Fallback for Sinhala Consonant Clusters

Harshula via Unicode unicode at unicode.org
Mon Oct 15 10:47:36 CDT 2018

Hi Richard,

On 15/10/18 6:53 pm, Richard Wordingham via Unicode wrote:
> 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?

I had not considered the missing glyph symbol. Perhaps that is the most
accurate solution when a font is missing a glyph during an *explicit*
conjunct lookup.

Note, touching letters are formed by <ZWJ><AL-LAKUNA>, so they should
not be displayed as a fallback for <AL-LAKUNA><ZWJ> conjuncts.


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