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# Rotation Animation - Slerping the Kool-Aid Part 2

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Hey guys, I'm having some SLERP issues and was hoping one of you guys' could help me with your expertise...

(The following code is the Update method of a component whose responsibility is to rotates it's actor towards a given target using SLERP over the course of multiple frames...)

The problem I'm having is that the following causes the actor to rotate on some arbitrary axis when it is facing any direction other than the negative Z axis...

public override void Update(GameTime gameTime)
{
if (this.Status == AnimationStatus.Running)
{
TransformComponent transformComponent = this.Owner.GetComponent<TransformComponent>();

Vector3 position = transformComponent.Transform.Translation;

Vector3 forwardVector = Vector3.Normalize(transformComponent.Transform.Forward);

float dot = (float)Math.Round(
Vector3.Dot(
forwardVector,
Vector3.Normalize(_targetVector)),
4);

if (dot == 1)
{
this.Status = AnimationStatus.Finished;

return;
}

_targetVector.Normalize();

Vector3 rotationAxis =
Vector3.Normalize(
Vector3.Cross(
_targetVector,
forwardVector));

float rotationAngle = MathUtilities.GetSignedAngle(
forwardVector,
_targetVector);

_slerpAmount += (float)gameTime.ElapsedGameTime.Milliseconds / 20000f;

Quaternion qStart = Quaternion.CreateFromRotationMatrix(transformComponent.Transform);

Quaternion qEnd = Quaternion.CreateFromAxisAngle(rotationAxis, rotationAngle);

Quaternion qSlerp = Quaternion.Slerp(
qStart,
qStart * qEnd,
_slerpAmount);

Matrix rotation = Matrix.CreateFromQuaternion(qSlerp);

rotation.Translation = position;

PhysicsComponent physicsComponent = this.Owner.GetComponent<PhysicsComponent>();

physicsComponent.KinematicMove(rotation);
}
}


...and if anyone's curious, here's the GetSignedAngle()  method...

public static float GetSignedAngle(
Vector3 a,
Vector3 b)
{
a.Normalize();

b.Normalize();

float theta = (float)Math.Acos(
Vector3.Dot(
a,
b));

Vector3 normal = Vector3.Normalize(
Vector3.Cross(
b,
a));

Vector3 v3 = Vector3.Normalize(
Vector3.Cross(
a,
normal));

if (Vector3.Dot(
v3,
b) > 0)
{
theta *= -1;
}

return theta;
}


....the really odd thing (and proof that SLERP isn't working as I'd expect it to), is that if I replace the code that performs the SLERP...

                _slerpAmount += (float)gameTime.ElapsedGameTime.Milliseconds / 20000f;

Quaternion qStart = Quaternion.CreateFromRotationMatrix(transformComponent.Transform);

Quaternion qEnd = Quaternion.CreateFromAxisAngle(rotationAxis, rotationAngle);

Quaternion qSlerp = Quaternion.Slerp(
qStart,
qStart * qEnd,
_slerpAmount);

Matrix rotation = Matrix.CreateFromQuaternion(qSlerp);


...with a regular old rotation transformation like so...

Matrix rotation = transformComponent.Transform *
Matrix.CreateFromAxisAngle(
rotationAxis, rotationAngle);


...then the object is rotated correctly (hoever it is instant and so I lose the rotation "animation").

If anyone has any ideas as to how this could be happening or just a suggestion on how I can make this code better please let me know.

Thanks!

- Bullgoose311

Edited by Nick Joseph
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Not too sure why the above doesn't work as expected but I'm assuming it has to do my assumptions involving quaternions.  I had assumed multiplying quaternions was the same thing as multiplying rotation matrices but apparently not.  Instead of using the Quaternion's CreateFromAxisAngle I used the Matrix's and it resolved the issue...

Quaternion qStart = Quaternion.CreateFromRotationMatrix(transformComponent.Transform);

Quaternion qEnd =
Quaternion.CreateFromRotationMatrix(
transformComponent.Transform *
Matrix.CreateFromAxisAngle(
rotationAxis,
rotationAngle));

Quaternion qSlerp = Quaternion.Slerp(
qStart,
qEnd,
_slerpAmount);

Matrix rotation = Matrix.CreateFromQuaternion(qSlerp);


...thanks.

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