Box2D - Firing bullet from rotating gun - c++

Question

i have a little trouble as title said: i can't figure out how to shoot the bullet toward the direction the gun is pointing at.
Here's short version code, for my bullet firing:
b2BodyDef bd;
bd.type = b2_dynamicBody;
bd.bullet = true;
bd.position = gun->GetPosition();//bullet start at the middle of the gun
m_bullet = m_world->CreateBody(&bd);
m_bullet->CreateFixture(&fd);
m_bullet->ApplyLinearImpulseToCenter( ??? ,true);
At first i thought first param is the direction you want the body go toward so i put in gun->GetWorldPoint(b2Vec2(0.0f,-5.0f)) (middle of the gun's muzzle). Big miss! After a while i thought i should try put in the vector of current gun's rotation degrees b2Vec2 vect = b2Vec2(cos(angle * PI/180), sin(angle * PI/180)); but then the bullet won't fly at all. Now i am all out of ideas. Please, some light.
Full version of code:
public:
b2Body* m_bullet = NULL;
b2Body* gun;
b2RevoluteJoint* joint1;
b2FixtureDef fd;
TestingStuff()
{
{
//body
b2CircleShape circle1;
circle1.m_radius = 1.6f;
fd.shape = &circle1;
fd.density = 1.0f;
fd.filter.groupIndex = -1;
b2BodyDef bd1;
bd1.type = b2_staticBody;
bd1.position.Set(-5.0f, 9.0f);
b2Body* body1 = m_world->CreateBody(&bd1);
body1->CreateFixture(&fd);
//gun
b2PolygonShape box;
box.SetAsBox(0.5f, 5.0f);
fd.shape = &box;
fd.density = 1.0f;
fd.filter.groupIndex = -1;
b2BodyDef bd2;
bd2.type = b2_dynamicBody;
bd2.position.Set(-5.0f, 8.0f);
gun = m_world->CreateBody(&bd2);
gun->CreateFixture(&fd);
//joint
b2RevoluteJointDef jd1;
jd1.Initialize(gun, body1, bd1.position);
jd1.enableMotor = true;
jd1.maxMotorTorque = 90;
jd1.motorSpeed = 180 * DEGTORAD;//DEGTORAD=0.0174532925199432957f
joint1 = (b2RevoluteJoint*) m_world->CreateJoint(&jd1);
}
}
void Keyboard(int key)
{
switch (key)
{
case GLFW_KEY_COMMA:
if (m_bullet != NULL)
{
m_world->DestroyBody(m_bullet);
m_bullet = NULL;
}
{
//bullet
b2CircleShape shape;
shape.m_radius = 0.25f;
fd.shape = &shape;
fd.density = 1;
fd.restitution = 0.05f;
fd.filter.groupIndex = -1;
b2BodyDef bd;
bd.type = b2_dynamicBody;
bd.bullet = true;
bd.position = gun->GetPosition();
m_bullet = m_world->CreateBody(&bd);
m_bullet->CreateFixture(&fd);
m_bullet->ApplyLinearImpulseToCenter( ??? ,true);
}
break;
}
}

Answer 1

After a good sleep i found the solution after few more trial and error.
//bullet
float degAngle = joint1->GetJointAngle() * RADTODEG + 180;
b2Vec2 vect = b2Vec2(sin(degAngle* (b2_pi / 180)) * 10, cos(degAngle* (b2_pi / 180)) * 10);
m_bullet->ApplyLinearImpulseToCenter(vect ,true);
The * 10 is to increase the impulse, make the bullet fly faster and farther, for the sake of testing i just make it 10. Notice that this code is just for testing, if you want to make it more realistic, increase the impulse as well as make the bullet go toward the muzzle instead of go toward the vector it had been fired at.