Re: [motion-path] More natural names for 'auto' and 'reverse'

My main concern with the keyword `reverse` is that, in the shorthand
property, it would be unclear what it applies to.  If I was not already
familiar, I would assume that it meant travel along the path starting at
the end and moving back to the beginning.

Auto-reverse isn't much better on that point, but at least has the benefit
of consistency with an existing spec.  However, I can't say I've ever used
that option in SVG, and I *have* had need of the auto+angle option, so I
wouldn't be disappointed if auto-reverse didn't make the cut at all.

Keywords also complicate the animatability of the property.  And since this
property is all about animation, that should be a significant factor.  The
spec doesn't have details on how these values will be interpolated.  I'm
assuming that a transition from `auto` to `auto 180deg` would be perfectly
natural, by treating the first one as `auto 0deg`.  You would have to
decide if the same transition applies for `auto` to `auto-reverse`.

Think of the following keyframes sequence

0% {motion-offset: 0;}
40% {motion-offset: 100%;}
50% {motion-offset: 100%; motion-rotation: auto-reverse;}
90% {motion-offset: 0; motion-rotation: auto-reverse;}
100% {motion-offset: 0; motion-rotation: auto;}

In other words, drive to the end of the path, then turn around in place and
drive back along the other side of the line, then turn around again at the
beginning.  So long as this works logically, I'm okay with auto-reverse
being a shorthand for auto 180deg.

(Whether you should be able to transition from auto to a fixed angle is a
whole other question, of course!  It would be neat if you could, but it
would definitely complicate things.).

Best,
ABR

On 21 June 2015 at 21:32, Shane Stephens <shans@google.com> wrote:

> Hi list,
>
> Issue 2 of the motion-path specification requests more natural names for
> ‘auto’ and ‘reverse’. This is in the context of the motion-rotation
> property: http://dev.w3.org/fxtf/motion-1/#motion-rotation.
>
> A specification of ‘motion-rotation: auto’ causes the targeted element to
> rotate so that its horizontal axis is aligned with the tangent to the
> motion path. This matches the SVG definition for animateMotion elements’
> rotate attributes:
> http://www.w3.org/TR/SVG/animate.html#AnimateMotionElement.
> (rotate=”auto” has the same behavior).
>
> ‘motion-rotation: reverse’ causes the targeted element to rotate so that
> its horizontal axis is aligned opposite to the tangent of the motion path.
> This is specified in SVG as rotate=”auto-reverse”.
>
> Additionally, motion-rotation takes an angle (by default, 0deg) which
> applies on top of any automatic rotation.
>
> Does anyone have suggestions for better names than ‘auto’ and ‘reverse’,
> or do these names seem reasonable? Would it be best to align with SVG and
> use 'auto' and 'auto-reverse'?
>
>
> It's also true that 'reverse' provides identical behavior to 'auto
> 180deg'. Should we provide just 'auto', in the interests of brevity?
>
> Cheers,
>    -Shane
>