[mpeg-OTspec] Re: [OpenType] RE: Proposal: deprecate ReqFeatureIndex

[John Hudson]

Peter Constable wrote:

> So, are you suggesting that people invent arbitrary tags to use in that case?

That is how I understood Behdad's suggestion and Eric's comment on it, 
and also why Behdad suggested that this could be done in such a way that 
the arbitrary tag wouldn't 'pollute the feature tag space'. If I 
understood Behdad correctly, the ReqFeatureIndex doesn't seem to require 
that the indexed required feature be associated with the language system 
alongside the usual layout features: it simply needs to reference an 
enumerated feature in the font.

The general principle is that the feature referenced by ReqFeatureIndex 
doesn't need to be a particular registered feature, but only needs to be 
a collection of lookups that the font developer wishes to be mandatory 
for a particular language system. Whether that feature is associated 
with the language system via the feature tree or only via the 
ReqFeatureIndex is an implementation question.

If it turns out to be necessary for such a feature to have a tag that is 
associated with a language system in a typical way, i.e. if Behdad's 
method is unworkable, and if you are concerned about arbitrary tag 
names, then one could easily define a standard approach to naming such 
tags, e.g. replicating the language system tag as a feature tag. 
[Indeed, one could have devised the functionality of ReqFeatureIndex by 
such a tag naming methodology: if a feature tag exists that matches the 
language system tag name to which it is associated, treat it as a 
required feature.]

JH


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