XNA Creating a directional vector from two other vectors

Question

Could somebody please tell me what I'm missing?

I have a scene where the camera is fixed in the sky looking down on a plane. On that plane is a 3D model. I want to move the 3D model in the direction of the mouse whenever the person clicks. I have accurately converted the mouse coordinates into 3D coordinates and then according to my understanding all I had to do was the following:

direction = new Vector3(mouseX - modelX, 0, mouseZ - modelZ);
direction = Vector3.Normalize(direction);

However this doesn't quite work. If I remove the normalize part and divide it by 100 it works as in it moves towards the mouse but the speed is constantly changing (as it should) since I'm just dividing by 100, so I sorta-kinda-know the rest is fine. But if I get rid of that and keep the normalize part the character goes crazy and moves towards the mouse but it moves very fast and is constantly flashing in different positions around the mouse. In 2D I had to use sin and cosine after subtracting the values from one another, is there something similar I have to do in 3D?

Thanks!

Edit - code for figuring out the mouse position:

nearsourceV = graphicsDevice.Viewport.Unproject(new Vector3(mouse.X, mouse.Y, 0f), camera.Projection, camera.View, camera.World);
farsourceV = graphicsDevice.Viewport.Unproject(new Vector3(mouse.X, mouse.Y, 1f), camera.Projection, camera.View, camera.World);

fullMouseCoord = Vector3.Normalize(farsourceV - nearsourceV);

mY = -fullMouseCoord.Y;
mY = zoomAmount / mY;
fullMouseCoord  = fullMouseCoord  * mY;

And then the code for updating the model position:

public void MoveVertices(float x2, float y2, float z2)
{            
    for (int i = 0; i < vertices.Length; i++)
    {
        vertices[i].Position.X += x2;
        vertices[i].Position.Z += z2;
    }
}

Answer 1

I think you need to add a delta time, and movement speed into the equations in MoveVertices. Without these, you are moving one unit per update. Something like the following will make it N units per second:

public void MoveVertices(float x2, float y2, float z2, float movementSpeed, float deltaTime)
{            
    for (int i = 0; i < vertices.Length; i++)
    {
        vertices[i].Position.X += x2 * movementSpeed * deltaTime;
        vertices[i].Position.Z += z2 * movementSpeed * deltaTime;
    }
}

where movementSpeed is units/second and deltaTime is the number of fractional seconds that have passed since the last update. This will slow your character down.

You can read more about delta times here: When should I use a fixed or variable time step? and here: http://gafferongames.com/game-physics/fix-your-timestep/

Answer 2

1) Figuring out mouse position:

I think is easier using a Ray/Plane intersection. Seems that your plane is Y=0, then the code is:

  nearsourceV = graphicsDevice.Viewport.Unproject(new Vector3(mouse.X, mouse.Y, 0f), camera.Projection, camera.View, camera.World);
  farsourceV = graphicsDevice.Viewport.Unproject(new Vector3(mouse.X, mouse.Y, 1f), camera.Projection, camera.View, camera.World);

  direction = Vector3.Normalize(farsourceV - nearsourceV);
  ray = new Ray(nearSource, direction);
  var plane = new Plane(Vector3.UnitY,0);
  var distance = ray.Intersects(plane);
  if (distance.HasValue)
      fullMouseCoord = ray.Position + ray.Direction * distance.Value;

2) really you need to move the verticesd that way?

When you draw a model is usual to pass a model transform matrix as argument to the shader, use it to translate your model.

  // This is a good place to rotate and scale your model too
  var ModelTransform =  Matrix.CreateTranslation( model_position ); 

3) As pointed before if you want move with constant speed towards mouse point:

  model_movement_direction = Vector3.Normalize(fullMouseCoord-model_position);
  model_position += model_movement_direction * speed * elapsed_frame_time;

or doing some easing

  model_position +=  (fullMouseCoord-model_position) * 0.05f;