SVG - From Window coordinates to ViewBox coordinates

Basically I have an svg "SecondSVG" into an svg "FirstSVG" into an svg "MainSVG".Every svg has its own ViewBox. This page can be loaded anywhere on the screen by another page.

So basically how can i find the scr开发者_开发技巧een x for viewBox for"SecondSVG" knowing that this svg can be loaded basically anywhere based on the calling page? event.clientX gives myself the x coordinate for the screen. If I don't know the coordinate for ViewBox of "SecondSVG" then how can I find out the x coordinate inside the ViewBox of "SecondSVG"?

I am using Firefox 3.6.3 and I do have an event object from which I can extract clientX, clientY and other coordinates that are relative to the screen. However what I need are the coordinates inside the ViewBox.

function click(evt)
{

var root = document.getElementById('your svg');

        var uupos = root.createSVGPoint();

        uupos.x = evt.pageX;

        uupos.y = evt.pageY;

        var ctm = evt.target.getScreenCTM();

        if (ctm = ctm.inverse())

            uupos = uupos.matrixTransform(ctm);



 ///the new x y are : uupos.x , uupos.y 
}