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Re: Proposal: Fix frozen to-animation

From: Brian Birtles <birtles@gmail.com>
Date: Fri, 8 Apr 2011 10:13:55 +0900
Message-ID: <BANLkTinW6pFBCk+_YEB+5umfdknJxsCo9Q@mail.gmail.com>
To: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>
Cc: www-svg@w3.org
Dear Olaf,

Thanks again for taking time to get back to me on this. I do
appreciate your help.

2011/4/6 Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>:
>> The point of this change is not to add a variant but to remove one.
>
> Unfortunately this would remove possible applications as well,
> therefore not a good idea from the authors point of view ;o)

I believe neither the current behaviour nor the proposed behaviour can
be easily produced by other means. (However, the current behaviour is
more easily simulated since it requires calculating only one value.
The proposed behaviour cannot be easily simulated.)

I am still not aware of any use case for the current behaviour hence I
propose adopting the more intuitive and consistent behaviour.

> For to-animation, because it is neither additive or not-additive, one has
> always to use the specific defined algorithm to get the intended
> effect, therefore it works anyway different from values-animations,
> including the frozen value. The complete effect is not additive,
> why should the frozen value suddenly be additive, this is not
> intuitive and does not fit to the model of the to-animation.

Yes, to-animation is "is a kind of mix of additive and non-additive
animation".[1] As SMIL 3.0 describes,

  "The underlying value is used as a starting point as with additive
  animation, however the ending value specified by the to attribute
  overrides the underlying value as though the animation was
  non-additive."[1]

Using this description we could think of to-animation as becoming less
additive as it progresses through the simple duration (since the
starting point is additive and the end point is non-additive).

If the animation is frozen part-way through the simple duration, the
currently defined behaviour causes the to-animation to suddenly go
from being partly additive to completely non-additive.

The change I am proposing means there is no sudden change. A partly
additive to-animation remains partly additive. That is why I claim
this is more intuitive and more consistent with to-animation.

> Well, I you have a look for example into the CSS transitions draft,
> this might give an idea.
> It is not perfect either, but indicates, that several people assume,
> that something like this has use cases.

Please provide specific reference. I can't see any case in CSS
transitions where the transition is adding to a value in change, stops
mid-interval, and then maintains that intermediate value masking
underlying changes.

> By the way, doesn't values like 'currentColor' and 'inherit' complicate
> the model in a similar way? Those can behave similar to to-animations
> (ok, the frozen part is excluded here) and some viewers do not
> manage them correctly as well.

These issues are confined to the animation model (including the
automatic fallback to calcMode=discrete). The difficulty with the
frozen to-animation is coupling between the two models.

At any rate, it's too late to get this into SVG 1.1 now. It seems
likely that implementations will simply have to deviate from the
specification here.

Thanks again for your help Olaf.

Best regards,

Brian

[1] http://www.w3.org/TR/SMIL/smil-animation.html#animationNS-ToAnimation
Received on Friday, 8 April 2011 01:14:23 GMT

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