[HTML5]使用Box2dWeb模擬飛行箭矢

Box2d是一個2D遊戲物理引擎，由Erin Catto開發，於2007年發佈。很多2D遊戲都用過Box2d，其中最有名的自然是憤怒的小鳥。Box2d本身是C++編寫，但在不同平臺都有它的衍生版本，像Flash版的Box2dFlash，JS版的Box2dJS和Box2dWeb。最近偶然看到一篇使用Box2dFlash模擬箭矢飛行效果的文章：

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

很有意思，想嘗試使用下Box2d。

之前從沒接觸過Flash，選擇JS版的Box2d，而Box2dJS已經很久沒更新，所以使用Box2dWeb重寫箭矢飛行效果。

網上有不少Box2d教程，不過介紹其應用的多些。對於Box2d基本概念和原理，推薦阿蕉的博客，他將Box2d C++的系列教程譯成中文。雖然C++和JS不同，但是Box2d原理是相通的，可以參考。

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

首先下載Box2dWeb

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

壓縮包裏只有四個文件，這裏只需要Box2dWeb-2.1.a.3.min.js（也可以用Box2dWeb-2.1.a.3.js，方便了解Box2DWeb的各個函數）。

按照下面的目錄結構創建各個文件即可：

|-js/

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

| |-game.js

|-arrow.html

編輯arrow.html，引用javascript文件並創建一個canvas標籤，代碼如下：

<!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>

接下來編輯game.js。從arrow.html中可以看到，頁面載入後調用init方法模擬整個過程，添加下面的代碼：

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 */
};

以上代碼是爲了方便使用Box2d中的對象。接着就是設置全局屬性，像canvas的大小、Box2d的世界參數、鼠標消息響應方法等。

// 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);

設置好Box2d的世界之後就可以放置各種模型。接下來的代碼創建四面牆壁來封閉區域：

// Create bottom wall
createBox(640,30,320,480,b2Body.b2_staticBody,null);
// Create top wall
createBox(640,30,320,0,b2Body.b2_staticBody,null);
// Create left wall
createBox(30,480,0,240,b2Body.b2_staticBody,null);
// Create right wall
createBox(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);
}

希望點擊鼠標時，從左下角向鼠標點擊的位置發射一支箭。所以在鼠標點擊消息的響應方法中調用createArrow方法，根據傳入的座標生成一支箭，並賦給它初速度，代碼如下：

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);
 }

接下來就是系統方法，繪製物體並定時更新：

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方法中調用的updateArrow方法，它負責模擬箭矢在空中運動形態，讓整個過程更加真實。

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);
}

最後是getElementPosition方法，用於獲得canvas的偏移座標：

//http://js-tut.aardon.de/js-tut/tutorial/position.html
function 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};
}

程序的運行結果（Chrome 25.0.1323.1下測試）如下圖所示：

源代碼下載地址：http://download.csdn.net/detail/raymondcode/5151381

[HTML5]使用Box2dWeb模擬飛行箭矢

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