[MPEG-OTSPEC] 'curs' feature: required?

[John Hudson]

On 14092020 11:29 am, Peter Constable wrote:
>
> Is the ‘curs’ feature considered a required feature for Arabic or 
> other cursively-connecting scripts?
>
> I would have expected it’s required. It’s clear the intent is for 
> triggering cursive attachment positioning (GPOS type 3), and 
> Microsoft’s Arabic implementation spec 
> <https://docs.microsoft.com/en-us/typography/script-development/arabic> 
> lists it among features to be applied in GPOS processing. But the 
> description in the OTL feature registry 
> <https://docs.microsoft.com/en-us/typography/opentype/spec/features_ae#tag-curs> 
> is worded in a way that makes it sound optional, at the user’s discretion:
>
> “/UI suggestion:/ This feature could be made active or inactive by 
> default, at the user’s preference.”
>
Based on Arabic fonts I have worked on or examined, I would say this 
definitely should not be optionally active, and if present the curs 
feature is expected to be applied by default. Indeed, a lot of fonts 
simply break if curs is not applied.

If a font contains support for different kinds of Arabic layout, e.g. a 
simplified layout with only horizontal connections and a more 
sophisticated display with some amount of vertical connections or 
cascading connections, then the curs support required for the latter 
still needs to be applied by default, and the differentiation of the 
layout styles would be implemented, somehow, in GSUB, setting up the 
glyphs that either will or will not require curs attachment in GPOS.

JH

-- 

John Hudson
Tiro Typeworks Ltd    www.tiro.com
Salish Sea, BC        tiro at tiro.com

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