Inertia computation

[DIMOSUS.X]

Greetings, could you please help me?
There is a physical agent, being placed into free space and having an angular damping not equal to zero. A constant rotational moment is applied to the agent. As a result, the agent is rotating with some constant speed.
I need to find an angle of the agent’s rotation under inertia exposure, after the rotational moment is gone.
How can I compute it?

[Julio Jerez]

you can just get the quaternion rotation before and after the step and extract the angule for the instant rotaion around the body angular velocity vector
you can get the angle using this function adapted from the dMatrix class in the SDK

Code: Select all

dFloat dQuaternion::CalcAverageAngle (const dQuaternion &QB) const
{
   dFloat dirMag;
   dFloat dirMag2;
   dFloat omegaMag;
   dFloat dirMagInv;

   dQuaternion dq (Inverse() * QB);
   dVector omegaDir (dq.m_q1, dq.m_q2, dq.m_q3);

   dirMag2 = omegaDir % omegaDir;
   if (dirMag2   < dFloat(dFloat (1.0e-5f) * dFloat (1.0e-5f))) {
      return dVector (dFloat(0.0f), dFloat(0.0f), dFloat(0.0f), dFloat(0.0f));
   }

   dirMagInv = dFloat (1.0f) / dSqrt (dirMag2);
   dirMag = dirMag2 * dirMagInv;

   omegaMag = dFloat(2.0f) * dAtan2 (dirMag, dq.m_q0) / dt;
   return dirMagInv * omegaMag;
}




Quertion r0 = BoadyGetRoation();
UpdateBody
Quertion r1 = BoadyGetRoation();

angle = r0.CalcAverageAngle (r1);

this will guve you the angle the body rotated around the axis of rotation given by normalizing its angular velocity vector.

[DIMOSUS.X]

This angle must be predicted before the body stops. Is there any way to do it?
And, if possible, we need to know how long body will rotate by inertia after applying force.

[Julio Jerez]

that will be hard, you can crate and extra world with a copy of the body and advance for few frames

[stnal]

Good idea to have a copy of the world, how expensive is it? Do I need to copy all object in world as well?
I can see it is useful for physics projection in the client of a multiplayer game.

[JernejL]

It should not be very prohibitive, i think some shapes can be actually reused across separate newton worlds.