这里的“var x_max = d_width-b_width-step; 该咋解释啊？【求大神指教】

Question

window.onload=function(){
  document.body.onkeyup=function(evt){
   var step=20;
   var box=document.getElementById("box");
   var img=document.getElementById("img");
   
   var left = parseInt(img.style.left);
   /*获得图片左边距的数值*/
   var  top = parseInt(img.style.top);
   /*获得图片高的数值*/
   
   var d_width = box.offsetWidth;
   var d_height = box.offsetHeight;
   /*获得box控件的的绝对（宽度，高度）*/
   
   var b_width = img.width;
   var b_height = img.height;
   /*获得图片的（宽度，高度）*/
   
   var x_max = d_width-b_width-step;
   var y_max = d_height-b_height-step;
   /**/
   var e = evt ? evt: window.event;
   if(e.keyCode == 37 && left >=step){
     img.style.left = left-step+"px";
   }
   if(e.keyCode == 38 && top >=step){
     img.style.top = top-step+"px";
   }
   if(e.keyCode == 39 && left<= x_max){
   img.style.left = left+step+"px";
   } 
   if(e.keyCode == 40 && top <=y_max){
     img.style.top = top+step+"px";
   }
  }
}

Answer 1

这是自定义的偏移，step都没说是干什么用的