XNA 4.0 Camera Question

I'm having trouble understanding how the camera works in my test application. I've been able to piece together a working camera - now I am trying to make sure I understand how it all works. My camera is encapsulated in its own class. Here is the update method that gets called from my Game.Update() method:

public void Update(float dt)
    {
      Yaw += (200 - Game.MouseState.X) * dt * .12f;
      Pitch += (200 - Game.MouseState.Y) * dt * .12f;

      Mouse.SetPosition(200, 200);

      _worldMatrix = Matrix.CreateFromAxisAngle(Vector3.Right, Pitch) * Matrix.CreateFromAxisAngle(Vector3.Up, Yaw);

      float distance = _speed * dt;

      if (_game.KeyboardState.IsKeyDown(Keys.E))
        MoveForward(distance);
      if (_game.KeyboardState.IsKeyDown(Keys.D))
        MoveForward(-distance);
      if (_game.KeyboardState.IsKeyDown(Keys.S))
        MoveRight(-distance);
      if (_game.KeyboardState.IsKeyDown(Keys.F))
        MoveRight(distance);
      if (_game.KeyboardState.IsKeyDown(Keys.A))
        MoveUp(distance);
      if (_game.KeyboardState.IsKeyDown(Keys.Z))
        MoveUp(-distance);

      _worldMatrix *= Matrix.CreateTranslation(_position);
      _viewMatrix 开发者_如何转开发= Matrix.Invert(_worldMatrix);      // What's gong on here???
    }

First of all, I understand everything in this method other than the very last part where the matrices are being manipulated. I think the terminology is getting in my way as well. For example, my _worldMatrix is really a Rotation Matrix. What really baffles me is the part where the _viewMatrix is calculated by inverting the _worldMatrix. I just don't understand what this is all about.

In prior testing, I always used Matrix.CreateLookAt() to create a view matrix, so I'm a bit confused. I'm hoping someone can explain in simple terms what is going on.

Thanks,

-Scott

One operation the view matrix does for the graphics pipeline is that it converts a 3d point from world space (the x, y, z, we all know & love) into view (or camera) space, a space where the camera is considered to be the center of the world (0,0,0) and all points/objects are relative to it. So while a point may be at 1,1,1 relative to the world, what are it's cordinates relative to the camera location? Well, as it turns out, to find out, you can transform that point by the inverse of a matrix representing the camera's world space position/rotation.

It kinda makes sense if you think about it... let's say the camera position is 2,2,2. An arbitrary point is at 3,3,3. We know that the point is 1,1,1 away from the camera, right? so what transformation would you apply to the point 3,3,3 in order for it to become 1,1,1 (it's location relative to the camera)? you would transform 3,3,3 by -2,-2,-2 to result in 1,1,1. -2,-2,-2 is also the camera's inverted position. That example was for translation because it is relatively easy to groc but basically the same happens for rotation. But don't expect to be able to simply negate all basis vectors to invert a matrix... there is a little more going on with that for rotation.

The Matrix.CreateLookAt() method automatically returns the inverted matrix so you don't really notice it happening unless you reflect its code.

Taking that one step further, the Projection matrix then takes that point in view space and projects it onto a flat surface and that point that started out in 3d space is now in 2d space.