XNA: Rotate a model to face a target

Question

What rotation Matrix can I use to get my Model face a certain point?

My model is a character, and I want it to face the camera position.

I tried Matrix.LookAt, but it doesn't work.

Answer 1

Let's assume that lookVector is the vector in model space that defines the view direction of the character. Usually this is one of the principal axes (e.g. the z-axis). Let's further assume that the character is positioned at characterPosition and that the target point is target.

The view direction towards the target in world space is

var view = target - characterPosition;

Now all we need to do is find a rotation matrix that maps lookVector to view. The solution is not unique, but we are looking for the solution with a minimal rotation angle.

The rotation axis can be found with the cross product, the rotation angle with the dot product:

var rotationAxis = Vector3.Cross(lookVector, view);
var rotationAngle = (float)Math.Acos(Vector3.Dot(lookVector, view) / lookVector.Length() / view.Length());

Now we can construct the rotation matrix:

var rotationMatrix = Matrix.CreateFromAxisAngle(rotationAxis, rotationAngle);

Answer 2

NO Wait! Matrix.CreateLookAt Works!

Think you have only used something like Matrix.CreateLookAt(Ship.Position, Target.Position, Ship.ModelRotation.Up)

But you have to add a Matrix.CreateTranslation to get it work. Or it will change it's shape instead of rotation!

This is the code

Ship.ModelRotation = Matrix.CreateTranslation(Ship.Position) * Matrix.CreateLookAt(Ship.Position, Target.Position, Ship.ModelRotation.Up)

I was in a serious trouble with this in two days. But this stupid code sloved that

Edit:

I don't know why, but this is the code working for me currently

Ship.ModelRotation = Matrix.invert( Matrix.CreateTranslation(Ship.Position) * Matrix.CreateLookAt(Ship.Position, Target.Position, Ship.ModelRotation.Up))

Note - This is a Visual basic code; Think you could just make it into C#