Fallback for Sinhala Consonant Clusters

Richard Wordingham via Unicode unicode at unicode.org
Mon Oct 15 02:53:59 CDT 2018

On Mon, 15 Oct 2018 01:55:24 +1100
Harshula via Unicode <unicode at unicode.org> wrote:

> 3) However, what you have observed is an issue with *explicit*
> conjunct creation. After the segmentation is completed, the
> layout/shaping engine needs to first check if there is a
> corresponding lookup for the explicit conjunct, if not, then it needs
> to remove the ZWJ and redo the segmentation and lookup(s). Perhaps
> that is not happening in Harfbuzz.

This indeed seems to be the problem with HarfBuzz and with Windows 7
Uniscribe.  Curiously, they almost adopt this behaviour when touching
letters are not available.  (The ZWJ seems not to be completely removed
- in HarfBuzz at least it can result in the al-lakuna not interacting
properly with the base character.)

But where is this usually useful behaviour specified?

1.  There may be nothing but time and money to stop fallbacks being
built into the font.  For example, what prohibits the rendering of a
conjunct falling back to touching letters or a missing glyph symbol?

2. One could argue that the current behaviour falls back to a <consonant,
al-lakuna, consonant> display; Pali in Thai script does use sequences
of <left matra, consonant, vowel-killer, consonant>. The problem is
that al-lakuna also acts as a vowel modifier.

3. What stops one arguing that a conjunct is an abstract
character and that to render it with a sequence using a visible
al-lakuna would violate its identity? 


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