These appear to be bugs in WebKit and Opera.

I've annotated the file with a rectangle showing the bounding box of the
containing group:

http://brian.sol1.net/svg/tests/bbox.svg

In Firefox the red circle matches the centre of the rectangle which
shifts as expected.

In WebKit the bounding box returned for the containing group is not the
tightest fitting axis-aligned rectangle, there is often extra space.

In Opera the bounding box does not incorporate the transformation
applied to the child element, i.e. does not change.

Based on [1] I think Firefox is doing the right thing here.

Best regards,

Brian

[1] http://www.w3.org/TR/SVG11/types.html#__svg__SVGLocatable__getBBox

(2012/01/26 8:47), dirk_w_schulze wrote:
>
>
> I didn't see that you take the bounding box of the grouping element at
> the beginning :P
>
> But you are rotating around the central point of the shape, that's why
> both circles stay as they are. The circle would be shifting if the
> center point of the shape is not exactly on the zero coordinate of its
> userspace.
>
> That it shifts on FF seems to be a bug in FF for polygon elements. You
> should report it at bugs.mozilla.org.
>
> Dirk
>
> --- In svg-developers@yahoogroups.com
> <mailto:svg-developers%40yahoogroups.com>, "th_w@..." <th_w@...> wrote:
> >
> > --- In svg-developers@yahoogroups.com
> <mailto:svg-developers%40yahoogroups.com>, "th_w@" <th_w@> wrote:
> > >
> > > --- In svg-developers@yahoogroups.com
> <mailto:svg-developers%40yahoogroups.com>, Cameron McCormack <cam@> wrote:
> > > >
> > > > David Dailey:
> > > > > Oh good! The center of mass (centroid or first central moment)
> is also a
> > > > > graphically relevant construct, so we should think about that
> too! It’s
> > > > > easily scriptable for integrable curves (well, heck, I suppose they
> > > > > don’t even need to be integrable* do they?) but declarative
> access to
> > > > > the centroid would be good for realistic rotation (even sans
> gravity).
> > > >
> > > > Yeah, I should've said that the percentages in transform-origin
> will be
> > > > the bounding box of the object. (There is still the issue of *which*
> > > > bounding box to use -- the geometry one that getBBox() provides, one
> > > > that include the stroke, markers, etc...) Centroids may also be
> useful,
> > > > but supporting bounding boxes is a good simple first step.
> > > >
> > >
> > > Another problem I see is that during the animation the center of
> the bounding box may constantly be shifting.
> > >
> > > http://wwwpub.zih.tu-dresden.de/~s9783698/svg/bbRotate.svg
> > >
> > >
> > > <?xml version="1.0" encoding="UTF-8"?>
> > > <svg xmlns="http://www.w3.org/2000/svg" width="100%" height="100%"
> onload="init()">
> > > <script type="text/ecmascript">
> > > var polygon
> > > var polygonContainer
> > > var bboxCenterMarker
> > >
> > > var angle = 0
> > > var angleIncrement = 5
> > > var interval = 50
> > >
> > > function init() {
> > > polygonContainer = document.getElementById('polygonContainer')
> > > polygon = polygonContainer.getElementsByTagName('polygon')[0]
> > > bboxCenterMarker = document.getElementById('bboxCenterMarker')
> > > animate()
> > > }
> > >
> > > function animate() {
> > > polygon.setAttribute('transform','rotate('+angle+')')
> > > angle += angleIncrement
> > > var bbox = polygonContainer.getBBox()
> > > bboxCenterMarker.setAttribute('cx',bbox.x + .5 * bbox.width)
> > > bboxCenterMarker.setAttribute('cy',bbox.y + .5 * bbox.height)
> > > window.setTimeout('animate()',interval)
> > > }
> > > </script>
> > > <g transform="translate(100,100)" id="polygonContainer">
> > > <polygon points="-50,-20 50,-20 -30,20, -50,20" stroke="black"
> fill="none"/>
> > > <circle r="2" fill="green"/>
> > > <circle r="2" stroke="red" fill="none" id="bboxCenterMarker"/>
> > > </g>
> > > <text x="10" y="200">Green circle: Rotation center (initial
> bounding box center)</text>
> > > <text x="10" y="220">Red circle: Momentary bounding box center</text>
> > > </svg>
> > >
> >
> > Sorry, there must be something wrong here. In Firefox, the red circle
> is shifting around as I expected, but not in Opera and Chromium.
> >
>
>