How to specify the x-y rotation point when using animateTransform? - animation

Question

I want to use animateTransform to rotate an SVG image continuously. So here we go:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
width="1024px" height="768px" enable-background="new 0 0 100 100" xml:space="preserve">
<g transform="translate(100, 100)">
<rect fill="#FE9FFF" width="100px" height="100px">
<animateTransform attributeName="transform" type="rotate"
from="0" to="360" dur="20s" repeatDur="indefinite"/>
</rect>
</g>
</svg>
This works.
Now: I would like to change the above, so that the block rotates around its center and not its top left corner. I know that if I want to rotate the block statically around its center, I can do this:
<g transform="rotate(30, 50, 50)">
<rect fill="#FE9FFF" width="100px" height="100px">
</rect>
</g>
My question is - how do I manage a continuous animated rotation around the block's center? I have looked at the spec and a couple of other related questions on SO, but I'm having trouble implementing the explanations supplied.
Thanks in advance.

Answer 1

http://www.w3.org/TR/SVG/animate.html#AnimateTransformElement
The ‘from’, ‘by’ and ‘to’ attributes take a value expressed using the same syntax that is available for the given transformation type:
(...)
For a type="rotate", each individual value is expressed as <rotate-angle> [<cx> <cy>]
You can specify the center for the rotation if you provide 2 additional values, cx and cy.
So, for your piece of code, I add "50 50" in the "from" and "to" attribute :
<rect fill="#FE9FFF" width="100px" height="100px">
<animateTransform attributeName="transform" type="rotate"
from="0 50 50" to="360 50 50" dur="20s" repeatDur="indefinite"/>
</rect>
It work with latest version of Opera, Safari and Chrome, also Firefrox 4 Beta and Batik.

Answer 2

To be clear: what we are trying to achieve is rotation around a center which is itself being translated. I find that if I have an <animateTransform type=rotate>, I cannot use <animateMotion>, nor <animateTransform type=translate> to perform simultaneous translation. It (latest Chrome or Firefox) does not interpolate the center of rotation as desired, resulting in a "loop de loop" instead. However, using a simple <animate> for each of the x,y coordinates does work as desired; in this case, <animateTransform type=rotate> interpolates the center along the x,y path, as long as you set the from= parameter to the start angle, start x and start y position, and set the to= parameter to the ending values.