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Re: [css3-transforms] neutral element for addition - by animation

From: Cyril Concolato <cyril.concolato@telecom-paristech.fr>
Date: Fri, 01 Jun 2012 08:39:21 +0200
Message-ID: <4FC86399.4090503@telecom-paristech.fr>
To: www-svg@w3.org
Le 5/31/2012 10:35 PM, Cyril Concolato a écrit :
> Hi Dirk,
>
> See my comment inline.
>
> ----- Mail original -----
> De: "Dirk Schulze"<dschulze@adobe.com>
> À: "SVG public list"<www-svg@w3.org>, public-fx@w3.org
> Envoyé: Samedi 26 Mai 2012 18:56:27
> Objet: [css3-transforms] neutral element for addition - by animation
>
> Hi SVG folks,
>
> I have a question to by animations on transforms. See the simple example here:
>
> <rect width="100" height="100">
>    <animateTransform attributeName="transform" attributeType="XML"
>                      type="scale" by="1" dur="5s" fill="freeze"/>
> </rect>
>
> This example scales from 0 to 1, since the neutral element for scale is 0. This is relevant because of the definition in SMIL Animation:
>
> ""
> Normative: A by animation with a by value vb is equivalent to the same animation with a values list with 2 values, the neutral element for addition for the domain of the target attribute (denoted 0) and vb, andadditive="sum". Any other specification of the additive attribute in a by animation is ignored.
> "" [1]
>
> [CC] Adding 1 in the scale transformation means going from scale(X) to scale(X+1), therefore the neutral element is scale(0) which is the identity matrix.
I made a mistake. I meant to say scale(1) is the identity matrix. This 
does not change my conclusion.
scale(X+0)=scale(X)scale(1)=scale(X)

Cyril
>
>
> And according to SVG animation, the animated transform gets post multiplied to the userspace transform of the element (which is the identity matrix).
>
> My question is the following: Every transform function (independent if translate, scale or rotate) is just a shorthand for the underlaying transformation matrix (3x3/(3x2) or 4x4). Therefore, shouldn't the neutral element for addition on every transform function be the null matrix? If that is the case, what happens for the following example:
>
> <rect width="100" height="100">
>    <animateTransform attributeName="transform" attributeType="XML"
>                      type="translate" by="100,100" dur="5s" fill="freeze"/>
> </rect>
>
> Shouldn't the neutral element here be the zero matrix?
> [CC] No. Adding 100,100 to a translate means going from translate(tx,ty) to translate(tx+100,ty+100), therefore the neutral element is translate(0) which is also the identity matrix.
>
>   And shouldn't it mean that at the beginning of the animation the userspace transform gets post multiplied by this null matrix and therefore the element gets invisible? And as a consequence, shouldn't the element scale from 0 to 1 and translate from 0 to 100 at the same time?
> [CC] No.
>
> Non of the tested implementations scale the rect for the second example. I think this is incorrect from the mathematical point of view.
> [CC] No, I think they interpret it as I indicated.
>
> Of course it is also possible to define the neutral element for every single transform function. But then the null element doesn't really make sense for me.
> [CC] The neutral element is the identity matrix for transformation 'addition' (i.e. composition). It works for scale, rotate, translate, skew ...
>
> Cyril
>
>
> I prefixed the subject with [css3-transforms] since I want to specify it in CSS3 transforms and believe that the current definition is incorrect [2].
>
> Greetings,
> Dirk
>
> [1] http://www.w3.org/TR/SMIL3/smil-animation.html#q34
> [2] http://dev.w3.org/csswg/css3-transforms/#neutral-element
>
Received on Friday, 1 June 2012 06:39:56 GMT

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