# I think my rigid body classes are incorrect. Error laden behaviour

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Hi, I've been mulling over a few rigid body papers the last week or so, and getting my head around the concepts and the crazy notation. Now I finally understand (or I think I do) the inertia tensor and how torque influences angular velocity I have written a few simple classes, a kind-of abstract rigid body class and a cube and sphere class that derive from it. I'm also using the glm library - it has a few handy functions for working out inertia tensors for non-uniform cubes, spheres etc.

The parent class has these properties:

[CODE]
float m_mass;
vec3 m_position;
vec3 m_linearMomentum;
vec3 m_angularMomentum;
mat3x3 m_orientation;
mat3x3 m_inertiaTensor;
mat3x3 m_inverseInertiaTensor;
vec3 m_force;
vec3 m_torque;
[/CODE]

and the important functions are as follows:

[CODE]
void RigidBody::Update(float dt)
{

m_linearMomentum += m_force * dt;
m_position += m_linearMomentum/m_mass * dt;

//angular update
m_angularMomentum += m_torque * dt;
mat3x3 R(m_orientation);
mat3x3 RT = glm::transpose(R);
mat3x3 I = R * m_inverseInertiaTensor * RT;
vec3 angularVelocity = I._inverse() * m_angularMomentum;

float angle = sqrt(glm::dot(angularVelocity,angularVelocity)) * dt;

R = (mat3x3)glm::rotate(glm::mat4x4(R),angle,glm::normalize(angularVelocity));

m_orientation = R;
}

//takes a force and a contact point relative in local space,
//so if the force is gravity it should act on the center of mass (0,0,0) for these
//simple simulations

void RigidBody::ApplyForce(vec3& force,vec3& contactPoint)
{
//force is dP/dt
//so...
m_force += force;

//torque is force * distance to r so...
m_torque += glm::cross(contactPoint-m_position,force);

//use a numeric threshold - if the new values of force and/or
//torque are under the threshold in any direction set that direction's
//total to 0.0f

float threshold = 0.0001f;
for(int direction = 0; direction < 3; direction++)
{
if(m_force[direction] < threshold) m_force[direction] = 0.0f;
if(m_torque[direction] < threshold) m_torque[direction] = 0.0f;
}

}
[/CODE]

I'm 100% sure that the above code is the culprit, but I am not sure what exactly I'm doing wrong.

The derived class are so simple I'll just post their constructors:

[CODE]
CubeRigidBody(float mass,vec3& position,mat3x3& orientation,vec3& dimensions):m_Dimensions(dimensions),
RigidBody(mass,position,orientation)
{
m_inertiaTensor = (glm::boxInertia3(mass,dimensions));
m_inverseInertiaTensor = m_inertiaTensor._inverse();
};
{
m_inverseInertiaTensor = m_inertiaTensor._inverse();
};
[/CODE]

Anyway, when I apply a force to a cube in a test, when getting the orientation matrix after an update many elements of the matrix are NANs or -NANs, so I can't render using the orientation matrix. Position update seems ok though.

Any ideas? Thanks in advance, and I'll be back. Edited by Lil_Lloyd

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I don't know if this has anything to do with your problem, but you should change this:

if(m_force[direction] < threshold) m_force[direction] = 0.0f;
if(m_torque[direction] < threshold) m_torque[direction] = 0.0f;

to:

if(abs(m_force[direction]) < threshold) m_force[direction] = 0.0f;
if(abs(m_torque[direction]) < threshold) m_torque[direction] = 0.0f;

because the components of the force can take minus values..

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Ahhh yes! Thanks, I had actually noticed and changed that since but thank you!