碰撞检测

HTML

<div id="box"></div>
<img src="img/2.jpg"/>

CSS

#box,img{
	 100px;
	height: 100px;
	background: red;
	position: absolute;
}
#box{
	left: 300px;
	top: 200px;
}

JS

var Box=document.getElementById("box");
var oImg=document.getElementsByTagName("img")[0];



oImg.onmousedown=function(ev){
	var ev=ev||event;
		
		
	var disX=ev.clientX-this.offsetLeft;
	var disY=ev.clientY-this.offsetTop;
		
	//设置全局捕获
	//所有的onmousemove或者onmouseup都被oImg截获，然后作用在oImg身上
	if(oImg.setCapture){
		oImg.setCapture();
	}
		
	document.onmousemove=function(ev){
		var ev=ev||event;
			
		var L=ev.clientX-disX;
		var T=ev.clientY-disY;
			
		//限制拖拽范围
		//横向
		if(L<0){
			L=0;
		}else if(L>document.documentElement.clientWidth-oImg.offsetWidth){
			L=document.documentElement.clientWidth-oImg.offsetWidth;
		}
		//纵向
		if(T<0){
			T=0;
		}else if(T>document.documentElement.clientHeight-oImg.offsetHeight){
			T=document.documentElement.clientHeight-oImg.offsetHeight;
		}
		
		
		//碰撞检测
		var L1=oImg.offsetLeft;
		var B1=oImg.offsetLeft+oImg.clientWidth;
		var T1=oImg.offsetTop;
		var B2=oImg.offsetTop+oImg.clientHeight;
		
		var L2=Box.offsetLeft;
		var B3=Box.offsetLeft+Box.clientWidth;
		var T2=Box.offsetTop;
		var B4=Box.offsetTop+Box.clientHeight;
		
		if(B1<L2||B3<L1||B2<T2||B4<T1){
			Box.style.background="red";
		}else{
			Box.style.background="green";
		}
			
		oImg.style.left=L+'px';
		oImg.style.top=T+'px';
	}
		
	document.onmouseup=function(){
		document.onmousemove=null;
		//释放全局捕获
		if(oImg.releaseCapture){
			oImg.releaseCapture();
		}
	}
		
	//清除默认事件
	return false;
}