[MPEG-OTSPEC] Is it valid to use a GSUB sequence with a leading ZWJ character please?

[John Hudson]

I don’t see any reason why beginning a GSUB input sequence with ZWJ 
should not be valid, but that isn’t the same thing as implying that you 
might not run into problems in particular OTL implementations.

ZWJ and other formatting control characters with representative glyphs — 
properly only displayed in editing environments, not in text 
presentation — should be available to GSUB, but then suppressed from the 
resulting glyph string for GPOS and display purposes. That behaviour is 
mostly reliable in complex script shaping engines, but it is possible 
that the path emoji take in some environments may run into unexpected 
issues with some part of this procedure.

JH


> An interesting discussion about glasses emoji is taking place in the 
> Unicode public mailing list.
>
>
> https://corp.unicode.org/pipermail/unicode/2022-October/date.html
>
>
> Thinking about this, earlier this morning here in England I adapted 
> one of my own fonts to add some additional glyphs to run a test, and 
> it worked. I used the High-Logic FontCreator program to make the font 
> and Serif Affinity Publisher to run the test.
>
>
> The font that I adapted is the Mariposa.otf font that is available 
> from the following web page, though in fact I started with a copy of 
> the unpublished High-Logic project file for the font.
>
>
> http://www.users.globalnet.co.uk/~ngo/mariposa_novel.htm
>
>
> To the font I added glyphs for three Unicode characters and for one 
> unmapped glyph.
>
>
> U+200D ZERO WIDTH JOINER though I used a wide visible glyph design
>
>
> U+1F453 EYEGLASSES
>
>
> U+1F469 WOMAN
>
>
> unmapped ceyeglasses, a copy of the glyph used for eyeglasses, moved 
> 2048 font units to the left and the glyph having zero advance width.
>
>
> Within the liga table I added the following line.
>
>
> sub uni200D u1F453 -> ceyeglasses;
>
>
> that is, for "combining eyeglasses".
>
>
> Please note that the substitution starts with a ZERO WIDTH JOINER
>
>
> In the test in Affinity Publisher I used the Glyph Browser to enter 
> the three characters
>
>
> U+1F469 U+200D U+1F4F3
>
>
> and the result displayed was of a woman wearing the eyeglasses as the 
> display was of the glyph for WOMAN with the glyph for ceyeglasses 
> superimposed upon it.
>
>
> So if it is valid to use a glyph substitution with a leading ZWJ in 
> this way then the issue appears to be solved.
>
>
> Is it valid to use a glyph substitution with a leading ZWJ in this way 
> please?
>
>
> One could go further with a sequence also involving a pink square and 
> a large circle to produce stylish large round pink rimmed glasses for 
> the lady to be wearing.
>
>
> Indeed the technique could be adapted so that the lady is also wearing 
> a U+1F452 WOMANS HAT in a stylish purple colour.
>
>
> William Overington
>
>
> Friday 14 October 2022
>
>
>
>
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-- 

John Hudson
Tiro Typeworks Ltdwww.tiro.com

Tiro Typeworks is physically located on islands
in the Salish Sea, on the traditional territory
of the Snuneymuxw and Penelakut First Nations.

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