Display mesh in front of camera

Question

Im trying to display a mesh in front of the camera and FACING the camera ( I'm developing an small editor in wpf using helix3d).

Pretty much something like this question

I load my mesh into a ModelVisual3D and I set it 20 units in front of the camera with this code:

var camera = ((PerspectiveCamera) viewport.Camera);
var distanceFromCamera = 20;

var direction = camera.LookDirection;
direction.Normalize();
direction = direction * distanceFromCamera;

var meshPosition = camera.Position + direction;


 var compositeTransform = new Transform3DGroup();

 compositeTransform.Children.Add(new RotationTransform(???));
 compositeTransform.Children.Add(new TranslateTransform3D(meshPosition));



ModelVisual3D mesh = LoadMesh(...);
mesh.Transform = compositeTransform

It already shows correctly in front of the camera, but I'm missing the rotation, how can I extract the rotation of the camera and set it to my mesh so is facing the camera?

Edit

I tried to use the following function to get the rotation matrix but is not working either:

    static Matrix3D CalculateLookAt(Vector3D eye, Vector3D at, Vector3D up)
    {
        if (Math.Abs(up.Length - 0.0) < double.Epsilon) up = new Vector3D(0, 1, 0);
        var zaxis = (at - eye);
        zaxis.Normalize();
        var xaxis = Vector3D.CrossProduct(up, zaxis);
        xaxis.Normalize();
        var yaxis = Vector3D.CrossProduct(zaxis, xaxis);


        return new Matrix3D(
            xaxis.X, yaxis.X, zaxis.X, 0,
            xaxis.Y, yaxis.Y, zaxis.Y, 0,
            xaxis.Z, yaxis.Z, zaxis.Z, 0,
            Vector3D.DotProduct(xaxis, -eye), Vector3D.DotProduct(yaxis, -eye), Vector3D.DotProduct(zaxis, -eye), 1
            );
    }

Answer 1

I had to ask a friend that is good at 3D graphics this is what he did to make it right:

 ProjectionCamera camera = (ProjectionCamera) viewport.Camera;

Vector3D ax1 = camera.LookDirection.Normalized();
Vector3D ax2 = Vector3D.CrossProduct(ax1, camera.UpDirection).Normalized() * -1;
Vector3D ax3 = camera.UpDirection.Normalized();

Vector3D translation = (ax1*20 + camera.Position).ToVector3D();
Quaternion rotation = new Quaternion(ax3, -90) * QuaternionUtils.CreateFromRotationAxes(ax1,ax2,ax3);

Transform3DGroup transformGroup = new Transform3DGroup();

transformGroup.Children.Add(new RotateTransform3D(new QuaternionRotation3D(rotation)));
transformGroup.Children.Add(new TranslateTransform3D(translation));

mesh.Transform = transformGroup;