So as a learning process I'm trying to create some matrix functions to use in place of the XNA Math functions in DirectX.

Right now I'm stuck on MatrixLookAtLH which would replace XMMatrixLookAtLH.

Here is my function:

inline void MatrixLookAt( Matrix4F& matrixOut, Vector3F& eye, Vector3F& at, Vector3F& up )
{
    Vector3F zAxis( at.x() - eye.x(), at.y() - eye.y(), at.z() - eye.z() );
    VectorNormalize( zAxis );

    Vector3F xAxis = VectorCrossProduct( up, zAxis );
    VectorNormalize( xAxis );

    Vector3F yAxis = VectorCrossProduct( zAxis, xAxis );
    VectorNormalize(yAxis);


    MatrixSetRow( matrixOut, 0, xAxis );
    MatrixSetRow( matrixOut, 1, yAxis );
    MatrixSetRow( matrixOut, 2, zAxis );

    Vector3F wAxis( -VectorDotProduct(xAxis,eye), -VectorDotProduct(yAxis,eye), -VectorDotProduct(zAxis,eye) );
    MatrixSetRow( matrixOut, 3, wAxis );
}

And here is some code that calls it and the XM function:

   XMVECTOR Eye = XMVectorSet( 0.0f, 3.0f, -6.0f, 0.0f );
    XMVECTOR At = XMVectorSet( 0.0f, 1.0f, 0.0f, 0.0f );
    XMVECTOR Up = XMVectorSet( 0.0f, 1.0f, 0.0f, 0.0f );

    Vector3F Eye2 = Vector3F(0.0f, 3.0f, -6.0f);
    Vector3F At2 = Vector3F(0.0f, 1.0f, 0.0f);
    Vector3F Up2 = Vector3F(0.0f, 1.0f, 0.0f);

    Matrix4F view;
    MatrixLookAt(view, Eye2, At2, Up2);

    XMMATRIX xmView = XMMatrixLookAtLH( Eye, At, Up );
 

The results of the functionis are simiar but not exact.

This is the matrix "xmView" after calling the XNA XMMatrixLookAtLH function:

1 0 0 0
0 0.94868326 -0.31622776 0
0 0.31622776 0.94868326 0
0 -0.94868326 6.6407828 1

And this is the matrix "view" after calling my MatrixLookAtLH function:

1 0 0 0

0 0.94868326 0.31622776 0
0 -0.31622776 0.94868326 0
0 -0.94868326 6.6407828 1

As you can see, [1][2] and [2][1] seem to be reversed in the output from my code and everything else is the same.

Is there something fundamental in my calculation that I'm doing wrong, or is it possible that the step that I am missing is just reversing those 2 values?

Thanks!

Brilliant, thanks so much for the answer and the explanation. I created a function to transpose the 3x3 portion of a 4x4 matrix in-place, and it works like a champ now.