XNA WorldMatrix and ViewMatrix

I have created a Camera class that allows me to move around a scene in first person. The camera has worked just fine until I decided to use it as a location to add something to the 3D world. What I am trying to do is add a cube to the world when I press a mouse button. I want to cube to eventually travel away from the camera, but for now I just want to create it right in front of it. Sometimes it works and sometimes it creates it to one side or the other. It all depends on how much I've rotated and translated the camera.

I am tryinng to find the vector in front of my camera by using the View Matrix like so:

Vector3 inFront = Camera.ViewMatrix.Forward;

I plan to use the vector to add some physics behind the cube and have it travel away from the camera. For now I am ju开发者_开发问答st wanting to get a correct Vector.

I know you normally draw thing in the world using the WorldMatrix, but I can't figure out how to convert my ViewMatrix into a WorldMatrix. Still learing :-)

What am I doing wrong?

-Scott

First of all, there is no real difference between a "World Matrix" and a "View Matrix", they are both transformation matrices and the distinction is somewhat arbitrary. Some systems even combine the two (OpenGL simply has a "ModelView" matrix).

Traditionally the "world matrix" is used to move individual models from "model space" to "world space". Then the "view matrix" is used to move all the models from world space into their relative positions in front of the camera (which, in effect, "moves the camera"). And finally the "Projection Matrix" converts the 3D positions into their 2D positions on the screen (generally with a perspective projection). Because they are matrices, they can be multiplied together into a single matrix that can transform points in a single step.

First of all, take a look at the properties of the Matrix struct.

What you also need to realise is that Matrix.Forward returns a Vector3. A Vector3 can represent either a position or a scalar and a direction. You need two of them to represent a position and a direction.

Now, my 3D matrix maths is a bit rusty, but I'm pretty sure that what you want is the Matrix.Translation as the position of the camera in world space. And Matrix.Forward as the forward direction of the camera in world space.

Unless your camera/view matrix is performing a scaling operation on the world (and really it shouldn't), then the Vector3 you get back from Matrix.Forward will have unit length - in other words just a direction (no scalar). Use this to give a direction to move your object in.

I assume you have to location of the camera. Have you tried something like this (I haven't done Matrix/Vector math in a few years so this might be off):

float scalar = 10; // how far away from the camera you want to move the object
Vector3 camPos = ???; // supplied from somewhere elese
Vector3 inFront = Camera.ViewMatrix.Forward;
Vector3 newPos = camPos + inFront * scalar;