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Proposal: Fix frozen to-animation

From: Brian Birtles <birtles@gmail.com>
Date: Fri, 25 Mar 2011 11:16:02 +0900
Message-ID: <AANLkTimvkFipmy=uitGoxvfZoLZ8NUN_AuLCFiVBp6eC@mail.gmail.com>
To: www-svg <www-svg@w3.org>
Dear all,

I would like to propose an amendment to SVG 1.1 regarding the
behaviour of frozen to-animation.

Currently, SMIL Animation defines the following behaviour for a
to-animation when it becomes frozen:

  The value for F(t) when a to-animation is frozen (at the end of the
  simple duration) is just the to value. If a to-animation is frozen
  anywhere within the simple duration (e.g., using a repeatCount of
  "2.5"), the value for F(t) when the animation is frozen is the value
  computed for the end of the active duration. Even if other, lower
  priority animations are active while a to-animation is frozen, the
  value for F(t) does not change.[1]

SMIL 3.0 defines the same behaviour but with different wording:

  A frozen to animation takes on the value at the time it is frozen,
  masking further changes in the underlying value. This matches the
  dominance of the to value at the end of the simple duration. Even if
  other, lower priority animations are active while a to animation is
  frozen, the value does not change.[2]

I find this behaviour to be counter-intuitive when a to-animation is
frozen part way through its simple duration (e.g. using
repeatCount=0.5). In such a case I would expect the partially applied
to animation to continue to constrain the underlying value but not
mask it completely.

See the following test case:

The dotted line shows the behaviour defined by SMIL.

Current status of implementation:

WebKit -- Sets the 'to' value when frozen even if frozen part-way
through the simple duration (i.e. ignores the 'at the time it is
frozen' condition).
Opera -- The value continues to change but is constrained by the
partly applied to-animation (i.e. ignores the 'masking further changes
in the underlying value' condition)
Firefox -- Follows the spec I believe (there is code specially for
this condition) but fails when seeking the document or if the
frame-rate is low.

Not only is this behaviour counter-intuitive (or at least, that's my
contention), but it is difficult to implement. Consider what the user
agent must do when a seek is performed:

1. Detect any such to-animations that become frozen within the seek interval
2. Seek to the time when the animation becomes frozen
3. Sample all underlying animations at that point in time
4. Store the underlying animation value
5. Proceed to the next such point, and so on before finally proceeding
to the seek time

In fact, this is not only the case for seeking but is necessary on
every sample since a low sample rate may mean we skip over the time
when the animation became frozen.

If we simply say that a frozen partially-complete to-animation
constrains but does not mask the underlying value then this
complication is avoided and no special casing is required.

Proposed change:

In section 19.2.9 'Attributes that define animation values over time'
change the paragraph:

  The processing rules for the variants of from/by/to animations are described
  in Animation function values.


  The processing rules for the variants of from/by/to animations are
  described in Animation function values with the following exception.

  For the case of a to-animation that is frozen within its simple
  duration, the frozen animation continues to constrain the underlying
  value rather than masking it completely as SMIL Animation defines.
  That is, the result of F(t) will continue to change as the underlying
  value, v_cur, continues to change, but the value of t used for
  interpolation remains fixed at the end of the active duration.

That last sentence is a bit cludgy. It's a bit hard to fix the wording
of SMIL Animation when it comes to these formulae (since SMIL
Animation contains a number of inaccuracies rectified in subsequent
specifications such as describing v_cur in terms of the base value
rather than the underlying value). For SMIL 3.0 it's simple, just drop
the f_f(t,u) = v_f formula altogether, no special case is required.


In summary the current behaviour is:
* Counter-intuitive (as evidenced by the special casing required in
the specification and in implementations)
* Hard to implement
* Not implemented correctly (anywhere I'm aware of)

The proposed behaviour removes the need for special casing and is
already implemented in Opera.

Best regards,


[1] http://www.w3.org/TR/smil-animation/#FromToByAndAdditive
[2] http://www.w3.org/TR/SMIL/smil-animation.html#animationNS-ToAnimation
Received on Friday, 25 March 2011 02:16:48 GMT

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