Question

I am trying to make an easy demo where I have a cube in the space and a first person camera that moves around. I made similar thing in the past in c++ & opengl.

My problem is:

• I can position my cube in any place in the space with no problem
• I can create the camera and move
• But when I create both at the same time, the cube is always in the same position relative to camera.

I suspect the problem is with world matrix. Camera is manipulating matrix and affecting cube. In OpenGL I used glPushMatrix and glPopMatrix so Matrix were stored and not affected.

//Draw Cube in openGL
    glPushMatrix();
        glTranslatef(1000,0,1000);
        glRotatef(90,0,1,0);
        DrawCube();
    glPopMatrix();

• How can I achieve similar funcionality to Push/Pop Matrix in XNA?

• Do you know any similar demo (first camera + object) I can look? (I am using MSDN Microsoft tutorials but most of links are broken 🙁 )

You can see what I have tried so far. I have pasted relevant methods.

    protected override void Update(GameTime gameTime)
    {
        // Allows the game to exit
        if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
            this.Exit();

        float timeDifference = (float)gameTime.ElapsedGameTime.TotalMilliseconds / 1000.0f;
        ProcessInput(timeDifference);

        base.Update(gameTime);
    }
    protected override void Draw(GameTime gameTime)
    {
        GraphicsDevice.Clear(Color.CornflowerBlue);

        //Text
        DrawText();

        //CUBE
        // Set the World matrix which defines the position of the cube
        cubeEffect.World = Matrix.CreateTranslation(modelPosition);
        // Set the View matrix which defines the camera and what it's looking at
        cubeEffect.View = Matrix.CreateLookAt(cameraPositionCube, modelPosition, Vector3.Up);
        // Set the Projection matrix which defines how we see the scene (Field of view)
        cubeEffect.Projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.PiOver4, aspectRatio, 1.0f, 1000.0f);
        // Enable textures on the Cube Effect. this is necessary to texture the model
        cubeEffect.TextureEnabled = true;
        cubeEffect.Texture = cubeTexture;
        // Enable some pretty lights
        cubeEffect.EnableDefaultLighting();
        // apply the effect and render the cube
        foreach (EffectPass pass in cubeEffect.CurrentTechnique.Passes)
        {
            pass.Apply();
            cubeToDraw.RenderToDevice(GraphicsDevice);
        }

        base.Draw(gameTime);
    }

    private void UpdateViewMatrix()
    {
        Matrix cameraRotation = Matrix.CreateRotationX(updownRot) * Matrix.CreateRotationY(leftrightRot);

        Vector3 cameraOriginalTarget = new Vector3(0, 0, -1);
        Vector3 cameraRotatedTarget = Vector3.Transform(cameraOriginalTarget, cameraRotation);
        Vector3 cameraFinalTarget = cameraPosition + cameraRotatedTarget;

        Vector3 cameraOriginalUpVector = new Vector3(0, 1, 0);
        Vector3 cameraRotatedUpVector = Vector3.Transform(cameraOriginalUpVector, cameraRotation);

        viewMatrix = Matrix.CreateLookAt(cameraPosition, cameraFinalTarget, cameraRotatedUpVector);
    }

    private void ProcessInput(float amount)
    {
        MouseState currentMouseState = Mouse.GetState();
        if (currentMouseState != originalMouseState)
        {
            float xDifference = currentMouseState.X - originalMouseState.X;
            float yDifference = currentMouseState.Y - originalMouseState.Y;
            leftrightRot -= rotationSpeed * xDifference * amount;
            updownRot -= rotationSpeed * yDifference * amount;
            Mouse.SetPosition(device.Viewport.Width / 2, device.Viewport.Height / 2);
            UpdateViewMatrix();
        }

        Vector3 moveVector = new Vector3(0, 0, 0);
        KeyboardState keyState = Keyboard.GetState();
        if (keyState.IsKeyDown(Keys.Up) || keyState.IsKeyDown(Keys.W))
            moveVector += new Vector3(0, 0, -1);
        if (keyState.IsKeyDown(Keys.Down) || keyState.IsKeyDown(Keys.S))
            moveVector += new Vector3(0, 0, 1);
        if (keyState.IsKeyDown(Keys.Right) || keyState.IsKeyDown(Keys.D))
            moveVector += new Vector3(1, 0, 0);
        if (keyState.IsKeyDown(Keys.Left) || keyState.IsKeyDown(Keys.A))
            moveVector += new Vector3(-1, 0, 0);
        if (keyState.IsKeyDown(Keys.Q))
            moveVector += new Vector3(0, 1, 0);
        if (keyState.IsKeyDown(Keys.Z))
            moveVector += new Vector3(0, -1, 0);
        AddToCameraPosition(moveVector * amount);
    }

    private void AddToCameraPosition(Vector3 vectorToAdd)
    {
        Matrix cameraRotation = Matrix.CreateRotationX(updownRot) * Matrix.CreateRotationY(leftrightRot);
        Vector3 rotatedVector = Vector3.Transform(vectorToAdd, cameraRotation);
        cameraPosition += moveSpeed * rotatedVector;
        UpdateViewMatrix();
    }

Answer 1

Your UpdateViewMatrix() creates a matrix, which is never used. Use this matrix as the effect’s view matrix instead of creating a new one in Draw().