Re: Can we change default radius to 0 in the touch events extensions

On 06/05/2014 10:12 AM, Rick Byers wrote:
> On Thu, Jun 5, 2014 at 12:33 PM, Matt Brubeck <mbrubeck@mozilla.com 
> <mailto:mbrubeck@mozilla.com>> wrote:
>
>     On 06/05/2014 09:13 AM, Rick Byers wrote:
>
>         Secondly, using X and Y for the radius is somewhat limiting.
>          X11 and Android instead use an ellipse model of touch contact
>         with touchMajor, touchMinor and touchRotationAngle.  I've just
>         learned that the Nexus 10 does a really good job of supporting
>         this (you can see it by enabling "show touch locations" in
>         developer options).  I'm not aware of any device that returns
>         touch width/height so in practice chrome always reports
>         radiusX==radiusY.  So I'd prefer we change this to
>         radiusMajor, radiusMinor and rotationAngle properties.  OSes
>         like Windows 8 that report contact geometry in terms of
>         width/height can still be properly supported - rotationAngle
>         would just always be 0, but the inverse is not true.
>
>     Earlier versions of the v2 spec had a "rotationAngle" property
>     that worked like this:
>
>     https://dvcs.w3.org/hg/webevents/raw-file/c8eccbb92b31/touchevents.html#widl-Touch-rotationAngle
>
>     It was removed (by accident?) when the document was changed from
>     Touch Events v2 to Touch Events v1 Extensions.
>
>
> Yeah I think we removed it because we weren't aware of any 
> implementations that used it for anything reasonable (eg. Chrome 
> hard-coded it to 0), so didn't want to encourage use of it by 
> websites.  In particular, the spec didn't explicitly say how radiusX / 
> radiusY should be interpreted when there is a non-zero rotation angle. 
>  Eg. if X11/Android returns Major=10, Minor=5, Angle=45deg, then 
> presumably the browser should use some math to calculate the distance 
> to the ellipse edge along each axis.

No, in the old v2 spec that would simply be equivalent to { radiusY: 
10/2, radiusX: 5/2, rotationAngle: Math.PI/8 }.  To draw the ellipse 
described by these properties, you start with an ellipse whose axes are 
parallel to the X and Y axes, with the given lengths, *then* you rotate 
it.  So the math to convert to/from Android's PointerCoords is very 
simple.  I think the Android names are definitely clearer, though.