今天要学习的是cocos2dx中的Box2d物理引擎。
Box2D是一款开元的C++编写的物理引擎,起初是用于Flash的游戏开发中,随后推出了C++版本,其开发和升级工作一直非常活跃,几乎成了2D游戏产品的必备物理引擎。Box2D引擎为开发者提供了一个二维刚体的物理模拟库,借助Box2D的强大功能,游戏中的动画可以更加真实,让游戏世界更具交互性。
Box2D物理引擎所创建的物理引擎具有下面的三个特点:
1.物理世界存在一个重力场;
2.物理世界可以是一个存在边界的范围(有碰撞效果);
3.在世界中加入静态和动态的物体,符合现实的运动规律;
今天要学习的内容还是以实例为主。有不会的细节留言或者自行google。
先看最终效果
这个例子的实现思路如下:
1.定义物理世界,定义地面;
2.响应触控操作,在点击的位置创建一个物理世界中的正方块,并用debugdraw显示;
3.将带纹理的精灵和物理世界中的正方块相连接;
关于DebugDraw
当开发者使用物理引擎开发时,需要将物理世界绘制出来,但是我们都知道物理引擎中只存在碰撞检测和物理模拟,没有任何绘制功能,为了方便调试,就加入了调试绘图功能,这个绘图都是一些先狂,并不能作为游戏的画面,只是为了方便开发者,看到物体世界的样子。
利用DebugDraw,开发者可以轻易地看到物体的形状,关节,用于连续碰撞的核心形状,AABB包围盒,接触,质心。
还是按前面两篇教程那样创建工程,然后将cocos2d-x-2.2/samples/Cpp/TestCpp/Classes/Box2DTestBed/GLES-Render.h和cocos2d-x-2.2/samples/Cpp/TestCpp/Classes/Box2DTestBed/GLES-Render.cpp复制到工程的Classes文件夹中,修改pro.linux 的MakeFile和pro.android 的Android.mk,将文件包含进去,接下来是具体的代码实现。
HelloWorld.h
#ifndef __HELLOWORLD_SCENE_H__ #define __HELLOWORLD_SCENE_H__ #include "cocos2d.h" #include "Box2D/Box2D.h" #include "GLES-Render.h" using namespace cocos2d; class HelloWorld : public cocos2d::CCLayer { private: b2World *world; GLESDebugDraw * m_debugDraw; CCTexture2D* m_pSpriteTexture; public: HelloWorld(); ~HelloWorld(); // Here's a difference. Method 'init' in cocos2d-x returns bool, instead of returning 'id' in cocos2d-iphone virtual bool init(); void initPhysics(); virtual void draw(); void addNewSpriteAtPosition(CCPoint p); void update(float dt); virtual void ccTouchesEnded(CCSet* touches, CCEvent* event); // there's no 'id' in cpp, so we recommend returning the class instance pointer static cocos2d::CCScene* scene(); // a selector callback void menuCloseCallback(CCObject* pSender); // implement the "static node()" method manually CREATE_FUNC(HelloWorld); }; #endif // __HELLOWORLD_SCENE_H__
public的函数里面,有三个函数要注意,update是更新物体的位置,ccTouchesEnded是在触控结束,也就是手指离开屏幕时触发。addNewSpiriteAtPosition是在某个点添加一个Spirite。函数实现如下:
HelloWorld.cpp
#include "HelloWorldScene.h" USING_NS_CC; #define PTM_RATIO 32 enum { kTagParentNode = 1, }; HelloWorld::HelloWorld() { setTouchEnabled( true ); setAccelerometerEnabled( true ); this->initPhysics(); m_pSpriteTexture = CCTextureCache::sharedTextureCache()->addImage("blocks.png"); scheduleUpdate(); } HelloWorld::~HelloWorld() { CC_SAFE_DELETE(world); world = NULL; delete m_debugDraw; } CCScene* HelloWorld::scene() { // 'scene' is an autorelease object CCScene *scene = CCScene::create(); // 'layer' is an autorelease object HelloWorld *layer = HelloWorld::create(); // add layer as a child to scene scene->addChild(layer); // return the scene return scene; } // on "init" you need to initialize your instance bool HelloWorld::init() { ////////////////////////////// // 1. super init first if ( !CCLayer::init() ) { return false; } CCSize visibleSize = CCDirector::sharedDirector()->getVisibleSize(); CCPoint origin = CCDirector::sharedDirector()->getVisibleOrigin(); ///////////////////////////// // 2. add a menu item with "X" image, which is clicked to quit the program // you may modify it. // add a "close" icon to exit the progress. it's an autorelease object CCMenuItemImage *pCloseItem = CCMenuItemImage::create( "CloseNormal.png", "CloseSelected.png", this, menu_selector(HelloWorld::menuCloseCallback)); pCloseItem->setPosition(ccp(origin.x + visibleSize.width - pCloseItem->getContentSize().width/2 , origin.y + pCloseItem->getContentSize().height/2)); // create menu, it's an autorelease object CCMenu* pMenu = CCMenu::create(pCloseItem, NULL); pMenu->setPosition(CCPointZero); this->addChild(pMenu, 1); ///////////////////////////// // 3. add your codes below... // add a label shows "Hello World" // create and initialize a label CCLabelTTF* pLabel = CCLabelTTF::create("Box2d Test", "Arial", 24); // position the label on the center of the screen pLabel->setPosition(ccp(origin.x + visibleSize.width/2, origin.y + visibleSize.height - pLabel->getContentSize().height)); // add the label as a child to this layer this->addChild(pLabel, 1); setTouchEnabled( true ); setAccelerometerEnabled( true ); // init physics this->initPhysics(); return true; } void HelloWorld::initPhysics() { CCLOG("Init physics!"); b2Vec2 gravity; gravity.Set(0.0f, -10.0f); world = new b2World(gravity); // Do we want to let bodies sleep? world->SetAllowSleeping(true); world->SetContinuousPhysics(true); m_debugDraw = new GLESDebugDraw( PTM_RATIO ); world->SetDebugDraw(m_debugDraw); uint32 flags = 0; flags += b2Draw::e_shapeBit; m_debugDraw->SetFlags(flags); // Define the ground body. b2BodyDef groundBodyDef; groundBodyDef.position.Set(0, 0); // bottom-left corner CCSize screenSize = CCDirector::sharedDirector()->getVisibleSize(); // Call the body factory which allocates memory for the ground body // from a pool and creates the ground box shape (also from a pool). // The body is also added to the world. b2Body* groundBody = world->CreateBody(&groundBodyDef); // Define the ground box shape. b2PolygonShape groundBox; groundBox.SetAsBox(screenSize.width, 1.0f); groundBody->CreateFixture(&groundBox, 0.0f); } void HelloWorld::menuCloseCallback(CCObject* pSender) { #if (CC_TARGET_PLATFORM == CC_PLATFORM_WINRT) || (CC_TARGET_PLATFORM == CC_PLATFORM_WP8) CCMessageBox("You pressed the close button. Windows Store Apps do not implement a close button.","Alert"); #else CCDirector::sharedDirector()->end(); #if (CC_TARGET_PLATFORM == CC_PLATFORM_IOS) exit(0); #endif #endif } void HelloWorld::update(float dt) { int velocityIterations = 8; int positionIterations = 1; world->Step(dt, velocityIterations, positionIterations); for (b2Body* b = world->GetBodyList(); b; b = b->GetNext()) { if (b->GetUserData() != NULL) { CCSprite* myActor = (CCSprite*)b->GetUserData(); //获取精灵 myActor->setPosition( CCPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO) ); myActor->setRotation( -1 * CC_RADIANS_TO_DEGREES(b->GetAngle()) ); } } } void HelloWorld::draw() { // This is only for debug purposes // It is recommend to disable it CCLayer::draw(); ccGLEnableVertexAttribs( kCCVertexAttribFlag_Position ); kmGLPushMatrix(); world->DrawDebugData(); kmGLPopMatrix(); } void HelloWorld::ccTouchesEnded(CCSet* touches, CCEvent* event) { //Add a new body/atlas sprite at the touched location CCSetIterator it; CCTouch* touch; for( it = touches->begin(); it != touches->end(); it++) { touch = (CCTouch*)(*it); if(!touch) break; CCPoint location = touch->getLocation(); addNewSpriteAtPosition( location ); } } void HelloWorld::addNewSpriteAtPosition(CCPoint p) { CCLOG("Add sprite %0.2f x %02.f",p.x,p.y); // Define the dynamic body. //Set up a 1m squared box in the physics world b2BodyDef bodyDef; bodyDef.type = b2_dynamicBody; bodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO); b2Body *body = world->CreateBody(&bodyDef); // Define another box shape for our dynamic body. b2PolygonShape dynamicBox; dynamicBox.SetAsBox(.5f, .5f);//These are mid points for our 1m box // Define the dynamic body fixture. b2FixtureDef fixtureDef; fixtureDef.shape = &dynamicBox; fixtureDef.density = 1.0f; fixtureDef.friction = 0.3f; body->CreateFixture(&fixtureDef); // //We have a 64x64 sprite sheet with 4 different 32x32 images. The following code is // //just randomly picking one of the images int idx = (CCRANDOM_0_1() > .5 ? 0:1); int idy = (CCRANDOM_0_1() > .5 ? 0:1); CCSprite *sprite = CCSprite::createWithTexture(m_pSpriteTexture,CCRectMake(32 * idx,32 * idy,32,32)); this->addChild(sprite,1); body->SetUserData(sprite); }
具体解析下addNewSpiriteAtPosition。
CCLOG("Add sprite %0.2f x %02.f",p.x,p.y);
b2BodyDef bodyDef; bodyDef.type = b2_dynamicBody; bodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO); b2Body *body = world->CreateBody(&bodyDef);
// Define another box shape for our dynamic body. b2PolygonShape dynamicBox; dynamicBox.SetAsBox(.5f, .5f);//These are mid points for our 1m box // Define the dynamic body fixture. b2FixtureDef fixtureDef; fixtureDef.shape = &dynamicBox; fixtureDef.density = 1.0f; fixtureDef.friction = 0.3f; body->CreateFixture(&fixtureDef);
int idx = (CCRANDOM_0_1() > .5 ? 0:1); int idy = (CCRANDOM_0_1() > .5 ? 0:1); CCSprite *sprite = CCSprite::createWithTexture(m_pSpriteTexture,CCRectMake(32 * idx,32 * idy,32,32)); this->addChild(sprite,1);
通过纹理创造一个物体(注意不说刚体),idx和idy随机选择0.5和1,因为纹理是64*64的,而正方形是32*32的,需要再屏幕上体现出来,得创造一个精灵,因为刚体并不能具象的在屏幕上表示。
body->SetUserData(sprite);
还要注意一下Update里面的一句:
int velocityIterations = 8; int positionIterations = 1; world->Step(dt, velocityIterations, positionIterations);
Box2D的物理世界通常包括这样几个对象
world:一个物理世界,所有的刚体都将存在在这个世界里面,这个世界以米为距离单位。尽量贴近真实世界的度量。
body:刚体,存在在物理世界的理想物体,比任何物体都硬,不会发生形变。body对应着一个bodyDef(刚体定义),刚体定义指定了刚体的类型(动态、静态、轨迹运动的)和刚体的位置,world通过刚体定义创建刚体。
fixture:刚体修饰物,描述刚体的一些特征。fixture对应着fixtureDef(修饰物定义),它将形状绑定到刚体上,使刚体具有一些表现特征,如密度、摩擦系数、弹性等等。body通过fixtureDef创建fixture。
shape:一个几何形状,比如圆和多边形。形状是修饰物fixture的一个属性,描述了刚体的碰撞边界。
解释一下b2World, b2Body, b2BodyDef, b2Fixture, b2FixtureDef, shpae之间的关系
1.b2World通过b2BodyDef创建b2Body,没有b2BodyDef,b2Body不知道是什么类型,放在世界什么位置。
2.b2Body通过b2FixtureDef创建b2Fixture,没有b2Fixture,b2Body不知道是什么形状,摩擦、弹性、密度都不知道。shpae提供了碰撞检测的外边框。
学习 Box2D 个人笔记(二)b2body - http://blog.csdn.net/adrianous/article/details/8435156
Cocos2d-x游戏开发技术精解
Cocos2d-x by Example Beginner's Guide
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