[MPEG-OTSPEC] Note on VARC conditions and condition set negation

[Skef Iterum]

A note on conditions as defined in WG03-varc-and-other-updates-03.pdf 
(up for review tomorrow morning).

When I sketched out the VARC condition idea I had them arranged in 
if/else if/.../else sequences. From my reading of the document they're 
currently in "if" form -- a condition set can be attached to a component 
and if it is it will only be used if the condition set evaluates to true.

There's nothing wrong with that semantic; it may be preferable because 
it's simpler. However, the most common case with this system is that you 
want one shape to render in some region of design space (defined by the 
condition set) and another shape to render outside of that. And negating 
a condition /set/ can be expensive and/or tricky. So I would recommend 
that one of these two things change:

  * The if/else if/.../else semantic is restored
  * Some way of negating a condition set is added to the specification

We talked about the possibility of condition set negation in a previous 
meeting but didn't come to any conclusions. It's tricky because the 
table has no versioning. If we wanted to hack negations into the current 
format the way to do that might be to add a new condition format 
(possibly 5 if newfeatvar_spec.pdf is accepted) that looks something like:

    ConditionTableFormat5

    *Type                    Name Description*

    uint16                  Format Format (set to 5)

    When present in a condition set this condition indicates the set
    should apply when
    at least one of the other conditions is false and not otherwise.
    That is, it makes the
    condition set apply when it would normally not and vice-versa. There
    should be at
    most one format 5 condition in a condition set and it should be the
    first.

If this is preferred it could be used to replace/simplify the 
trueLookupIndexListOffset/falseLookupIndexListOffset pair in the 
LookupCondition record of newfeatvar_spec.pdf to just 
lookupIndexListOffset (because one could just follow the record
with the "positive" condition set with another record with the negated 
one when needed).

Skef
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