Question

我正在尝试创建一个在Box2D中围绕静态体旋转的动态体。我有一个零重力世界，和DistanceJoint连接两个身体。我已经消除了身体和关节的所有摩擦力和阻尼，并对动态体施加了初始线速度。结果是身体开始绕行，但是它的速度随着时间的推移而减小 - 在零重力环境中我没有预料到没有摩擦。

我做错了吗？是应该在每一步重建线性速度，还是可以将这项工作委托给Box2D？

以下是相关代码：

// positions of both bodies

Vector2 planetPosition = new Vector2(x1 / Physics.RATIO, y1 / Physics.RATIO);
Vector2 satellitePosition = new Vector2(x2 / Physics.RATIO, y2 / Physics.RATIO);


// creating static body

BodyDef planetBodyDef = new BodyDef();
planetBodyDef.type = BodyType.StaticBody;
planetBodyDef.position.set(planetPosition);
planetBodyDef.angularDamping = 0;
planetBodyDef.linearDamping = 0;

planetBody = _world.createBody(planetBodyDef);

CircleShape planetShapeDef = new CircleShape();
planetShapeDef.setRadius(40);

FixtureDef planetFixtureDef = new FixtureDef();
planetFixtureDef.shape = planetShapeDef;
planetFixtureDef.density = 0.7f;
planetFixtureDef.friction = 0;

planetBody.createFixture(planetFixtureDef);

// creating dynamic body

BodyDef satelliteBodyDef = new BodyDef();
satelliteBodyDef.type = BodyType.DynamicBody;
satelliteBodyDef.position.set(satellitePosition);
satelliteBodyDef.linearDamping = 0;
satelliteBodyDef.angularDamping = 0;

satelliteBody = _world.createBody(satelliteBodyDef);

CircleShape satelliteShapeDef = new CircleShape();
satelliteShapeDef.setRadius(10);

FixtureDef satelliteFixtureDef = new FixtureDef();
satelliteFixtureDef.shape = satelliteShapeDef;
satelliteFixtureDef.density = 0.7f;
satelliteFixtureDef.friction = 0;

satelliteBody.createFixture(satelliteFixtureDef);

// create DistanceJoint between bodies

DistanceJointDef jointDef = new DistanceJointDef();        
jointDef.initialize(satelliteBody, planetBody, satellitePosition, planetPosition);
jointDef.collideConnected = false;
jointDef.dampingRatio = 0;

_world.createJoint(jointDef);

// set initial velocity

satelliteBody.setLinearVelocity(new Vector2(0, 30.0f)); // orthogonal to the joint

Answer 1

从物理上讲，你是对的。能量守恒应确保身体的速度保持不变。

然而，Box2D无法完美地代表物理学。每一帧都会有一个小错误，这些错误加起来。我不知道Box2D如何处理关节，但是如果它将对象的位置投射到圆上，这将导致框架中行进的距离略小于没有关节的情况。

底线：期望速度与您的速度保持完全相同是不合理的，您需要进行补偿。根据您的需要，您可以每帧手动设置速度，也可以使用电机固定在行星上的旋转关节。

Answer 2

尝试检查linearDamping和angularDamping数字并将它们设为零。这可能会解决问题

零重力box2d世界的速度降低

2 个答案: