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Re: Deferring SVG DOM to a separate module

From: Nikos Andronikos <nikos.andronikos@cisra.canon.com.au>
Date: Thu, 11 Jun 2015 08:31:15 +0200
Message-ID: <55792B33.6090406@cisra.canon.com.au>
To: <www-svg@w3.org>


On 11/06/2015 3:46 am, Brian Birtles wrote:
> On 2015/06/11 0:55, Amelia Bellamy-Royds wrote:
>> For now, one specific change we discussed was neutering "animVal" so
>> that it is always an exact synonym for "baseVal".  From the
>> implementation side, this makes it much easier to move forward with web
>> animation optimizations on a separate thread, if they don't have to keep
>> updating data accessible from the main thread.
>
> I don't understand this. How is it different to CSS where animations
> not happening on the main thread still need to update the computed
> style returned on the main thread?
This was in reference to a comment I made, although I didn't mention
threading.
The issue I raised was that in WebKit, the SVG implementation is so
overly complex and horrible, that it blocks implementing a lot of things.
So it's a chicken and egg thing, we want Web Animations so that we have
a replacement, but the current architecture means that implementing Web
Animations is a lot more difficult.
If we aliased animVal to baseVal it would simplify things and greatly
speed up the refactoring of the SVG code to support Web Animations.
We would want a way to obtain the animated value somehow though.
>
>> I am assuming that web animations will offer an alternative, thread-safe
>> way of accessing this information (e.g., when an animation is
>> paused/interrupted because of user action, you need to figure out where
>> everything currently is before setting up a new animation), but I'm not
>> sure how this would work (maybe by passing a specific time value to the
>> animation, instead of asking for "current value"?).
>
> Exposing this is really an issue for SVG. For CSS we can (sort-of) get
> the animated value using getComputedStyle (which may not actually be
> the animated value since it could be overwritten by !important rules
> etc. but it answers the question, "what color is this thing now?").
>
> For any attributes SVG defines as animatable that are not presentation
> attributes, SVG needs to define how to get the animated value, if at all.
>
> We've talked about adding API to Web Animations that would let you
> ask, "What's the contribution of this particular animation to property
> x?", possibly providing the time as an argument, but that's a long way
> off and would need a wrapper to make it useful in this case.
>
> With regards to multiple threads/processes, in Gecko (and I'm pretty
> sure other engines are the same here) we run the animation twice in
> effect. On the compositor thread/process we try to run it at 60fps and
> on the main thread we throttle it so that we only update style when we
> need to (e.g. someone calls getComputedStyle).
>
> Obviously that means there will be some discrepancy between the values
> you get back from getComputedStyle and whatever the compositor is
> currently generating (particularly if either thread/process is bogged
> down) but I don't see an easy way around that. For applications where
> that discrepancy is unacceptable we might need to provide a way for
> authors to force their animations to run on the same thread as their JS.
>
> Best regards,
>
> Brian
>

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Received on Thursday, 11 June 2015 06:31:33 UTC

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