相信各位同学对于Cocos2D的物理引擎box2d都不是很熟悉，本篇教程将带你深入了解一下Cocos2D的物理引擎box2d。
一、准备工作
引入box2d包，在需要使用box2d的文件中加入box2d的头文件;由于box2d是C++编写的，所以要把引入box2d的所有文件后缀名都改为.mm
二、box2d中的一些重要参数
1、gravity，重力加速度，同现实世界中的g，向量
2、shape，形状，形状是有大小的
3、density，密度
4、friction，摩擦力
5、restitution，恢复，此参数用于碰撞，如果两个物体有不同的restitution，box2d总是选择比较大的restitution进行计算
6、meter，距离单位，灵活定义你的meter，当对象为0.1至10meters的时候，box2d可以很好的处理它们，
三、box2d之hello world
让我们先创建一个box2d项目。创建好之后运行：
每当我们点击屏幕时，会落下一个小方块，ok，然我们来详细看下生成的代码。
-(id) init
{
// always call "super" init
// apple recommends to re-assign "self" with the "super" return value
if( (self=[super init])) {
// enable touches
self.isTouchEnabled = YES;
// enable accelerometer
self.isAccelerometerEnabled = YES;
CGSize screenSize = [CCDirector sharedDirector].winSize;
CCLOG(@"Screen width %0.2f screen height %0.2f",screenSize.width,screenSize.height);
// Define the gravity vector.
b2Vec2 gravity;
gravity.Set(0.0f, -10.0f);
// Do we want to let bodies sleep?
// This will speed up the physics simulation
bool doSleep = true;
// Construct a world object, which will hold and simulate the rigid bodies.
world = new b2World(gravity, doSleep);
world->SetContinuousPhysics(true);
// debug Draw functions
m_debugDraw = new GLESDebugDraw( PTM_RATIO );
world->SetDebugDraw(m_debugDraw);
uint32 flags = 0;
flags += b2DebugDraw::e_shapeBit;
// flags += b2DebugDraw::e_jointBit;
// flags += b2DebugDraw::e_aabbBit;
// flags += b2DebugDraw::e_pairBit;
// flags += b2DebugDraw::e_centerOfMassBit;
m_debugDraw->SetFlags(flags);
// Define the ground body.
b2BodyDef groundBodyDef;
groundBodyDef.position.Set(0, 0); // bottom-left corner
// 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;
// bottom
groundBox.SetAsEdge(b2Vec2(0,0), b2Vec2(screenSize.width/PTM_RATIO,0));
groundBody->CreateFixture(&groundBox,0);
// top
groundBox.SetAsEdge(b2Vec2(0,screenSize.height/PTM_RATIO), b2Vec2(screenSize.width/PTM_RATIO,screenSize.height/PTM_RATIO));
groundBody->CreateFixture(&groundBox,0);
// left
groundBox.SetAsEdge(b2Vec2(0,screenSize.height/PTM_RATIO), b2Vec2(0,0));
groundBody->CreateFixture(&groundBox,0);
// right
groundBox.SetAsEdge(b2Vec2(screenSize.width/PTM_RATIO,screenSize.height/PTM_RATIO), b2Vec2(screenSize.width/PTM_RATIO,0));
groundBody->CreateFixture(&groundBox,0);
//Set up sprite
CCSpriteBatchNode *batch = [CCSpriteBatchNode batchNodeWithFile:@"blocks.png" capacity:150];
[self addChild:batch z:0 tag:kTagBatchNode];
[self addNewSpriteWithCoords:ccp(screenSize.width/2, screenSize.height/2)];
CCLabelTTF *label = [CCLabelTTF labelWithString:@"Tap screen" fontName:@"Marker Felt" fontSize:32];
[self addChild:label z:0];
[label setColor:ccc3(0,0,255)];
label.position = ccp( screenSize.width/2, screenSize.height-50);
[self schedule: @selector(tick:)];
}
return self;
}
init方法首先创建了重力加速度，加速度是一个向量，-10是因为加速度朝向y轴负方向。之后创建world以及word的四个边缘，防止物体跑出屏幕。再然后是创建精灵和一个label，然后schedule tick方法
创建body的步骤
创建fixture步骤
知道了这些，让我们来看看如何创建一个box2d世界中的物体
-(void) addNewSpriteWithCoords:(CGPoint)p
{
CCLOG(@"Add sprite %0.2f x %02.f",p.x,p.y);
CCSpriteBatchNode *batch = (CCSpriteBatchNode*) [self getChildByTag:kTagBatchNode];
//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 spriteWithBatchNode:batch rect:CGRectMake(32 * idx,32 * idy,32,32)];
[batch addChild:sprite];
sprite.position = ccp( 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);
bodyDef.userData = sprite;
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);
}
可以看到，代码中的步骤跟我们上面的步骤一样，唯一不同的就是多了一个userData，这个属性用来绑定精灵否则你会看到精灵在你点击的地方不动，而一个粉色的方块掉了下去。fixture是相对于body的位置来的
下面这段代码相对固定，可以先不管
-(void) draw
{
// Default GL states: GL_TEXTURE_2D, GL_VERTEX_ARRAY, GL_COLOR_ARRAY, GL_TEXTURE_COORD_ARRAY
// Needed states: GL_VERTEX_ARRAY,
// Unneeded states: GL_TEXTURE_2D, GL_COLOR_ARRAY, GL_TEXTURE_COORD_ARRAY
glDisable(GL_TEXTURE_2D);
glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
world->DrawDebugData();
// restore default GL states
glEnable(GL_TEXTURE_2D);
glEnableClientState(GL_COLOR_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
}
接下来是tick方法
-(void) tick: (ccTime) dt
{
//It is recommended that a fixed time step is used with Box2D for stability
//of the simulation, however, we are using a variable time step here.
//You need to make an informed choice, the following URL is useful
//http://gafferongames.com/game-physics/fix-your-timestep/
int32 velocityIterations = 8;
int32 positionIterations = 1;
// Instruct the world to perform a single step of simulation. It is
// generally best to keep the time step and iterations fixed.
world->Step(dt, velocityIterations, positionIterations);
//Iterate over the bodies in the physics world
for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
{
if (b->GetUserData() != NULL) {
//Synchronize the AtlasSprites position and rotation with the corresponding body
CCSprite *myActor = (CCSprite*)b->GetUserData();
myActor.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
myActor.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
}
}
}
先说下step方法，velocityIterations和positionIterations这两个参数越大，box2d就能进行更好的模拟，但是性能就会下降，这两个参数你应该自己把握以适合你的游戏。
下面的循环是为了让你的sprite与box2d中的对象同步，你可以注释掉这段代码看下效果，你会发现粉方块掉下去了，你的sprite没掉下去。
