碰撞检测（九宫格法）

两个元素如图所示：

                var L = ev.clientX - disX;
                var T = ev.clientY - disY;
                
                var L1 = L;
                var R1 = L + obj.offsetWidth;
                var T1 = T;
                var B1 = T + obj.offsetHeight;
                
                var L2 = oImg.offsetLeft;
                var R2 = L2 + oImg.offsetWidth;
                var T2 = oImg.offsetTop;
                var B2 = T2 + oImg.offsetHeight;
                
                if ( R1 < L2 || L1 > R2 || B1 < T2 || T1 > B2 ) {
                    oImg.src = '1.jpg';
                } else {
                    oImg.src = '2.jpg';
                }