three.js (四)离散层次细节level of details

LOD 处理比较大的外部地面场景中比较有用， 一般用于绘制地形。 首先通过可视体的切割删除不用的地形块，接着通过LOD 对照相机不同距离的地形块进行层次细节调整。
这里采用最简单的LOD 方法。

首先地形有n*n 的块构成， 这些块共同构成一个大平面；
首先根据每个块到照相机的距离 计算细节层次，
例如假设由5*5 个块构成地形， 每个块1*1大小， 有4个细节层次，
当块距离照相机 小于2 层次 0
距离小于4 层次 1
小于6 层次 2
其它层次 3

首先构造一个Object3D 作为整个地面的代表。

myGame.Earth = function(){
THREE.Object3D.call(this);
this.curPatches = [];
this.patches = [];
this.width = 5;
this.height = 5;
this.patch_width = 1;
this.patch_height = 1;
for(var i = 0; i < this.height; i++)
{
for(var j = 0; j < this.width; j++)
{
this.patches.push(1);//distance ---> detail 0 1*1
}
}
};

myGame.Earth.prototype = new THREE.Object3D();

var earth = new myGame.Earth();

earth中的每一个块是 一个Mesh 对象，底层的几何体是一个PlaneGeometry

curPatches 用于存放当前组成earth的块， 当细节层次需要改变的时候这些块将被丢弃，而重新构造新的块。
patches用于存储当前块的细节层次。

关键的setDetails 函数用于调节块的细节， 首先计算所有块到照相机的距离 得到细节层次； 接着删除旧的所有平面， 接着再构建新的块加入到场景中。

myGame.Earth.prototype.setDetails = function(camera){
var diff = new THREE.Vector3();
var standard = this.patch_width;
var pos = new THREE.Vector3();
for(var i = 0; i < this.height; i++)
{
for(var j = 0; j < this.width; j++)
{
pos.set(-this.width/2*this.patch_width+j*this.patch_width+this.patch_width/2,
0,
-this.height/2*this.patch_height+i*this.patch_height+this.patch_height/2);

var dist = diff.sub(camera.position, pos).length();
if(dist < 2*standard)
this.patches[i*this.width+j] = 0;
else if(dist < 4*standard)
this.patches[i*this.width+j] = 1;
else if(dist < 6*standard)
this.patches[i*this.width+j] = 2;
else
this.patches[i*this.width+j] = 3;
}
}
for(var i = 0; i < this.curPatches.length; i++)
{
this.remove(this.curPatches[i]);
}
var mat = new THREE.MeshBasicMaterial({color:0xff0000, wireframe:true});
for(var i = 0; i < this.patches.length; i++)
{
var pl;
var detail = this.patches[i];
console.log(detail);
if(detail == 0)
pl = new THREE.PlaneGeometry(this.patch_width, this.patch_height, 10, 10);
else if(detail == 1)
pl = new THREE.PlaneGeometry(this.patch_width, this.patch_height, 5, 5);
else if(detail == 2)
pl = new THREE.PlaneGeometry(this.patch_width, this.patch_height, 2, 2);
else
pl = new THREE.PlaneGeometry(this.patch_width, this.patch_height, 1, 1);
var obj = new THREE.Mesh(pl, mat);
obj.position.set(-this.width/2*this.patch_width+i%this.width*this.patch_width+this.patch_width/2,
-this.height/2*this.patch_height+ (this.height-~~(i/this.width))*this.patch_height+this.patch_height/2,
0);
this.curPatches.push(obj);
this.add(obj);
}
};

在每帧更新的时候， 通过检测照相机新旧位置的距离差， 如果足够大 则更新整个场景的块。

function animate()
{
requestAnimationFrame(animate);
controls.update(clock.getDelta());
var vec = new THREE.Vector3();
var dist = vec.sub(camera.position, camera.oldPosition).length();
if(dist > earth.patch_width)
{
camera.oldPosition.copy(camera.position);
earth.setDetails(camera);
}
render();
}