Public/10.0.0/charts/CodeCharts.pdf) one may
feel quite comfortable with assigning the property values:

1143E..1143F ; Top # Mn [2] NEWA VOWEL SIGN E..NEWA VOWEL SIGN AI

11440..11441 ; Right # Mc [2] NEWA VOWEL SIGN O..NEWA VOWEL SIGN AU

However, on consulting Section 3.6 of Anshuman Pandey's 'Proposal to
Encode the Newar Script in ISO/IEC 10646'
http://www.unicode.org/L2/L2012/12003r-newar.pdf, one finds that after
the seven headless consonants GA, NYA, TTHA, NNA, THA, DHA and SHA, the
dependent vowels take forms more appropriate to the property values

1143E ; Left # Mn NEWA VOWEL SIGN E

1143F ; Top_and_Left # Mn NEWA VOWEL SIGN AI

11440..11441 ; Left_and_Right # Mc [2] NEWA VOWEL SIGN O..NEWA VOWEL
SIGN AU

Now, I have no idea what the effect of a right-to-left
directional override should be on the combining marks, but in general I
believe gc=Mn makes more sense for U+1143E and U+1143F, so I am not
challenging that property assignment.

However, I do wonder what the best property values are for a renderer,
such as Microsoft's Universal Shaping Engine. It seems to me that it
may be better to start with the properties involving 'Left' and use
contextual substitutions to convert the dependent vowels to components
of the correct position. However, this does seem more complicated than
the general decomposition of multipart vowels. In particular, for
headed consonants, a glyph substitution is required to replace the head
by a part of the vowel symbol; the default glyph will not be
appropriate. It is entirely possible that a font will simply replace a
headed consonant and any of these four vowels by a ligature glyph,
leaving reordering to be considered only for the seven headless
consonants.

Has this matter been considered? If so, is the rationale recorded
anywhere?

Richard.
Received on Mon Apr 17 2017 - 10:24:24 CDT