在上一篇里，我們學(xué)習(xí)了“自主角色”的一些基本行為：尋找(seek)、避開(flee)、到達(dá)(arrive)、追捕(pursue)、躲避(evade)、漫游(wander)。這一篇將繼續(xù)學(xué)習(xí)其它更復(fù)雜，更高級(jí)的行為。

一、對(duì)象回避(object avoidance)

對(duì)象回避的正式解釋為：角色預(yù)測(cè)出對(duì)象的行動(dòng)路徑，然后避開他們。

也可以通俗的描述為：假如有一個(gè)"灰太狼抓喜羊羊"的游戲場(chǎng)景，“喜羊羊"在草地上四處游蕩的時(shí)候要盡量避免被隨處找羊的"灰太狼"抓住。好象聽起來并不復(fù)雜，跟前面提到的"避開(flee)"或"躲避(evade)"甚至第一章提到的碰撞檢測(cè)差不多，只要調(diào)頭走開、改變路線甚至檢測(cè)這二者是否發(fā)生碰撞即可。

但如果仔細(xì)考慮的話，這個(gè)并不象上面想的這么簡(jiǎn)單，如果“羊”跟“狼”的距離足夠遠(yuǎn)(或者“狼”運(yùn)動(dòng)得太慢)，以至于“狼”在預(yù)測(cè)的時(shí)間范圍內(nèi)根本不可能抓住“羊”，那么“羊”可能根本無需調(diào)整方向（或僅需做小的調(diào)整）；如果“狼”突然出現(xiàn)在離“羊”很近的地方，“羊”做出的反應(yīng)是急轉(zhuǎn)90度，換個(gè)方向跑開。（問：為什么不是轉(zhuǎn)180度反方向跑呢？答：如果大家經(jīng)常看動(dòng)物世界里非洲草原上“獵豹追羚羊"的片段，應(yīng)該就能理解了，大多數(shù)情況下，急轉(zhuǎn)彎比反向跑，更能有效避開覓食者）另外，該行為的另一個(gè)特征是預(yù)測(cè)可能要發(fā)生的碰撞，而非實(shí)際發(fā)生的碰撞，所以碰撞檢測(cè)也不太適合。

ok，直接看算法示意圖吧：

首先把目標(biāo)（障礙）物體認(rèn)為是一個(gè)有半徑范圍的圓形對(duì)象(當(dāng)然這是一種簡(jiǎn)化問題的理想模型)；

然后得出自己與目標(biāo)的距離向量difference；

接下來將自身的速度向量單位化，得到單位向量header（即長(zhǎng)度為1，且與速度同方向的向量）；

計(jì)算header與difference的點(diǎn)積(也叫點(diǎn)乘、內(nèi)積、標(biāo)量積)，得到一個(gè)值dotProd = |header| * |difference| * cos(θ) = |difference| * cos(θ) (注：header為單位向量，大小為1，所以可省去)，很明顯如果該值小于0，則表示障礙物就在前方，準(zhǔn)備做調(diào)整；

將header放大一個(gè)系數(shù)，模擬出自身的觸角feeler(相當(dāng)于物體自身向前方伸出去一個(gè)觸須試探試探)；

將difference投影在feeler上，得到向量projection，并進(jìn)一步得到距離dist(即projection末端與difference末端的距離)；

如果dist小于圓半徑，且projection長(zhǎng)度小于feeler的長(zhǎng)度（即觸角碰到了目標(biāo)了），則轉(zhuǎn)90度逃開；

private var _avoidDistance:Number=300;//發(fā)現(xiàn)障礙物的有效視野 private var _avoidBuffer:Number=20;//機(jī)車在準(zhǔn)備避開時(shí)，自身和障礙物間的預(yù)留距離。public function set avoidDistance(value:Number):void {_avoidDistance=value; } public function get avoidDistance():Number {return _avoidDistance; }public function set avoidBuffer(value:Number):void {_avoidBuffer=value; } public function get avoidBuffer():Number {return _avoidBuffer; }//對(duì)象回避 public function avoid(circles: Array):void {for (var i: int=0; i < circles.length; i++) {var circle:Circle=circles[i] as Circle;var heading:Vector2D=_velocity.clone().normalize();// 障礙物和機(jī)車間的位移向量var difference:Vector2D=circle.position.subtract(_position);var dotProd:Number=difference.dotProd(heading);// 如果障礙物在機(jī)車前方if (dotProd>0) {// 機(jī)車的“觸角”var feeler:Vector2D=heading.multiply(_avoidDistance);// 位移在觸角上的映射var projection:Vector2D=heading.multiply(dotProd);// 障礙物離觸角的距離var dist:Number=projection.subtract(difference).length;// 如果觸角（在算上緩沖后）和障礙物相交// 并且位移的映射的長(zhǎng)度小于觸角的長(zhǎng)度// 我們就說碰撞將要發(fā)生，需改變轉(zhuǎn)向if (dist < circle.radius + _avoidBuffer && projection.length < feeler.length) {// 計(jì)算出一個(gè)轉(zhuǎn)90度的力var force:Vector2D=heading.multiply(_maxSpeed);force.angle+=difference.sign(_velocity)*Math.PI/2;// 通過離障礙物的距離，調(diào)整力度大小，使之足夠小但又能避開force=force.multiply(1.0-projection.length/feeler.length);// 疊加于轉(zhuǎn)向力上_steeringForce=_steeringForce.add(force);// 剎車——轉(zhuǎn)彎的時(shí)候要放慢機(jī)車速度，離障礙物越接近，剎車越狠。_velocity=_velocity.multiply(projection.length/feeler.length);}}} }

將以上代碼加入SteeredVehicle.as，當(dāng)然，在測(cè)試前還要有一個(gè)Circle類來模擬障礙物

package {import flash.display.Sprite;public class Circle extends Sprite {private var _radius:Number;private var _color:uint;private var _vx:Number;private var _vy:Number;public function Circle(radius:Number, color:uint = 0x000000) {_radius=radius;_color=color;graphics.lineStyle(0, _color);graphics.beginFill(_color);graphics.drawCircle(0, 0, _radius);graphics.endFill();}public function get radius():Number {return _radius;}public function get position():Vector2D {return new Vector2D(x, y);}public function get vx():Number {return _vx;}public function set vx(value:Number):void {_vx = value;}public function get vy():Number {return _vy;}public function set vy(value:Number):void {_vy = value;}} }

測(cè)試：

package {import flash.display.Sprite;import flash.display.StageAlign;import flash.display.StageScaleMode;import flash.events.Event;public class AvoidTest extends Sprite {private var _vehicle:SteeredVehicle;private var _circles:Array;private var _numCircles:int=5;public function AvoidTest():void {stage.align=StageAlign.TOP_LEFT;stage.scaleMode=StageScaleMode.NO_SCALE;_vehicle = new SteeredVehicle(0xff0000);_vehicle.edgeBehavior=Vehicle.BOUNCE;addChild(_vehicle);//初始化障礙物_circles = new Array();for (var i:int = 0; i < _numCircles; i++) {var c:Circle=new Circle(Math.random()*30+15,0x0000ff);c.x=Math.random()*stage.stageWidth;c.y=Math.random()*stage.stageHeight;c.vx=Math.random()-0.5;c.vy=Math.random()-0.5;if (c.x<c.radius) {c.x=c.radius;} else if (c.x>stage.stageWidth-c.radius) {c.x=stage.stageWidth-c.radius;}if (c.y<c.radius) {c.y=c.radius;} else if (c.y>stage.stageHeight-c.radius) {c.y=stage.stageHeight-c.radius;}addChild(c);_circles.push(c);}addEventListener(Event.ENTER_FRAME, onEnterFrame);}private function onEnterFrame(e:Event):void { _vehicle.wander();//處理障礙物的運(yùn)動(dòng)以及邊界反彈for (var i:int = 0; i < _numCircles; i++) {var c:Circle=_circles[i] as Circle;c.x+=c.vx;c.y+=c.vy;if (c.x<c.radius) {c.x=c.radius;c.vx*=-1;} else if (c.x>stage.stageWidth-c.radius) {c.x=stage.stageWidth-c.radius;c.vx*=-1;}if (c.y<c.radius) {c.y=c.radius;c.vy*=-1;} else if (c.y>stage.stageHeight-c.radius) {c.y=stage.stageHeight-c.radius;c.vy*=-1;}}_vehicle.avoid(_circles);_vehicle.update();}} }

二、路徑跟隨(path following)

對(duì)于玩過星際之類游戲的朋友們，這種行為應(yīng)該最熟悉了。隨便選一個(gè)神族的狂熱者(俗稱叉叉兵)，然后在幾個(gè)指定的位置點(diǎn)擊一下，它們就會(huì)沿著指定的位置來回巡邏。這就是路徑跟隨：角色盡可能的沿著指定的路徑移動(dòng)。

在算法上的處理很簡(jiǎn)單：用數(shù)組保存一組位置(每個(gè)位置其實(shí)就是一個(gè)Vector2D對(duì)象)，然后加一個(gè)索引變量(指針)，用于指示當(dāng)前移動(dòng)到了哪一個(gè)位置，最終將機(jī)車以seek行為移動(dòng)以下一個(gè)位置。

但有一個(gè)要注意的細(xì)節(jié)：seek行為會(huì)讓機(jī)車最終在目標(biāo)位置來回反復(fù)運(yùn)動(dòng)停不下來，為了讓代碼能知道機(jī)車已經(jīng)經(jīng)過了當(dāng)前位置，接下來應(yīng)該去下一個(gè)位置，需要引入一個(gè)距離閾值，用于判斷機(jī)車是否已經(jīng)接近目標(biāo)點(diǎn)。

private var _pathIndex:int=0;//路徑索引private var _pathThreshold:Number=20;//路徑跟隨中的距離閾值public function set pathIndex(value:int):void {_pathIndex=value;}public function get pathIndex():int {return _pathIndex;}public function set pathThreshold(value:Number):void {_pathThreshold=value;}public function get pathThreshold():Number {return _pathThreshold;}public function set avoidDistance(value:Number):void {_avoidDistance=value;}public function get avoidDistance():Number {return _avoidDistance;}public function set avoidBuffer(value:Number):void {_avoidBuffer=value;}public function get avoidBuffer():Number {return _avoidBuffer;}//路徑跟隨public function followPath(path:Array,loop:Boolean=false):void {var wayPoint:Vector2D=path[_pathIndex];if (wayPoint==null) {return;}if (_position.dist(wayPoint)<_pathThreshold) {if (_pathIndex>=path.length-1) {if (loop) {_pathIndex=0;}} else {_pathIndex++;}}if (_pathIndex>=path.length-1&&! loop) {arrive(wayPoint);} else {seek(wayPoint);}}

測(cè)試：

package {import flash.display.Sprite;import flash.display.StageAlign;import flash.display.StageScaleMode;import flash.events.Event;import flash.events.MouseEvent;public class PathTest extends Sprite {private var _vehicle:SteeredVehicle;private var _path:Array;public function PathTest() {stage.align=StageAlign.TOP_LEFT;stage.scaleMode=StageScaleMode.NO_SCALE;_vehicle=new SteeredVehicle ; addChild(_vehicle);_path=new Array ;stage.addEventListener(MouseEvent.CLICK,onClick);addEventListener(Event.ENTER_FRAME,onEnterFrame);}private function onEnterFrame(event:Event):void {_vehicle.followPath(_path,true);_vehicle.update();}private function onClick(event:MouseEvent):void {graphics.lineStyle(0,0,.25);if (_path.length==0) {graphics.moveTo(mouseX,mouseY);}graphics.lineTo(mouseX,mouseY);graphics.drawCircle(mouseX,mouseY,10);graphics.moveTo(mouseX,mouseY);_path.push(new Vector2D(mouseX,mouseY));}} }

拿鼠標(biāo)在上面隨便點(diǎn)一下，就能看到效果了

三、群落(flock)行為

群落行為是指類似鳥群這樣的復(fù)合行為。它由三個(gè)子行為組成：

分離(separation)：鳥群中每個(gè)角色都試著和相鄰角色保持一定的距離(即：一只鳥與其它鳥太靠近時(shí)，主動(dòng)退讓一定的距離，以避免碰到)
凝聚(cohesion)：每個(gè)角色盡量不掉隊(duì)，不落下太遠(yuǎn)(即：盡量向鳥群靠攏)
隊(duì)列(alignment)：每個(gè)角色盡可能與相鄰角色行動(dòng)于同一方向（即：每只鳥的速度方向可能不完全相同，但大致跟隊(duì)伍的總體方向一致）

借用一句李建忠老師名言：原代碼就是最好的設(shè)計(jì)！ 直接上代碼吧：

private var _inSightDist:Number=200;//視野距離private var _tooCloseDist:Number=60;//防止群落靠得太近的安全距離//群落行為public function flock(vehicles:Array):void {var averageVelocity:Vector2D=_velocity.clone();//平均速度變量var averagePosition:Vector2D=new Vector2D ;//平均位置變量var inSightCount:int=0;//在視野中的機(jī)車數(shù)量for (var i:int=0; i<vehicles.length; i++) {var vehicle:Vehicle=vehicles[i] as Vehicle;if (vehicle!=this&&inSight(vehicle)) { //如果其它機(jī)車在視野中//累加速度與位置averageVelocity=averageVelocity.add(vehicle.velocity);averagePosition=averagePosition.add(vehicle.position);//如果其它機(jī)車太靠近，則避開(即分離行為[separation]的體現(xiàn))if (tooClose(vehicle)) {flee(vehicle.position);}inSightCount++; //累加在視野中的機(jī)車數(shù)}}if (inSightCount>0) {//計(jì)算平均位置averageVelocity=averageVelocity.divide(inSightCount);averagePosition=averagePosition.divide(inSightCount);seek(averagePosition);//向中心位置靠攏(即凝聚行為[cohesion]的體現(xiàn))_steeringForce.add(averageVelocity.subtract(_velocity));//根據(jù)平均速度校準(zhǔn)自身速度(即隊(duì)列[alignment]行為的體現(xiàn))}}public function set inSightDist(vaule:Number):void {_inSightDist=vaule;}public function get inSightDist():Number {return _inSightDist;}public function set tooCloseDist(value:Number):void {_tooCloseDist=value;}public function get tooCloseDist():Number {return _tooCloseDist;}//判斷(身后的其它)機(jī)車是否在視野范圍內(nèi)public function inSight(vehicle:Vehicle):Boolean {if (_position.dist(vehicle.position)>_inSightDist) {return false;}//---->start 下面這一段代碼甚至去掉也行，不過去掉后，群落的行為將有所不同var heading:Vector2D=_velocity.clone().normalize();var difference:Vector2D=vehicle.position.subtract(_position); var dotProd:Number=difference.dotProd(heading);if (dotProd<0) {return false;}//<-----endreturn true;}public function tooClose(vehicle:Vehicle):Boolean {return _position.dist(vehicle.position)<_tooCloseDist;}

重點(diǎn)關(guān)注下inSight方法，它直接影響到群落的行為，示意圖如下：

先檢測(cè)二只鳥的距離是否足夠近，然后僅關(guān)注身后的其它鳥。

測(cè)試代碼：

package {import flash.display.Sprite;import flash.display.StageAlign;import flash.display.StageScaleMode;import flash.events.Event;public class FlockTest extends Sprite {private var _vehicles:Array;private var _numVehicles:int=20;public function FlockTest() {stage.align=StageAlign.TOP_LEFT;stage.scaleMode=StageScaleMode.NO_SCALE;_vehicles=new Array ;for (var i:int=0; i<_numVehicles; i++) {var vehicle:SteeredVehicle=new SteeredVehicle(Math.random()*0xffffff);vehicle.position=new Vector2D(Math.random()*stage.stageWidth,Math.random()*stage.stageHeight);vehicle.velocity=new Vector2D(Math.random()*20-10,Math.random()*20-10);vehicle.edgeBehavior=Vehicle.BOUNCE;_vehicles.push(vehicle);addChild(vehicle);}addEventListener(Event.ENTER_FRAME,onEnterFrame);}private function onEnterFrame(event:Event):void {for (var i:int=0; i<_numVehicles; i++) {_vehicles[i].flock(_vehicles);_vehicles[i].update();}}} }

如果把inSight中檢測(cè)其它鳥是否在身后的代碼去掉，即簡(jiǎn)化成：

public function inSight(vehicle:Vehicle):Boolean {if (_position.dist(vehicle.position)>_inSightDist) {return false;} return true;}

預(yù)測(cè)一下最終的效果：這樣相當(dāng)于只要距離小于閾值的其它鳥，其速度和位置都會(huì)被計(jì)算在內(nèi)，最終整個(gè)群落將始終聚集在一定的范圍內(nèi)，不會(huì)發(fā)生分離，從而體現(xiàn)出了另外一種群落效果。

?最后，給出Vehicle.as及SteeredVehicle.as的完整代碼

package {import flash.display.Sprite;public class Vehicle extends Sprite {//邊界行為:是屏幕環(huán)繞(wrap)，還是反彈{bounce}protected var _edgeBehavior:String=WRAP;//質(zhì)量protected var _mass:Number=1.0;//最大速度protected var _maxSpeed:Number=10;//坐標(biāo)protected var _position:Vector2D;//速度protected var _velocity:Vector2D;//邊界行為常量public static const WRAP:String="wrap";public static const BOUNCE:String="bounce";public function Vehicle(color:uint=0xffffff) {_position=new Vector2D ;_velocity=new Vector2D ;draw(color);}protected function draw(color:uint=0xffffff):void {graphics.clear();graphics.lineStyle(0);graphics.beginFill(color);graphics.moveTo(10,0);graphics.lineTo(-10,5);graphics.lineTo(-10,-5);graphics.lineTo(10,0);graphics.endFill();}public function update():void {//設(shè)置最大速度_velocity.truncate(_maxSpeed);//根據(jù)速度更新坐標(biāo)向量_position=_position.add(_velocity); //處理邊界行為if (_edgeBehavior==WRAP) {wrap();} else if (_edgeBehavior==BOUNCE) {bounce();}//更新x,y坐標(biāo)值x=position.x;y=position.y;//處理旋轉(zhuǎn)角度rotation=_velocity.angle*180/Math.PI;}//反彈private function bounce():void {if (stage!=null) {if (position.x>stage.stageWidth) {position.x=stage.stageWidth;velocity.x*=-1;} else if (position.x<0) {position.x=0;velocity.x*=-1;}if (position.y>stage.stageHeight) {position.y=stage.stageHeight;velocity.y*=-1;} else if (position.y<0) {position.y=0;velocity.y*=-1;}}}//屏幕環(huán)繞private function wrap():void {if (stage!=null) {if (position.x>stage.stageWidth) {position.x=0;}if (position.x<0) {position.x=stage.stageWidth;}if (position.y>stage.stageHeight) {position.y=0;}if (position.y<0) {position.y=stage.stageHeight;}} }//下面的都是屬性定義public function set edgeBehavior(value:String):void {_edgeBehavior=value;}public function get edgeBehavior():String {return _edgeBehavior;}public function set mass(value:Number):void {_mass=value;}public function get mass():Number {return _mass;}public function set maxSpeed(value:Number):void {_maxSpeed=value;}public function get maxSpeed():Number {return _maxSpeed;}public function set position(value:Vector2D):void {_position=value;x=_position.x;y=_position.y;}public function get position():Vector2D {return _position;}public function set velocity(value:Vector2D):void {_velocity=value;}public function get velocity():Vector2D {return _velocity;}override public function set x(value:Number):void {super.x=value;_position.x=x;}override public function set y(value:Number):void {super.y=value;_position.y=y;}} } package {import flash.display.Sprite;//(具有)轉(zhuǎn)向(行為的)機(jī)車public class SteeredVehicle extends Vehicle {private var _maxForce:Number=1;//最大轉(zhuǎn)向力private var _steeringForce:Vector2D;//轉(zhuǎn)向速度private var _arrivalThreshold:Number=100;//到達(dá)行為的距離閾值(小于這個(gè)距離將減速)private var _wanderAngle:Number=0;private var _wanderDistance:Number=10;private var _wanderRadius:Number=5;private var _wanderRange:Number=1;private var _avoidDistance:Number=300;//發(fā)現(xiàn)障礙物的有效視野private var _avoidBuffer:Number=20;//機(jī)車在準(zhǔn)備避開時(shí)，自身和障礙物間的預(yù)留距離。private var _pathIndex:int=0;//路徑索引private var _pathThreshold:Number=20;//路徑跟隨中的距離閾值private var _inSightDist:Number=200;//視野距離private var _tooCloseDist:Number=60;//防止群落靠得太近的安全距離//群落行為public function flock(vehicles:Array):void {var averageVelocity:Vector2D=_velocity.clone();//平均速度變量var averagePosition:Vector2D=new Vector2D ;//平均位置變量var inSightCount:int=0;//在視野中的機(jī)車數(shù)量for (var i:int=0; i<vehicles.length; i++) {var vehicle:Vehicle=vehicles[i] as Vehicle;if (vehicle!=this&&inSight(vehicle)) { //如果其它機(jī)車在視野中//累加速度與位置averageVelocity=averageVelocity.add(vehicle.velocity);averagePosition=averagePosition.add(vehicle.position);//如果其它機(jī)車太靠近，則避開(即分離行為[separation]的體現(xiàn))if (tooClose(vehicle)) {flee(vehicle.position);}inSightCount++; //累加在視野中的機(jī)車數(shù)}}if (inSightCount>0) {//計(jì)算平均位置averageVelocity=averageVelocity.divide(inSightCount);averagePosition=averagePosition.divide(inSightCount);seek(averagePosition);//向中心位置靠攏(即凝聚行為[cohesion]的體現(xiàn))_steeringForce.add(averageVelocity.subtract(_velocity));//根據(jù)平均速度校準(zhǔn)自身速度(即隊(duì)列[alignment]行為的體現(xiàn))}}public function set inSightDist(vaule:Number):void {_inSightDist=vaule;}public function get inSightDist():Number {return _inSightDist;}public function set tooCloseDist(value:Number):void {_tooCloseDist=value;}public function get tooCloseDist():Number {return _tooCloseDist;}//判斷(身后的其它)機(jī)車是否在視野范圍內(nèi)public function inSight(vehicle:Vehicle):Boolean {if (_position.dist(vehicle.position)>_inSightDist) {return false;}//---->start 下面這一段代碼甚至去掉也行，不過去掉后，群落的行為將有所不同var heading:Vector2D=_velocity.clone().normalize();var difference:Vector2D=vehicle.position.subtract(_position); var dotProd:Number=difference.dotProd(heading);if (dotProd<0) {return false;}//<-----endreturn true;}public function tooClose(vehicle:Vehicle):Boolean {return _position.dist(vehicle.position)<_tooCloseDist;}public function set pathIndex(value:int):void {_pathIndex=value;}public function get pathIndex():int {return _pathIndex;}public function set pathThreshold(value:Number):void {_pathThreshold=value;}public function get pathThreshold():Number {return _pathThreshold;}public function set avoidDistance(value:Number):void {_avoidDistance=value;}public function get avoidDistance():Number {return _avoidDistance;}public function set avoidBuffer(value:Number):void {_avoidBuffer=value;}public function get avoidBuffer():Number {return _avoidBuffer;}//路徑跟隨public function followPath(path:Array,loop:Boolean=false):void {var wayPoint:Vector2D=path[_pathIndex];if (wayPoint==null) {return;}if (_position.dist(wayPoint)<_pathThreshold) {if (_pathIndex>=path.length-1) {if (loop) {_pathIndex=0;}} else {_pathIndex++;}}if (_pathIndex>=path.length-1&&! loop) {arrive(wayPoint);} else {seek(wayPoint);}}//對(duì)象回避public function avoid(circles:Array):void {for (var i:int=0; i<circles.length; i++) {var circle:Circle=circles[i] as Circle;var heading:Vector2D=_velocity.clone().normalize();// 障礙物和機(jī)車間的位移向量var difference:Vector2D=circle.position.subtract(_position);var dotProd:Number=difference.dotProd(heading);// 如果障礙物在機(jī)車前方if (dotProd>0) {// 機(jī)車的“觸角”var feeler:Vector2D=heading.multiply(_avoidDistance);// 位移在觸角上的映射var projection:Vector2D=heading.multiply(dotProd);// 障礙物離觸角的距離var dist:Number=projection.subtract(difference).length;// 如果觸角（在算上緩沖后）和障礙物相交// 并且位移的映射的長(zhǎng)度小于觸角的長(zhǎng)度// 我們就說碰撞將要發(fā)生，需改變轉(zhuǎn)向if (dist<circle.radius+_avoidBuffer&&projection.length<feeler.length) {// 計(jì)算出一個(gè)轉(zhuǎn)90度的力var force:Vector2D=heading.multiply(_maxSpeed);force.angle+=difference.sign(_velocity)*Math.PI/2;// 通過離障礙物的距離，調(diào)整力度大小，使之足夠小但又能避開force=force.multiply(1.0-projection.length/feeler.length);// 疊加于轉(zhuǎn)向力上_steeringForce=_steeringForce.add(force);// 剎車——轉(zhuǎn)彎的時(shí)候要放慢機(jī)車速度，離障礙物越接近，剎車越狠。_velocity=_velocity.multiply(projection.length/feeler.length);}}}}//漫游public function wander():void {var center:Vector2D=velocity.clone().normalize().multiply(_wanderDistance);var offset:Vector2D=new Vector2D(0);offset.length=_wanderRadius;offset.angle=_wanderAngle;_wanderAngle+=Math.random()-0.5*_wanderRange;var force:Vector2D=center.add(offset);_steeringForce=_steeringForce.add(force);}public function set wanderDistance(value:Number):void {_wanderDistance=value;}public function get wanderDistance():Number {return _wanderDistance;}public function set wanderRadius(value:Number):void {_wanderRadius=value;}public function get wanderRadius():Number {return _wanderRadius;}public function set wanderRange(value:Number):void {_wanderRange=value;}public function get wanderRange():Number {return _wanderRange;}public function set arriveThreshold(value:Number):void {_arrivalThreshold=value;}public function get arriveThreshold():Number {return _arrivalThreshold;}//構(gòu)造函數(shù)public function SteeredVehicle(color:uint=0xffffff) {_steeringForce=new Vector2D ;super(color);}public function set maxForce(value:Number):void {_maxForce=value;}public function get maxForce():Number {return _maxForce;}//尋找(Seek)行為public function seek(target:Vector2D):void {var desiredVelocity:Vector2D=target.subtract(_position);desiredVelocity.normalize();desiredVelocity=desiredVelocity.multiply(_maxSpeed);//注：這里的_maxSpeed是從父類繼承得來的var force:Vector2D=desiredVelocity.subtract(_velocity);_steeringForce=_steeringForce.add(force);}//避開(flee)行為public function flee(target:Vector2D):void {var desiredVelocity:Vector2D=target.subtract(_position);desiredVelocity.normalize();desiredVelocity=desiredVelocity.multiply(_maxSpeed);var force:Vector2D=desiredVelocity.subtract(_velocity);_steeringForce=_steeringForce.subtract(force);//這是唯一也seek行為不同的地方，一句話解釋：既然發(fā)現(xiàn)了目標(biāo)，那就調(diào)頭就跑吧！}//到達(dá)(arrive)行為public function arrive(target:Vector2D):void {var desiredVelocity:Vector2D=target.subtract(_position);desiredVelocity.normalize();var dist:Number=_position.dist(target);if (dist>_arrivalThreshold) {desiredVelocity=desiredVelocity.multiply(_maxSpeed);} else {desiredVelocity=desiredVelocity.multiply(_maxSpeed*dist/_arrivalThreshold);}var force:Vector2D=desiredVelocity.subtract(_velocity);_steeringForce=_steeringForce.add(force);}//追捕(pursue)行為public function pursue(target:Vehicle):void {var lookAheadTime:Number=position.dist(target.position)/_maxSpeed;//假如目標(biāo)不動(dòng)，追捕者開足馬力趕過去的話，計(jì)算需要多少時(shí)間var predictedTarget:Vector2D=target.position.add(target.velocity.multiply(lookAheadTime));seek(predictedTarget);}//躲避(evade)行為public function evade(target:Vehicle):void {var lookAheadTime:Number=position.dist(target.position)/_maxSpeed;var predictedTarget:Vector2D=target.position.add(target.velocity.multiply(lookAheadTime));flee(predictedTarget);}override public function update():void {_steeringForce.truncate(_maxForce);//限制為最大轉(zhuǎn)向速度，以避免出現(xiàn)突然的大轉(zhuǎn)身_steeringForce=_steeringForce.divide(_mass);//慣性的體現(xiàn)_velocity=_velocity.add(_steeringForce);_steeringForce=new Vector2D ;super.update();}} }

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