车身角度不变box2D C ++

Question

i'm making a side-scrolling car game with box2D. I'm currently working on the car and it seems that i'm stuck. The chassis of my car isn't rotating when for example the car is trying to climb a hill. I don't know if its normal or if i should set the angle of the body. Here's a quick video that shows the problem : https://streamable.com/d802n

This is my code :

b2BodyDef carBox = b2BodyDef();
carBox.position = b2Vec2(bodyCenterPosition.x, bodyCenterPosition.y);
carBox.type = b2_dynamicBody;
car = game->getWorld()->CreateBody(&carBox);

b2PolygonShape carPolygon = b2PolygonShape();
carPolygon.SetAsBox(bodySize.x, bodySize.y);
b2FixtureDef carFix = b2FixtureDef();
carFix.density = 0.0f;
carFix.shape = &carPolygon;
car->CreateFixture(&carFix);

b2PolygonShape headPolygon = b2PolygonShape();
headPolygon.SetAsBox(headSize.x, headSize.y);
for (int i = 0; i < 8; i++) {
    headPolygon.m_vertices[i].x -= 8.0f / RATIO;
    headPolygon.m_vertices[i].y -= 24.0f / RATIO;
}
b2FixtureDef headFix = b2FixtureDef();
headFix.density = 0.0f;
headFix.shape = &headPolygon;
car->CreateFixture(&headFix);


b2CircleShape circleShape;
circleShape.m_radius = 0.35f;
circleShape.m_p.SetZero();

b2FixtureDef fd;
fd.shape = &circleShape;
fd.density = 1.0f;
fd.friction = 0.9f;

b2BodyDef wheel1Def;
wheel1Def.type = b2_dynamicBody;
wheel1Def.position = b2Vec2(backWheelCenterPosition.x, backWheelCenterPosition.y);
backWheel = game->getWorld()->CreateBody(&wheel1Def);
backWheel->CreateFixture(&fd);

b2BodyDef wheel2Def;
wheel2Def.type = b2_dynamicBody;
wheel2Def.position = b2Vec2(frontWheelCenterPosition.x, frontWheelCenterPosition.y);
frontWheel = game->getWorld()->CreateBody(&wheel2Def);
frontWheel->CreateFixture(&fd);

b2WheelJointDef springDef1;
springDef1.dampingRatio = 50.0f;
springDef1.maxMotorTorque = 1.0f;
springDef1.frequencyHz = 15.0f;
springDef1.motorSpeed = 0.0f;
springDef1.enableMotor = true;
springDef1.Initialize(car, backWheel, backWheel->GetPosition(), sfVecToB2Vec(sf::Vector2f(0.0f, 1.0f)));
backSpring = (b2WheelJoint*) game->getWorld()->CreateJoint(&springDef1);

springDef1.Initialize(car, frontWheel, frontWheel->GetPosition(), sfVecToB2Vec(sf::Vector2f(0.0f, 1.0f)));
frontSpring = (b2WheelJoint*) game->getWorld()->CreateJoint(&springDef1);

Answer 1

It is normal for a body with a fixture having a zero density to behave like an infinite mass. Sounds like that's not what you intended however.

Looking at the code presented, we see that the carFix.density and headFix.density are being set to 0.0f . Set these to a positive non-zero value , like 1.0f , and the dynamic body they're created on should behave more like a physical mass would. That's assuming all of the other fixtures created on that body also have a density that's greater than zero as well.

For a tutorial on fixtures and density, I'd recommend taking a look at iforce2d's Box2D C++ tutorials - Fixtures .

Hope this solves the problem or at least helps.

By the way, I loved the artwork shown in the video!