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Re: transform animation and decomposition

From: Doug Schepers <schepers@w3.org>
Date: Thu, 26 Mar 2009 15:32:01 -0400
Message-ID: <49CBD831.3040604@w3.org>
To: Dean Jackson <dino@apple.com>
CC: SVG WG <public-svg-wg@w3.org>
Hi, Dean-

Thanks for holding my hand.  I think I follow you now.


Dean Jackson wrote (on 3/26/09 3:20 PM):
> On 26/03/2009, at 1:36 PM, Doug Schepers wrote:
>> Hi, Dean-
>> Dean Jackson wrote (on 3/25/09 5:52 PM):
>>> (see the following message for a real world example)
>>> I wonder if this decomposition should be added to SVG animation as well.
>>> It will "break" existing content, but only in a way that makes things
>>> better in the majority of cases. And I doubt there is much content out
>>> there that is animating matrices.
>> Sorry to be a bit dense... Could you clarify what you mean by
>> "breaking" existing content?
> Nothing really breaks, but animations might look slightly different as
> they run. If so, it will probably be better and more like what the user
> expects.
>> Is the problem that Dr. Olaf points out that there is mismatch between
>> a matrix animation and the corresponding animation if decomposed into,
>> say, a translate, rotate, and scale?
> No. He's saying CSS animations should NOT decompose matrices.
>> If so, I do understand his concern, but I would also agree with you
>> (if I understand you correctly) that this allows an author to achieve
>> different effects by using different methods of animation, which may
>> be more desirable than pure mathematical consistency. If this doesn't
>> prevent authors from achieving their goals in one manner or another
>> (though it may have occasional surprising behavior), then I am not
>> particularly bothered.
> I think you're misunderstanding.
> Here are some examples:
> 1.
> from - translate(10, 20) scale(5) rotate(100)
> to - translate(50, 60) scale(10) rotate(-100)
> This will always animate each function separately. This is very easy for
> the author to understand (and write). It also looks good during the
> animation. I would think this is the most common case, especially hand
> authoring. A tool should also be able to keep the transform functions
> separate in the majority of cases.
> 2.
> from - translate(10, 20) scale(5) rotate(100)
> to - rotate(-100) scale(10)
> This can not be animated separately by function, so you calculate the
> resulting matrix at the from and to stages and animate between those two
> matrices.
> 3.
> from - matrix(a1, b1, c1, d1, e1, f1)
> to - matrix(a2, b2, c2, d2, e2, f2)
> This is pretty much the same as how 2 ends up.
> The problem is that doing linear interpolation of matrix components
> usually looks wrong (and has the really bizarre edge cases I mention in
> the example to www-style). So, in CSS we specify that the matrix should
> be mathematically decomposed into translate, scale, rotation, etc
> matrices and those matrices have their components linearly interpolated.
> I'm not sure why Dr Olaf thinks this is an implementation convenience.
> It's definitely more work for the engine, but produces much better results.
>> I appreciate your posting the example... it might be useful and
>> compelling if you could put together some actual running code to
>> illustrate the various differences (preferably something that could be
>> used as a test or as a tutorial later).
> This is all implemented in WebKit TOT. You can try it out with a nightly
> build. It's pretty easy to write content that triggers each method, if
> you're willing to do the matrix multiplication yourself :)
> Dean
Received on Thursday, 26 March 2009 19:32:10 UTC

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