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SMIL Animation and CSS OM

From: Sjoerd Mullender <sjoerd@acm.org>
Date: Wed, 27 Jun 2007 13:01:57 +0200
Message-ID: <468243A5.6050604@acm.org>
To: www-smil@w3.org
CC: Cyril Concolato <cyril.concolato@enst.fr>, Bjoern Hoehrmann <derhoermi@gmx.net>, Cameron McCormack <cam-www-smil@aka.mcc.id.au>, Patrick Schmitz <cogit@ludicrum.org>
Several people have reported in the past that there is a contradiction
in SMIL 2.0 (and 2.1) Animation with respect to the CSS OM.  I am now
trying to address this issue, but I would like to get some feedback.

But first my apologies that this took so long.  Until recently there was
nobody within the Working Group really responsible for Animation.  I am
now trying to resolve these old issues in preparation for the next
version of SMIL.

The problem, as I understand it, is that in the Sandwich Model we say:
- the value on which the animation is to operate is read from the CSS OM
using getComputedStyle();
- the new, computed value is written to the override style sheet in the
CSS OM.

The problem with this is that getComputedStyle() (apparently -- I am not
a CSS expert) takes the override style sheet into account, and thus
there is at the very least confusion, and quite possibly a
contradiction, with the possible result that animations are applied more
than once.

This is of course not the intention.  SMIL Animations should be applied
once, so we should fix the text to make that clear(er).  Of course, we
should keep in mind that the relevant text is all only informative.

The current text says:

"When animation is applied to CSS properties of a particular element,
the base value to be animated is read using the (readonly)
getComputedStyle() method on that element. The values produced by the
animation are written into an override stylesheet for that element,
which may be obtained using the getOverrideStyle() method."

My proposal is to tack a sentence on to this:

"Note that it is assumed that before reading the value, the override
stylesheet is cleared so that the animation works on the original
document value."

My question to the various reporters (and others): would this be
sufficient to resolve the issue?

Links:

The relevant bit in the latest public Working Draft of SMIL 3.0:
http://www.w3.org/TR/2006/WD-SMIL3-20061220/smil-animation.html#animationNS-AnimationSandwichModel

The relevant bit in the current recommendation (SMIL 2.1):
http://www.w3.org/TR/2005/REC-SMIL2-20051213/animation.html#animationNS-AnimationSandwichModel

Messages in which the issue was reported:
http://lists.w3.org/Archives/Public/www-smil/2005OctDec/0027
http://lists.w3.org/Archives/Public/www-smil/2006JanMar/0017
http://lists.w3.org/Archives/Public/www-smil/2006JulSep/0019

-- 
Sjoerd Mullender


Received on Wednesday, 27 June 2007 11:07:42 GMT

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