[mpeg-OTspec] OFF "cv01"-"cv99", and a more general question

[John Hudson]

Peter Constable wrote:

> In the intended usage scenarios for cvXX, I can see more than one 
> feature being applied to the same text, but I would not expect more than 
> one feature to have lookups affecting any given character. E.g. if cv01 
> is used for variants of “a”, then I would expect all the variants of “a” 
> to be accessed using cv01 rather than having some variants of “a” 
> accessed using cv01 but other variants of “a” accessed using a different 
> cvXX feature.

So, with the option of using type 3 substitutions, the <cvXX> features 
are like precisely targeted versions of the <salt> feature.

That is certainly a reasonable theoretical model, but my concern is 
that, to my knowledge, no one has actually tried to implement <salt> 
type 3 lookup support at the application UI level (unless SIL have done 
it). And the process by which the user would select from the enumerated 
variants on offer in each feature seems far from clear. It has been more 
than a decade now since type 3 lookups were included in the GSUB spec, 
and neither MS nor Adobe has tackled the UI implementation problems.

I had imagined that the <cvXX> features would be spec'd in such a way as 
to avoid this implementation issue, providing one-to-one substitutions 
for different variants in different features (although that comes with 
the same interactivity issues as the <ssXX> features).

John Hudson


-- 

Tiro Typeworks        www.tiro.com
Gulf Islands, BC      tiro at tiro.com

I'm like that Umberto Eco guy, but without
the writing.   -- anonymous caller