//對lod的原始碼進行了註釋
/**
* @author mikael emtinger / http://gomo.se/
* @author alteredq / http://alteredqualia.com/
* @author mrdoob / http://mrdoob.com/
*/
//  目錄src/objects/LOD.js
<script>
	THREE.LOD = function () 
	{
		THREE.Object3D.call( this );
		this.objects = [];
	};


	THREE.LOD.prototype = Object.create( THREE.Object3D.prototype );
	//新增細節
	THREE.LOD.prototype.addLevel = function ( object, distance ) 
	{
		if ( distance === undefined ) 
			distance = 0;//未定義距離時為0
		distance = Math.abs( distance );//取距離的絕對值
		//將各個細節的距離進行從小到大的排序
		for ( var l = 0; l < this.objects.length; l ++ ) 
		{
			if ( distance < this.objects[ l ].distance ) 
			{
				break;
			}
		}

		this.objects.splice( l, 0, { distance: distance, object: object } );//?objects.splice
		this.add( object );

	};
	//根據距離返回對應層次的物件
	THREE.LOD.prototype.getObjectForDistance = function ( distance ) 
	{
		//排序由小到大遍歷
		for ( var i = 1, l = this.objects.length; i < l; i ++ )
		{
			if ( distance < this.objects[ i ].distance )
			{

				break;
			}
		}
		return this.objects[ i - 1 ].object;
	};
	//更新顯示的層次
	THREE.LOD.prototype.update = function () 
	{
	
		var v1 = new THREE.Vector3();//點1
		var v2 = new THREE.Vector3();//點2
		return function ( camera ) 
		{
			//當細節層次不為1時
			if ( this.objects.length > 1 ) 
			{
				v1.setFromMatrixPosition( camera.matrixWorld );//相機的矩陣
				v2.setFromMatrixPosition( this.matrixWorld );//LOD的矩陣
				var distance = v1.distanceTo( v2 );//相機到LOD的距離
				this.objects[ 0 ].object.visible = true;//第一層（細節最詳細）的可見性為true
				//向細節不詳細的層次距離遍歷
				for ( var i = 1, l = this.objects.length; i < l; i ++ ) 
				{
					//距離小的詳細程度高的可見度為false
					if ( distance >= this.objects[ i ].distance ) 
					{
						this.objects[ i - 1 ].object.visible = false;
						this.objects[ i ].object.visible = true;
					} 
					//到達對應的詳細程度的距離區間後跳出
					else 
					{
						break;
					}
				}	
				//對應級別向後的可見度為false
				for( ; i < l; i ++ ) 
				{
					this.objects[ i ].object.visible = false;
				}
			}
		};
	}();

	THREE.LOD.prototype.clone = function ( object ) 
	{	
		//未定義時宣告lod
		if ( object === undefined ) 
			object = new THREE.LOD();
		//將LOD複製到object中
		THREE.Object3D.prototype.clone.call( this, object); 
		//依次新增每個細節到object中
		for ( var i = 0, l = this.objects.length; i < l; i ++ )
		{
			var x = this.objects[i].object.clone();
			x.visible = i === 0;
			object.addLevel( x, this.objects[i].distance );
		}
		return object;
	};
</script>