[HTML5]使用Box2dWeb模拟飞行箭矢

来源：互联网发布：蓝牙app源码编辑：程序博客网时间：2024/05/16 19:33

Box2d是一个2D游戏物理引擎，由Erin Catto开发，于2007年发布。很多2D游戏都用过Box2d，其中最有名的自然是愤怒的小鸟。Box2d本身是C++编写，但在不同平台都有它的衍生版本，像Flash版的Box2dFlash，JS版的Box2dJS和Box2dWeb。最近偶然看到一篇使用Box2dFlash模拟箭矢飞行效果的文章：

http://www.emanueleferonato.com/2012/12/10/flying-arrows-simulation-with-box2d/

很有意思，想尝试使用下Box2d。

之前从没接触过Flash，选择JS版的Box2d，而Box2dJS已经很久没更新，所以使用Box2dWeb重写箭矢飞行效果。

网上有不少Box2d教程，不过介绍其应用的多些。对于Box2d基本概念和原理，推荐阿蕉的博客，他将Box2d C++的系列教程译成中文。虽然C++和JS不同，但是Box2d原理是相通的，可以参考。

http://blog.csdn.net/wen294299195/article/category/1227604

首先下载Box2dWeb

https://code.google.com/p/box2dweb/downloads/list

压缩包里只有四个文件，这里只需要Box2dWeb-2.1.a.3.min.js（也可以用Box2dWeb-2.1.a.3.js，方便了解Box2DWeb的各个函数）。

按照下面的目录结构创建各个文件即可：

|-js/

| |-Box2dWeb-2.1.a.3.min.js

| |-game.js

|-arrow.html

编辑arrow.html，引用javascript文件并创建一个canvas标签，代码如下：

<!DOCTYPE HTML><html>    <head>       <title>Box2DWeb Test</title>        <scripttype="text/javascript"src="js/Box2dWeb-2.1.a.3.min.js"></script>        <scripttype="text/javascript" src="js/game.js"></script>    </head>     <bodyonload="init();">        <canvasid="canvas" width="640" height="480"style="background-color:#333333;"></canvas>    </body></html>

接下来编辑game.js。从arrow.html中可以看到，页面载入后调用init方法模拟整个过程，添加下面的代码：

function init() {// Commen code for usingBox2D object.    var b2Vec2 =Box2D.Common.Math.b2Vec2;    var b2AABB =Box2D.Collision.b2AABB;    var b2BodyDef =Box2D.Dynamics.b2BodyDef;    var b2Body =Box2D.Dynamics.b2Body;    varb2FixtureDef = Box2D.Dynamics.b2FixtureDef;    var b2Fixture =Box2D.Dynamics.b2Fixture;    var b2World =Box2D.Dynamics.b2World;    varb2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape;var b2DebugDraw =Box2D.Dynamics.b2DebugDraw;/* 下文添加 */};

以上代码是为了方便使用Box2d中的对象。接着就是设置全局属性，像canvas的大小、Box2d的世界参数、鼠标消息响应方法等。

// Get canvas fordrawing.var canvas =document.getElementById("canvas");var canvasPosition =getElementPosition(canvas);var context =canvas.getContext("2d");   // World constants.var worldScale = 30;var dragConstant=0.05;var dampingConstant = 2;var world = newb2World(new b2Vec2(0, 10),true);   document.addEventListener("mousedown",onMouseDown);debugDraw();            window.setInterval(update,1000/60);

设置好Box2d的世界之后就可以放置各种模型。接下来的代码创建四面墙壁来封闭区域：

// Create bottom wallcreateBox(640,30,320,480,b2Body.b2_staticBody,null);// Create top wallcreateBox(640,30,320,0,b2Body.b2_staticBody,null);// Create left wallcreateBox(30,480,0,240,b2Body.b2_staticBody,null);// Create right wallcreateBox(30,480,640,240,b2Body.b2_staticBody,null); functioncreateBox(width,height,pX,pY,type,data){    var bodyDef = new b2BodyDef;    bodyDef.type = type;    bodyDef.position.Set(pX/worldScale,pY/worldScale);    bodyDef.userData=data;     var polygonShape = new b2PolygonShape;    polygonShape.SetAsBox(width/2/worldScale,height/2/worldScale);     var fixtureDef = new b2FixtureDef;    fixtureDef.density = 1.0;    fixtureDef.friction = 0.5;    fixtureDef.restitution = 0.5;    fixtureDef.shape = polygonShape;           var body=world.CreateBody(bodyDef);    body.CreateFixture(fixtureDef);}

希望点击鼠标时，从左下角向鼠标点击的位置发射一支箭。所以在鼠标点击消息的响应方法中调用createArrow方法，根据传入的坐标生成一支箭，并赋给它初速度，代码如下：

function onMouseDown(e) {    var evt = e||window.event;    createArrow(e.clientX-canvasPosition.x,e.clientY-canvasPosition.y);} function createArrow(pX,pY) {    // Set the left corner as the originalpoint.    var angle = Math.atan2(pY-450, pX);     // Define the shape of arrow.    var vertices = [];    vertices.push(new b2Vec2(-1.4,0));    vertices.push(new b2Vec2(0,-0.1));    vertices.push(new b2Vec2(0.6,0));    vertices.push(newb2Vec2(0,0.1));     var bodyDef = new b2BodyDef;    bodyDef.type = b2Body.b2_dynamicBody;    bodyDef.position.Set(40/worldScale,400/worldScale);    bodyDef.userData = "Arrow";     var polygonShape = new b2PolygonShape;    polygonShape.SetAsVector(vertices,4);     var fixtureDef = new b2FixtureDef;    fixtureDef.density = 1.0;    fixtureDef.friction = 0.5;    fixtureDef.restitution = 0.5;    fixtureDef.shape = polygonShape;           var body = world.CreateBody(bodyDef);    body.CreateFixture(fixtureDef);     // Set original state of arrow.    body.SetLinearVelocity(newb2Vec2(20*Math.cos(angle), 20*Math.sin(angle)));    body.SetAngle(angle);    body.SetAngularDamping(dampingConstant); }

接下来就是系统方法，绘制物体并定时更新：

function debugDraw() {    var debugDraw = new b2DebugDraw();    debugDraw.SetSprite(document.getElementById("canvas").getContext("2d"));    debugDraw.SetDrawScale(worldScale);    debugDraw.SetFillAlpha(0.5);    debugDraw.SetLineThickness(1.0);    debugDraw.SetFlags(b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit);    world.SetDebugDraw(debugDraw);}    function update() {     world.Step(1/60,10,10);    world.ClearForces();    for(var b = world.m_bodyList; b != null; b = b.m_next){       if(b.GetUserData() === "Arrow") {                updateArrow(b);            }    }            world.DrawDebugData();}

注意上面update方法中调用的updateArrow方法，它负责模拟箭矢在空中运动形态，让整个过程更加真实。

functionupdateArrow(arrowBody) {    // Calculate arrow's fligth speed.    var flightSpeed =Normalize2(arrowBody.GetLinearVelocity());     // Calculate arrow's pointingdirection.    var bodyAngle = arrowBody.GetAngle();    var pointingDirection = new b2Vec2(Math.cos(bodyAngle),-Math.sin(bodyAngle));     // Calculate arrow's flightingdirection and normalize it.    var flightAngle =Math.atan2(arrowBody.GetLinearVelocity().y,arrowBody.GetLinearVelocity().x);    var flightDirection = newb2Vec2(Math.cos(flightAngle), Math.sin(flightAngle));     // Calculate dot production.    var dot = b2Dot( flightDirection,pointingDirection );    var dragForceMagnitude = (1 -Math.abs(dot)) * flightSpeed * flightSpeed * dragConstant *arrowBody.GetMass();    var arrowTailPosition =arrowBody.GetWorldPoint(new b2Vec2( -1.4, 0 ) );    arrowBody.ApplyForce( newb2Vec2(dragForceMagnitude*-flightDirection.x,dragForceMagnitude*-flightDirection.y),arrowTailPosition );} function b2Dot(a, b) {    return a.x * b.x + a.y * b.y;} function Normalize2(b) {    return Math.sqrt(b.x * b.x + b.y *b.y);}

最后是getElementPosition方法，用于获得canvas的偏移坐标：

//http://js-tut.aardon.de/js-tut/tutorial/position.htmlfunction getElementPosition(element) {    var elem=element, tagname="",x=0, y=0;    while((typeof(elem) =="object") && (typeof(elem.tagName) != "undefined")){        y += elem.offsetTop;        x += elem.offsetLeft;        tagname = elem.tagName.toUpperCase();        if(tagname == "BODY"){            elem=0;        }        if(typeof(elem) =="object"){            if(typeof(elem.offsetParent) =="object"){                elem = elem.offsetParent;            }        }    }    return {x: x, y: y};}

程序的运行结果（Chrome 25.0.1323.1下测试）如下图所示：

源代码下载地址：http://download.csdn.net/detail/raymondcode/5151381