[MPEG-OTSPEC] lookup processing and 'chws', 'vchw'

[Peter Constable]

[On the "Related to this": I'm reminded of some feedback that was discussed at the last UTC meeting: a request to change the bidi category of U+060D ARABIC DATA SEPARATOR because, giving the current AL property, "it's impossible to apply kerning between Arabic script numbers and ADS." My input to UTC on that is that that is a text layout/shaping design/implementation issue that's outside the scope of UCD, and that the rationale in the original proposal for AL made sense.]


Peter

-----Original Message-----
From: mpeg-otspec <mpeg-otspec-bounces at lists.aau.at> On Behalf Of John Hudson
Sent: Thursday, August 20, 2020 2:18 PM
To: mpeg-otspec at lists.aau.at
Subject: Re: [MPEG-OTSPEC] lookup processing and 'chws', 'vchw'

On 20082020 12:43 am, Peter Constable wrote:
> I guess there’s another question I can ask: When using GPOS type 2 
> lookups for ‘kern’ (or ‘vkrn’, ‘chws’, ‘vchw’), do font developers / 
> font development tools usually make adjustments to the first glyph only?

Depends on text direction. LTR kern lookups typically adjust width of first glyph; RTL kern lookups adjust width and position of second glyph. 
If you don't handle RTL kerning this way, every-other pair in the run gets skipped during kern processing.

[Related to this: I have an open mental issue about incompatibility between HarfBuzz and Microsoft handling of directionality as it applies to bidi text. My understanding is that HarfBuzz processes e.g. all Arabic text as RTL, even embedded LTR character strings such as numbers, and hence doesn't segment glyph processing runs based on direction, but Microsoft puts bidi RTL and LTR sequences into different runs. Either model works, but I am concerned about what the inconsistency means for kerning between Arabic numerals, which I treat as LTR, but which Harfbuzz may be treating as RTL? This was discussed at one of the OTWG meetings a few years ago, but I never got it entirely sorted in my head.]

J.


-- 

John Hudson
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Salish Sea, BC        tiro at tiro.com

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