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Re: [css3-2d-transforms] Interpolation of transforms (for transitions/animations)

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Fri, 4 Nov 2011 12:39:05 -0700
Message-ID: <CAAWBYDB2yJOynjh9Q9JfG3CtdKpbA3sYS=Oa3ox554jURF6aXw@mail.gmail.com>
To: Øyvind Stenhaug <oyvinds@opera.com>
Cc: www-style@w3.org, public-fx@w3.org
On Fri, Nov 4, 2011 at 8:59 AM, Øyvind Stenhaug <oyvinds@opera.com> wrote:
> http://dev.w3.org/csswg/css3-2d-transforms/#animation
> or
> http://www.w3.org/TR/2009/WD-css3-2d-transforms-20091201/#animation
> and also
> http://dev.w3.org/Graphics-FX/modules/2D-transforms/spec/2DTransforms.html#animation
> This section gives rules for interpolating transform values for
> transition/animation purposes. However, it is unclear on some points.
> 1) A couple of points are conditional on functions being "of the same type".
> It is not clear what "type" means. E.g.:
>   a) Is translate(20px) the same type as translate(40px, 40px)?
>   b) translate(20px) and translateX(20px)?
>   c) translate(20px) and translate(80%)?

It's probably supposed to mean "with the same function name".  It's
underspecified, though.  With that assumption:

(a) is definitely the same, since the 1-argument form explains the
default value of the omitted argument.
(b) *should* be the same, but the current spec probably disallows it.
translateX/Y should be treated as just a translate function with the
appropriate argument being 0.
(c) is definitely the same.

> 2) Even assuming that having the "same type" simply means having identical
> FUNCTION tokens and the same number of parameters, we end up at "the
> individual components of the function are interpolated numerically", which
> doesn't make sense unless each pair of "components" have matching units.

Right; they should be interpreted as lengths, numbers, or angles, as
appropriate.  We can interpolate lengths with different units without
problem, though some cases require representing it with calc().

> 3) Even if considering parameters with different units to result in
> functions being different "type", the "Otherwise" step says to convert to
> matrix values. But transitions/animations operate on computed values, and
> percentages cannot be resolved at this level (they depend on used values).

Yes, this can't be done in general at computed-value time.  Instead,
these transforms should be represented using whatever mechanism we
decide on for interpolating other types of values that can't be done
without used-time values, like going from "height:auto" to
"height:0px".  Then this section can specify how that is actually

Received on Friday, 4 November 2011 19:40:04 UTC

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