Box2d SetLinearVelocity 奇怪的问题

Posted 2023-03-11

【中文标题】Box2d SetLinearVelocity 奇怪的问题

【英文标题】：Box2d SetLinearVelocity weird issue

【发布时间】：2013-08-28 22:22:16

【问题描述】：

我试图让一个 b2body 对象以恒定的速度跟随触摸位置，但我在“y”轴上遇到问题，在该轴上，身体转到“屏幕的另一半”上的位置 就像一个镜像......不过，“x”轴很好。 （我用的是cocos2d 1.0.0）

所有代码如下：

HelloWorldLayer.h

#import "cocos2d.h"
#import "Box2D.h"

#define PTM_RATIO 32.0

@interface HelloWorldLayer : CCLayer 
    b2World *_world;
    b2Body *_body;
    CCSprite *_ball;


+ (id) scene;

@end

HelloWorldLayer.m

#import "HelloWorldLayer.h"
#import "CGPointExtension.h"

static CGPoint location;

@implementation HelloWorldLayer

+ (id)scene 

    CCScene *scene = [CCScene node];
    HelloWorldLayer *layer = [HelloWorldLayer node];
    [scene addChild:layer];
    return scene;



- (id)init 

    if ((self=[super init])) 
        CGSize winSize = [CCDirector sharedDirector].winSize;

        // Create sprite and add it to the layer
        _ball = [CCSprite spriteWithFile:@"ball.png" rect:CGRectMake(0, 0, 52, 52)];
        _ball.position = ccp(100, 300);
        [self addChild:_ball];

        // Create a world
        b2Vec2 gravity = b2Vec2(0.0f, 0.0f);
        _world = new b2World(gravity);

        // Create edges around the entire screen
        b2BodyDef groundBodyDef;
        groundBodyDef.position.Set(0,0);

        b2Body *groundBody = _world->CreateBody(&groundBodyDef);
        b2EdgeShape groundEdge;
        b2FixtureDef boxShapeDef;
        boxShapeDef.shape = &groundEdge;

        //wall definitions
        groundEdge.Set(b2Vec2(0,0), b2Vec2(winSize.width/PTM_RATIO, 0));
        groundBody->CreateFixture(&boxShapeDef);

        groundEdge.Set(b2Vec2(0,0), b2Vec2(0,winSize.height/PTM_RATIO));
        groundBody->CreateFixture(&boxShapeDef);

        groundEdge.Set(b2Vec2(0, winSize.height/PTM_RATIO),
                       b2Vec2(winSize.width/PTM_RATIO, winSize.height/PTM_RATIO));
        groundBody->CreateFixture(&boxShapeDef);

        groundEdge.Set(b2Vec2(winSize.width/PTM_RATIO, winSize.height/PTM_RATIO),
                       b2Vec2(winSize.width/PTM_RATIO, 0));
        groundBody->CreateFixture(&boxShapeDef);

        // Create ball body and shape
        b2BodyDef ballBodyDef;
        ballBodyDef.type = b2_dynamicBody;
        ballBodyDef.position.Set(100/PTM_RATIO, 300/PTM_RATIO);
        ballBodyDef.userData = _ball;
        _body = _world->CreateBody(&ballBodyDef);

        b2CircleShape circle;
        circle.m_radius = 26.0/PTM_RATIO;

        b2FixtureDef ballShapeDef;
        ballShapeDef.shape = &circle;
        ballShapeDef.density = 1.0f;
        ballShapeDef.friction = 0.2f;
        ballShapeDef.restitution = 0.8f;
        _body->CreateFixture(&ballShapeDef);

        [self schedule:@selector(tick:)];
        [self setTouchEnabled:YES];
        [self setAccelerometerEnabled:YES];
    
    return self;


- (void)tick:(ccTime) dt 

    int32 velocityIterations = 8;
    int32 positionIterations = 1;
    _world->Step(dt, velocityIterations, positionIterations);

    for(b2Body *b = _world->GetBodyList(); b; b=b->GetNext()) 
        if (b->GetUserData() != NULL) 
            CCSprite *ballData = (CCSprite *)b->GetUserData();
            ballData.position = ccp(b->GetPosition().x * PTM_RATIO,
                                    b->GetPosition().y * PTM_RATIO);
            ballData.rotation = 1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
        
    



-(void)ccTouchesMoved:(NSSet *)touches withEvent:(UIEvent *)event

    UITouch *touch = [[event allTouches] anyObject];
     location = [touch locationInView:touch.view];
    NSLog(@"%@", NSStringFromCGPoint(location));



-(void) ccTouchesBegan:(NSSet *)touches withEvent:(UIEvent *)event

    UITouch *touch = [[event allTouches] anyObject];
    location = [touch locationInView:touch.view];

    [self schedule:@selector(asd:)];




-(void) ccTouchesEnded:(NSSet *)touches withEvent:(UIEvent *)event

   // [self unschedule:@selector(asd:)];
   // _body->SetLinearVelocity(b2Vec2(0,0));
   // _body->SetAngularVelocity(0);
    //_body->SetActive(false);


-(void)asd:(ccTime) dt 

    NSLog(@"%@", NSStringFromCGPoint(location));

    //float ca = location.y;

        b2Vec2 convertedLocation = b2Vec2(location.x/PTM_RATIO - _body->GetPosition().x, location.y/PTM_RATIO - _body->GetPosition().y);

        //b2Vec2 toTouchPoint = convertedLocation - _body->GetPosition();

        convertedLocation.Normalize();
        b2Vec2 impulse = 5 * convertedLocation;

        _body->SetLinearVelocity(impulse);

     //  _body->ApplyLinearImpulse(_body->GetMass() * impulse, _body->GetWorldCenter());







- (void)dealloc 
    delete _world;
    _body = NULL;
    _world = NULL;
    [super dealloc];


@end

提前致谢！

【问题讨论】：

调试你的代码。可能触摸位置的y值是负数

【参考方案1】：

当您使用 locationInView 通过触摸获取位置时，原点位于左上角。在 cocos2D 中，原点位于左下角。您可以翻转 y 值。或者您也可以使用 CCDirector 的 ConvertToGL 函数。我更喜欢后者，主要是因为它可以处理所有的纵向和横向模式。

CGPoint loc = [touch locationInView:touch.view]
location = [[CCDirector sharedDirector] convertToGL:loc];