Camera zooming to the center of a model

Now camera zoom performs like that: I track mouse pointer coordinates and camera zooms towards these coordinates

const Graphic3d_Vec2d aNewPos2d = myView->Window()->ConvertPointToBacking(Graphic3d_Vec2d(theEvent->x, theEvent->y));
const Aspect_ScrollDelta aScroll(Graphic3d_Vec2i(aNewPos2d + Graphic3d_Vec2d(0.5)), 1);
if (UpdateMouseScroll(aScroll))
{
myGLArea->queue_draw();
}

As a result the model, which is displayed in a viewer slides away, it is not comfortable at all. I want always zoom the model ( zoom the camera to a point that is the center of the model ), so I need to somehow 1) translate global viewer coordinates to 2d window coordinates 2) get the center of the displayed model ( the model is displayed with the help of AIS_ConnectedInteractive, so I have access to each model part, but not to the entire model )