ZWJ as a Ligature Suppressor

[Richard Wordingham]

On Mon, 10 Aug 2015 17:58:24 +0000
"Andrew Glass (WINDOWS)" <Andrew.Glass at microsoft.com> wrote:
 
I had asked:

>> According to the text just after TUS 7.0.0 Figure 23-3
>> (http://www.unicode.org/versions/Unicode7.0.0/ch23.pdf#G25237), ZWJ
>> suppresses ligatures in Arabic script.  Does this rule apply to other
>> normally cursive joined scripts, e.g. Syriac and Mongolian?

> To ligate or not to ligate is up to the font designer. Normally, GSUB
> lookups that perform ligation will be broken by the presence of ZWJ
> or ZWNJ. If a font designer wishes to ligate in the presence of a ZWJ
> or ZWNJ then they could choose to include appropriate glyph sequences
> in their ligation lookups. For example:

> glyphA glyphB -> glyphC
> glyphA ZWJ glyphB -> glyphC

So, any rule as to what ZWJ means is not implemented in the OpenType
engine, but rather in the font.  (As is the rule that 'a' does not
look like 'b'.)  For which scripts may a font designer defensibly omit
the duplicate with ZWJ?  The TUS says Arabic is one. Are there any
others?

Richard.