[HTML5]使用Box2dWeb模拟飞行箭矢
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。
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：
[html]
<html>
<head>
<title>Box2DWeb Test</title>
<scripttypescripttype="text/javascript"src="js/Box2dWeb-2.1.a.3.min.js"></script>
<scripttypescripttype="text/javascript" src="js/game.js"></script>
</head>
<bodyonloadbodyonload="init();">
<canvasidcanvasid="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：
[javascript]
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。
[javascript]
// 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：
[javascript]
// 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：
[javascript]
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：
[javascript]
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。
[javascript]
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：
[javascript]
//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; www.2cto.com
} www.2cto.com
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：