切割Box2D对象(一)
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转载自：http://www.9ria.com/news/2011/0714/23330.html
向你展示了我的video of my work in progress engine后，是时候教你用Box2D ver2.1来实现对象切割了
内容比较多，废话少说，马上开始
绘制激光刀
绘制激光飞船简单，它是成千上万个绘画游戏中的一个古老的话题“按下鼠标、移动鼠标、释放鼠标”。
当玩家按下鼠标时，从起点到当前鼠标位置绘制一条线段，表示激光线，当玩家松开鼠标时，显示最终的激光。
也就是说激光有两个端点：起点和终点。这些点存放在b2Segment变量中。当然变量名没有定性的要求，但是既然Box2D为我定义好了segment，为什么不用呢？
下面是代码：
package { import Box2D.Dynamics.*; import Box2D.Collision.*; import Box2D.Collision.Shapes.*; import Box2D.Common.Math.*; import flash.display.Sprite; import flash.events.MouseEvent; public class Main extends Sprite { private var worldScale:int=30; private var canvas:Sprite; private var laserSegment:b2Segment; private var drawing:Boolean=false; public function Main() { canvas = new Sprite(); addChild(canvas); stage.addEventListener(MouseEvent.MOUSE_DOWN,mousePressed); stage.addEventListener(MouseEvent.MOUSE_MOVE,mouseMoved); stage.addEventListener(MouseEvent.MOUSE_UP,mouseReleased); } private function mousePressed(e:MouseEvent):void { drawing=true; laserSegment=new b2Segment(); laserSegment.p1=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function mouseMoved(e:MouseEvent):void { if (drawing) { canvas.graphics.clear(); canvas.graphics.lineStyle(1,0xff0000); canvas.graphics.moveTo(laserSegment.p1.x*worldScale,laserSegment.p1.y*worldScale); canvas.graphics.lineTo(mouseX,mouseY); } } private function mouseReleased(e:MouseEvent):void { drawing=false; laserSegment.p2=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } }}我们来看一下用到的变量
worldScale:用来将Box2D中的“米”转换为像素，如果你对Box2D中的米还很陌生的话，请点击这里，在我们这个例子中1米表示30像素
canvas：用来绘制激光的画布
laserSegment：b2Segment类变量用来存储激光的起点和终点。
drawing：表示玩家是否正在绘画的一个布尔值变量
然后建立三个鼠标事件侦听器：一个是鼠标按下、一个是鼠标移动、另一个是鼠标弹起。
鼠标按下时：将drawing设置为true，因为实际我们已经开始绘制了，创建一个新的b2Segment实例，同时将它的p1属性(起点)设置为一个新的b2Vec2变量(可以理解为Flash的point对象)，该属性的作用是将当前的鼠标坐标转换为Box2D单位。
当鼠标移动时：首先检查是否正在绘画过程中，然后从起点(laserSegment的p1属性)到鼠标当前位置绘制一个激光（一条细长的红线）。
当鼠标弹起时：首先将drawing设置为false，因为我们已经停止绘制了，将p2属性（终点）设置为一个新的b2Vec2变量，存储转换为Box2D单位的鼠标坐标，这一点跟“鼠标按下”里的处理是一样的。
现在，你可以用鼠标绘制激光了，测试一下：
http://www.emanueleferonato.com/wp-content/uploads/2011/06/Laser.swf
点击并拖动鼠标绘制激光。
现在我们需要一些用来切割的东西。
添加对象
下面我要创建三个静止的元素：floor、box、circle。这一步中，我们只是添加对象，所有你可能会觉得有点无聊，但是我希望你能注意依稀我创建circle的方法：因为Box2D本身并不只是弧线，所以我用12个端点绘制了一个近似circle的多边形。当然你可以用24或36个端点来实现更好的效果，我用的端点比较少，是出于方便理解的考虑的。在日常的项目工作中，我都是用36个顶点来绘制多边形的。
目前的代码如下：
package { import Box2D.Dynamics.*; import Box2D.Collision.*; import Box2D.Collision.Shapes.*; import Box2D.Common.Math.*; import flash.display.Sprite; import flash.events.MouseEvent; import flash.events.Event; public class Main extends Sprite { private var world:b2World=new b2World(new b2Vec2(0,10),true); private var worldScale:int=30; private var canvas:Sprite; private var laserSegment:b2Segment; private var drawing:Boolean=false; public function Main() { debugDraw(); addStuff(); canvas = new Sprite(); addChild(canvas); addEventListener(Event.ENTER_FRAME, updateWorld); stage.addEventListener(MouseEvent.MOUSE_DOWN,mousePressed); stage.addEventListener(MouseEvent.MOUSE_MOVE,mouseMoved); stage.addEventListener(MouseEvent.MOUSE_UP,mouseReleased); } private function mousePressed(e:MouseEvent):void { drawing=true; laserSegment=new b2Segment(); laserSegment.p1=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function mouseMoved(e:MouseEvent):void { if (drawing) { canvas.graphics.clear(); canvas.graphics.lineStyle(1,0xff0000); canvas.graphics.moveTo(laserSegment.p1.x*worldScale,laserSegment.p1.y*worldScale); canvas.graphics.lineTo(mouseX,mouseY); } } private function mouseReleased(e:MouseEvent):void { drawing=false; laserSegment.p2=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function debugDraw():void { var debugDraw:b2DebugDraw = new b2DebugDraw(); var debugSprite:Sprite = new Sprite(); addChild(debugSprite); debugDraw.SetSprite(debugSprite); debugDraw.SetDrawScale(worldScale); debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit); debugDraw.SetFillAlpha(0.5); world.SetDebugDraw(debugDraw); } private function addStuff():void { var floorBody:b2BodyDef= new b2BodyDef(); floorBody.position.Set(11,16); var floorShape:b2PolygonShape = new b2PolygonShape(); floorShape.SetAsBox(15,0.5); var floorFixture:b2FixtureDef = new b2FixtureDef(); floorFixture.shape=floorShape; var worldFloor:b2Body=world.CreateBody(floorBody); worldFloor.CreateFixture(floorFixture); // var squareBody:b2BodyDef= new b2BodyDef(); squareBody.position.Set(16,5); var squareShape:b2PolygonShape = new b2PolygonShape(); squareShape.SetAsBox(2.5,2.5); var squareFixture:b2FixtureDef = new b2FixtureDef(); squareFixture.shape=squareShape; var worldSquare:b2Body=world.CreateBody(squareBody); worldSquare.CreateFixture(squareFixture); // var circleVector:Vector.<b2Vec2>=new Vector.<b2Vec2>(); var circleSteps:int=24; var circleRadius:Number=3 for (var i:int=0; i<circleSteps; i++) { circleVector.push(new b2Vec2(circleRadius*Math.cos(2*Math.PI/circleSteps*i),circleRadius*Math.sin(2*Math.PI/circleSteps*i))); } var circleBody:b2BodyDef= new b2BodyDef(); circleBody.position.Set(5,5); var circleShape:b2PolygonShape = new b2PolygonShape(); circleShape.SetAsVector(circleVector,circleSteps); var circleFixture:b2FixtureDef = new b2FixtureDef(); circleFixture.shape=circleShape; var worldCircle:b2Body=world.CreateBody(circleBody); worldCircle.CreateFixture(circleFixture); } private function updateWorld(e:Event):void { world.Step(1/30,10,10); world.ClearForces(); world.DrawDebugData(); } }}代码相当多啊，但是大部分都是用来实现”调试绘制”(第42~51行)和添加多边形(第52~85行)的。你只需要看一下我是如何用一组b2Vec2对象，经过几何运算创建多边形，来实现”circle”效果的就可以了。到这一步，实现的效果如下：
http://www.emanueleferonato.com/wp-content/uploads/2011/06/Laser1.swf
你仍然可以绘制激光，不过现在我们有了一些用来切割的东西了。
检测切入点
既然激光可以切割对象，那么每个被切割的对象肯定有一个切入点和一个切出点。一定要记得：激光可以一次切割多个对象。
谢天谢地，有了b2World的RayCast方法，这个实现起来就简单多了，我们来看一下RayCast的三个参数：
laserFired：切割触发的函数
laserSegment.p1：b2Vec2对象，表示切割开始的点
laserSegment.p2：b2Vec2对象，表示切割结束的点
laserFired回调函数包含了一个对我们很有用的参数：
fixture：被激光切割的对象
point：与激光接触的b2Vect2点(即我们要找的切入点)
normal：交叉点的单位向量
fraction：交互部分的激光长度，如果你需要知道交互激光长度占总长度比例的话，这个值会很有用。
记住每次激光切割对象时都会调用laserFired方法。
如果你不希望激光继续切割，返回0，那么激光会停止继续检测。
返回1，激光会继续切割其他的对象。
代码如下：
package { import Box2D.Dynamics.*; import Box2D.Collision.*; import Box2D.Collision.Shapes.*; import Box2D.Common.Math.*; import flash.display.Sprite; import flash.events.MouseEvent; import flash.events.Event; public class Main extends Sprite { private var world:b2World=new b2World(new b2Vec2(0,10),true); private var worldScale:int=30; private var canvas:Sprite; private var laserSegment:b2Segment; private var drawing:Boolean=false; public function Main() { debugDraw(); addStuff(); canvas = new Sprite(); addChild(canvas); addEventListener(Event.ENTER_FRAME, updateWorld); stage.addEventListener(MouseEvent.MOUSE_DOWN,mousePressed); stage.addEventListener(MouseEvent.MOUSE_MOVE,mouseMoved); stage.addEventListener(MouseEvent.MOUSE_UP,mouseReleased); } private function mousePressed(e:MouseEvent):void { drawing=true; laserSegment=new b2Segment(); laserSegment.p1=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function mouseMoved(e:MouseEvent):void { if (drawing) { canvas.graphics.clear(); canvas.graphics.lineStyle(1,0xff0000); canvas.graphics.moveTo(laserSegment.p1.x*worldScale,laserSegment.p1.y*worldScale); canvas.graphics.lineTo(mouseX,mouseY); } } private function mouseReleased(e:MouseEvent):void { drawing=false; laserSegment.p2=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function debugDraw():void { var debugDraw:b2DebugDraw = new b2DebugDraw(); var debugSprite:Sprite = new Sprite(); addChild(debugSprite); debugDraw.SetSprite(debugSprite); debugDraw.SetDrawScale(worldScale); debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit); debugDraw.SetFillAlpha(0.5); world.SetDebugDraw(debugDraw); } private function addStuff():void { var floorBody:b2BodyDef= new b2BodyDef(); floorBody.position.Set(11,16); var floorShape:b2PolygonShape = new b2PolygonShape(); floorShape.SetAsBox(15,0.5); var floorFixture:b2FixtureDef = new b2FixtureDef(); floorFixture.shape=floorShape; var worldFloor:b2Body=world.CreateBody(floorBody); worldFloor.CreateFixture(floorFixture); // var squareBody:b2BodyDef= new b2BodyDef(); squareBody.position.Set(16,5); var squareShape:b2PolygonShape = new b2PolygonShape(); squareShape.SetAsBox(2.5,2.5); var squareFixture:b2FixtureDef = new b2FixtureDef(); squareFixture.shape=squareShape; var worldSquare:b2Body=world.CreateBody(squareBody); worldSquare.CreateFixture(squareFixture); // var circleVector:Vector.<b2Vec2>=new Vector.<b2Vec2>(); var circleSteps:int=12; var circleRadius:Number=3; for (var i:int=0; i<circleSteps; i++) { circleVector.push(new b2Vec2(circleRadius*Math.cos(2*Math.PI/circleSteps*i),circleRadius*Math.sin(2*Math.PI/circleSteps*i))); } var circleBody:b2BodyDef= new b2BodyDef(); circleBody.position.Set(5,5); var circleShape:b2PolygonShape = new b2PolygonShape(); circleShape.SetAsVector(circleVector,circleSteps); var circleFixture:b2FixtureDef = new b2FixtureDef(); circleFixture.shape=circleShape; var worldCircle:b2Body=world.CreateBody(circleBody); worldCircle.CreateFixture(circleFixture); } private function updateWorld(e:Event):void { world.Step(1/30,10,10); world.ClearForces(); if (laserSegment && !drawing) { world.RayCast(laserFired,laserSegment.p1,laserSegment.p2); } world.DrawDebugData(); } private function laserFired(fixture:b2Fixture,point:b2Vec2,normal:b2Vec2,fraction:Number):Number { drawCircle(point,0xff0000); return 1; } private function drawCircle(origin:b2Vec2,color:Number):void { canvas.graphics.lineStyle(2,color); canvas.graphics.drawCircle(origin.x*worldScale,origin.y*worldScale,5); } }}现在要为laserFired方法添加功能，绘制一个红色的圆圈，这样我们可以清楚的看到切入点。
结果如下：
http://www.emanueleferonato.com/wp-content/uploads/2011/06/Laser2.swf绘制一个激光，让它穿过一个或多个对象，产看切入点位置的红色圆圈。现在我们要找到切出点。
因为RayCast方法只是实现激光的发生，而无法模拟激光穿过对象，所以Box2D无法找到切出点的位置。
好的，谢谢您的阅读。….
等、等、等一下…因为激光是从A到B的一条线，我们可以想象另外一条从B到A的线。B到A的切入点刚好是A到B的切除点。这不正是我们想要的吗(这个方法只有在激光长度大于物体宽度时才适用)！
新增RayCast方法详见第91行。
代码如下：
package { import Box2D.Dynamics.*; import Box2D.Collision.*; import Box2D.Collision.Shapes.*; import Box2D.Common.Math.*; import flash.display.Sprite; import flash.events.MouseEvent; import flash.events.Event; public class Main extends Sprite { private var world:b2World=new b2World(new b2Vec2(0,10),true); private var worldScale:int=30; private var canvas:Sprite; private var laserSegment:b2Segment; private var drawing:Boolean=false; public function Main() { debugDraw(); addStuff(); canvas = new Sprite(); addChild(canvas); addEventListener(Event.ENTER_FRAME, updateWorld); stage.addEventListener(MouseEvent.MOUSE_DOWN,mousePressed); stage.addEventListener(MouseEvent.MOUSE_MOVE,mouseMoved); stage.addEventListener(MouseEvent.MOUSE_UP,mouseReleased); } private function mousePressed(e:MouseEvent):void { drawing=true; laserSegment=new b2Segment(); laserSegment.p1=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function mouseMoved(e:MouseEvent):void { if (drawing) { canvas.graphics.clear(); canvas.graphics.lineStyle(1,0xff0000); canvas.graphics.moveTo(laserSegment.p1.x*worldScale,laserSegment.p1.y*worldScale); canvas.graphics.lineTo(mouseX,mouseY); } } private function mouseReleased(e:MouseEvent):void { drawing=false; laserSegment.p2=new b2Vec2(mouseX/worldScale,mouseY/worldScale); } private function debugDraw():void { var debugDraw:b2DebugDraw = new b2DebugDraw(); var debugSprite:Sprite = new Sprite(); addChild(debugSprite); debugDraw.SetSprite(debugSprite); debugDraw.SetDrawScale(worldScale); debugDraw.SetFlags(b2DebugDraw.e_shapeBit|b2DebugDraw.e_jointBit); debugDraw.SetFillAlpha(0.5); world.SetDebugDraw(debugDraw); } private function addStuff():void { var floorBody:b2BodyDef= new b2BodyDef(); floorBody.position.Set(11,16); var floorShape:b2PolygonShape = new b2PolygonShape(); floorShape.SetAsBox(15,0.5); var floorFixture:b2FixtureDef = new b2FixtureDef(); floorFixture.shape=floorShape; var worldFloor:b2Body=world.CreateBody(floorBody); worldFloor.CreateFixture(floorFixture); // var squareBody:b2BodyDef= new b2BodyDef(); squareBody.position.Set(16,5); var squareShape:b2PolygonShape = new b2PolygonShape(); squareShape.SetAsBox(2.5,2.5); var squareFixture:b2FixtureDef = new b2FixtureDef(); squareFixture.shape=squareShape; var worldSquare:b2Body=world.CreateBody(squareBody); worldSquare.CreateFixture(squareFixture); // var circleVector:Vector.<b2Vec2>=new Vector.<b2Vec2>(); var circleSteps:int=12; var circleRadius:Number=3; for (var i:int=0; i<circleSteps; i++) { circleVector.push(new b2Vec2(circleRadius*Math.cos(2*Math.PI/circleSteps*i),circleRadius*Math.sin(2*Math.PI/circleSteps*i))); } var circleBody:b2BodyDef= new b2BodyDef(); circleBody.position.Set(5,5); var circleShape:b2PolygonShape = new b2PolygonShape(); circleShape.SetAsVector(circleVector,circleSteps); var circleFixture:b2FixtureDef = new b2FixtureDef(); circleFixture.shape=circleShape; var worldCircle:b2Body=world.CreateBody(circleBody); worldCircle.CreateFixture(circleFixture); } private function updateWorld(e:Event):void { world.Step(1/30,10,10); world.ClearForces(); if (laserSegment && !drawing) { world.RayCast(laserFired,laserSegment.p1,laserSegment.p2); world.RayCast(laserFired,laserSegment.p2,laserSegment.p1); laserSegment=null; } world.DrawDebugData(); } private function laserFired(fixture:b2Fixture,point:b2Vec2,normal:b2Vec2,fraction:Number):Number { drawCircle(point,0xff0000); return 1; } private function drawCircle(origin:b2Vec2,color:Number):void { canvas.graphics.lineStyle(2,color); canvas.graphics.drawCircle(origin.x*worldScale,origin.y*worldScale,5); } }}运行结果如下：
http://www.emanueleferonato.com/wp-content/uploads/2011/06/Laser3.swf
绘制一条激光，让它穿过一个或这个更多的对象，看一下切入点和切出点效果。
以上是本文的全部内容。下次，我会继续讨论更多关于切割对象的内容。
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