[MPEG-OTSPEC] Feature Variations and Feature Parameters

[Skef Iterum]

I've been getting back to Feature Variations questions (in relation to 
the feature file format) recently, and I'm now wondering whether it 
would be good to add some editorial advice in the specification. Note 
that I'm not making a proposal, I'm aware that the spec is in an awkward 
stage for making changes, etc. I'm just thinking in terms of merit right 
now, not process or timing.

The current draft adds functionality related to Feature Variations for 
/lookups/ but leaves the current system more or less unchanged. Because 
that system is based on offsets to alternative Feature tables, one can 
use it to adjust (via conditions)/either/ the set of lookups active for 
a feature for different axis positions, /or/ its "parameters". Recall 
that the those parameters are specific to a given feature tag, currently 
size, ssXX, and cvXX (where XX are two-digit numbers).

As I recall there is basically no guidance in the spec about when, or 
whether, one might adjust feature parameter values at different axis 
ranges, or how an implementation should handle such changes. Wouldn't 
having at least one of these be a good idea? I suspect that any such 
adjustments would have no effect in current implementations, but I could 
be wrong. I'm therefore tempted by the thought of adding guidance that 
any changes in feature parameters via the Feature Variations system is 
strongly discouraged, at least until someone comes back to "us" with a 
solid motivation that we could then describe.

What do people think? Are there cases where this makes sense? Are they 
compelling enough that e.g. I should support them in the feature file 
grammar?

Skef
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