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Re: [css3-2d-transforms] matrix animation

From: Chris Marrin <cmarrin@apple.com>
Date: Fri, 27 Mar 2009 10:54:47 -0700
Cc: www-style@w3.org
Message-id: <60C423D1-ADA1-4DDC-930A-293E11E9CD81@apple.com>
To: "Dr. Olaf Hoffmann" <Dr.O.Hoffmann@gmx.de>

On Mar 27, 2009, at 3:14 AM, Dr. Olaf Hoffmann wrote:

> Dean Jackson:
>> I'm afraid I disagree. Our main goal is to make the most common case
>> both simple to author and look good.
>
> Matrix animation with composition by the author and some decomposition
> by the viewer will not be a common case, I guess. Most authors will  
> not
> be familiar with matrix calculations anyway and for the others it is  
> still
> more convenient simply to write down the complete set of intended
> transformations and let the viewer do the job of matrix calculations.
>
>> The way we have specified
>> transform animations meets this (I believe).
>>
>> A secondary goal is to give authors complete control. The current
>> specification also meets this. An author can avoid decomposition if
>> they want to (it requires more work, but if they are *that*  
>> determined
>> to control every aspect of their animation then they probably have it
>> all worked out already).
>
> Well, then explain a simple approach for authors to specify an
> animation (direct interpolation) between matrix A and matrix B
> with an accuracy better than one device pixel.
> If you can do it with the current draft, then I'm almost convinced,
> that at least nothing is missing, even if other things are available
> twice.

I'm not sure what you mean by "better than one device pixel". If you  
have a matrix with just translation values, for instance, then matrix  
animation would animate from one to the other just as accurately as  
animating the components directly. Same for scale and rotation. It's  
when you combine everything that things get interesting. And in that  
case, we believe the author's intent is to go from one "pose" to  
another in the most natural way. For instance, if one pose is the  
object at a scale of 0.5 with a slight cant up and to the left and the  
other pose is the object at a scale of 1 with a slight cant up and to  
the right, then we believe the author's intent is to smoothly scale up  
the object while at the same time pivoting from left to right. And  
that's what you would get with matrix animation. For instance:

     from: -webkit-transform: scale(0.5) rotateX(15deg) rotateY(-30deg);

     to:     -webkit-transform: rotateX(15deg) rotateY(30deg);

This can't do a component by component animation because the functions  
don't match. But a decomposed matrix animation would give you the  
desired result. And linear interpolation of the matrix values would  
give add a implicit converse scale that would interfere visibly with  
the smoothness of the scale from 0.5 to 1.

Do you have any practical use cases where a linear interpolation would  
do something reasonable (much less preferable) where a decomposed  
matrix would not?

>
>
> And it does not solve the possible mathematical problem with the
> inverse, just to believe, that it works ;o)
> I had already similar discussions to fix such believes for
> constrained transformations in SVGT1.2.

A non-invertible matrix would result in nothing rendered, so what  
practical use do you have for them? Again practical examples would be  
helpful.

-----
~Chris
cmarrin@apple.com
Received on Friday, 27 March 2009 17:55:29 GMT

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