function getExtentView(){
var c2 = new Cesium.Cartesian2(0,0);
var leftTop = scene.getCamera().controller.pickEllipsoid(c2, ellipsoid);
c2 = new Cesium.Cartesian2(scene.getCanvas().width, scene.getCanvas().height);
var rightDown = scene.getCamera().controller.pickEllipsoid(c2, ellipsoid);
if(leftTop != null && rightDown != null){
leftTop = ellipsoid.cartesianToCartographic(leftTop);
rightDown = ellipsoid.cartesianToCartographic(rightDown);
return new Cesium.Extent(leftTop.longitude, rightDown.latitude, rightDown.longitude, leftTop.latitude);
}else{//The sky is visible in 3D
return null;
}
}