不理解，请解答！！！

window.onload=function(){
               document.onmousemove=function(ev){
                   var oevent=ev||event;    这是什么意思？
                   var aimg=document.getElementsByTagName('img');
                   var odiv=document.getElementById('wrapper');
                   for(var i=0;i<aimg.length;i++){
                          var x=aimg[i].offsetLeft+aimg[i].offsetWidth/2;
                          var y=aimg[i].offsetTop+odiv.offsetTop+aimg[i].offsetHeight/2;
                          var a=x-oevent.clientX;
                          var b=y-oevent.clientY;
                          var dis=parseInt(Math.sqrt(a*a+b*b));
                          var scale=1-dis/200;
                        if(scale<0.5){
                            scale=0.5;
                        }
                        aimg[i].width=scale*128;
            
                   }
               }
            }