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

[Behdad Esfahbod]

Hi Skef,

I'm a bit uncomfortable encoding if/else in VARC. As you said on the call,
the current design seems like a practical compromise. As for negation, we
*can* add a flag to the component to negate the condition. Maybe that's the
right approach. But I feel like the conditionSet itself should contain the
negation. That would reduce sharing though.

A "negate the rest" condition type sounds good to me.

behdad
http://behdad.org/


On Mon, Apr 8, 2024 at 1:55 PM Skef Iterum via mpeg-otspec <
mpeg-otspec at lists.aau.at> wrote:

> 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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