Long standing problem with Vedic tone markers and post-base visarga/anusvara

[Richard Wordingham via Unicode]

On Thu, 2 Jan 2020 20:20:34 +0000
Richard Wordingham via Unicode <unicode at unicode.org> wrote:

> There's a project whose basis I can't find to convert Indian Indic
> rendering at least to use the USE.  Now, according to the
> specification of the USE, visarga, anusvara and cantillation marks
> are all classified as vowel modifiers, and are so ordered relative to
> one another in the Indian Indic order: left, top, bottom, right.  So,
> the problem should already be solved for Grantha, and, if the plans
> come to fruition, will work with a font whose Devanagari script tag
> is 'dev3'.  However, I may have overlooked a set of overrides to the
> USE categorisations.

I've now knocked up a partial* representation* of a Devanagari dev3 and
a Grantha font (which I'm dubbing 'Mock Indic 3').  The supported
orders of COMBINING DIGIT ONE and VISARGA, as in Firefox on
Linux, are:

dev2: कः꣡ <U+915 LETTER KA, U+903 VISARGA, U+A8E1 COMBINING DEVANAGARI
DIGIT ONE>

dev3: क꣡ः  <U+0915, U+A8E1, U+0903>
Grantha: (1) ������ <U+11315 LETTER KA, U+11367 COMBINING GRANTHA DIGIT
ONE, U+11303 VISARGA>
 (2) ������ <U+11315, U+11303, U+11367>
The second Grantha spelling is enabled by a Harfbuzz-only change to
the USE categorisations.  It treats Grantha visarga and spacing
anusvara as though inpc=Top rather than inpc=Right.  As I am using
Ubuntu 16.04, this override isn't supported in applications that use the
system HarfBuzz library, such as my email client.

We are now establishing incompatible Devanagari font-specific
encodings fully compliant with TUS!

Richard.

* Partial = much is not handled
* Representation = glyphs are wrong, merely showing arrangement.  (I've
  actually re-used a Tai Tham font.)