如何在 box2d 体内设置精灵的位置? [英] How to set the position of a sprite within a box2d body?

问题描述

基本上我的身体有 2 个多边形.当我为 userData 添加精灵时，纹理的位置不是我想要的位置.我想要做的是调整纹理在体内的位置.这是我设置的代码示例:

Basically I have 2 polygons for my body. When I add a sprite for userData, the position of the texture isn't where I want it to be. What I want to do is adjust the position of the texture within the body. Here's the code sample of where I am setting this:

CCSpriteSheet *sheet = (CCSpriteSheet*) [self getChildByTag:kTagSpriteSheet];
CCSprite *pigeonSprite = [CCSprite spriteWithSpriteSheet:sheet rect:CGRectMake(0,0,40,32)];
[sheet addChild:pigeonSprite z:0 tag:kPigeonSprite];

pigeonSprite.position = ccp( p.x, p.y);

bodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
bodyDef.userData = sprite;
b2Body *body = world->CreateBody(&bodyDef);

b2CircleShape dynamicCircle;
dynamicCircle.m_radius = .25f;
dynamicCircle.m_p.Set(0.0f, 1.0f);

        // Define the dynamic body fixture.
b2FixtureDef circleDef;
circleDef.shape = &dynamicCircle;   
circleDef.density = 1.0f;
circleDef.friction = 0.3f;

body->CreateFixture(&circleDef);

b2Vec2 vertices[3];
vertices[0].Set(-0.5f, 0.0f);
vertices[1].Set(0.5f, 0.0f);
vertices[2].Set(0.0f, 1.0f);
b2PolygonShape triangle;
triangle.Set(vertices, 3);

b2FixtureDef triangleDef1;
triangleDef1.shape = ▵ 
triangleDef1.density = 1.0f;
triangleDef1.friction = 0.3f;

body->CreateFixture(&triangleDef1);

推荐答案

我对objective-c不是很熟悉，但我会尝试一下.

I'm not that familiar with objective-c but I'll give it a try.

我所看到的是，您将一个指向 sprite 对象的指针存储在 body 的用户数据中，然后将其留在那里.如果您希望将身体的位置转移到精灵，您需要每帧更新它.

All I can see is that you are storing a pointer to the sprite object in the body's user data and then leaving it there. If you want the body's position to be transferred to the sprite you need to update it every frame.

在 C++ 中，这看起来像这样.

In C++ this would look something like this.

// To be called each time physics should be updated.
void physicsStep(float32 timeStep, int32 velocityIterations, int32 positionIterations) {
    // This is the usual update routine.
    world.Step(timeStep, velocityIterations, positionIterations);
    world.ClearForces();

    // SpriteClass can be replaced with any class you favor.
    // Assume there is a known pointer to the b2Body. Otherwise you'll have to get that,
    // or iterate over all bodies in the world.
    SpriteClass *sprite = (SpriteClass*)body->GetUserData();

    // Once you have the pointer you can transfer all the data.
    sprite.position = body->GetPosition();
    sprite.angle = body->GetAngle();
    // ... and so on
}

用户数据只是 b2Body 中的任意存储空间，Box2D 不知道您决定在其中存储什么.

User data is just an arbitrary storage space in the b2Body and Box2D has no idea about what you decide to store there.

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