right now, if multiple volumes are loaded, the rendering loop is not optimal when only some volumes are visible since it loops on the lowest level. instead we should loop only through toplevel objects and only if the toplevel object is visible, access the children.
right now, if multiple volumes are loaded, the rendering loop is not optimal when only some volumes are visible since it loops on the lowest level. instead we should loop only through toplevel objects and only if the toplevel object is visible, access the children.