[MPEG-OTSPEC] Negated condition sets

[Skef Iterum]

Vladimir reminded me that there were some outstanding issues relating to 
flags, conditions, and condition sets. I'm just going to give my views 
on this subject briefly.


        Do condition tables need flags?

The main use case for a flag we discussed was to negate the condition, 
and in my view using a different format number is both more compatible 
(in that the existing type doesn't change) and more compact. One could 
split the existing uint16 format field into a format and flags, but 
given that the existing format is 1 the flags would come in the first 
byte rather than the second.


        Do condition sets need to be negate-able?

The only use of condition sets in the working draft is part of the 
Feature Variations mechanism, and Adobe's proposal for updating that 
system supports something equivalent to a negated condition set (the set 
of lookups to add when at least one condition is false). So until 
condition sets are used elsewhere (perhaps in VARC) the need for 
explicit negated condition sets is speculative.


        How would we make condition sets negate-able?

Condition sets don't have flags for a format field, so there's no 
entirely clean way of altering their behavior. The least ugly way that 
I've thought of is to add a special offset value with that meaning to 
the list, e.g. 1 or 2.

(One could imagine not implementing explicitly negated conditions at 
all, and instead making an offset of 1 mean "negate the whole set", 2 
mean "negate the condition at the following offset" and perhaps even an 
offset of 3 mean "or". However, there are already other means of 
achieving what amounts to an "or" and this kind of a approach seems 
hacky. Still, just adding the 1 with that meaning might be the best 
solution for negating the set if we need that.)

Skef
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