环境纹理映射中，动态构建立方体纹理是的观察矩阵问题？

为了得到6个观察矩阵，
开始时先：
C/C++ code

D3DXMATRIXA16 matViewDir( g_matView );          
matViewDir._41 = 0.0f; matViewDir._42 = 0.0f; matViewDir._43 = 0.0f;//去掉平移量

然后再根据6个方向创建6个观察矩阵：
C/C++ code

D3DXMATRIX DXUTGetCubeMapViewMatrix( DWORD dwFace )
{
    D3DXVECTOR3 vEyePt   = D3DXVECTOR3( 0.0f, 0.0f, 0.0f );
    D3DXVECTOR3 vLookDir;
    D3DXVECTOR3 vUpDir;

    switch( dwFace )
    {
        case D3DCUBEMAP_FACE_POSITIVE_X:
            vLookDir = D3DXVECTOR3( 1.0f, 0.0f, 0.0f );
            vUpDir   = D3DXVECTOR3( 0.0f, 1.0f, 0.0f );
            break;
        case D3DCUBEMAP_FACE_NEGATIVE_X:
            vLookDir = D3DXVECTOR3(-1.0f, 0.0f, 0.0f );
            vUpDir   = D3DXVECTOR3( 0.0f, 1.0f, 0.0f );
            break;
        case D3DCUBEMAP_FACE_POSITIVE_Y:
            vLookDir = D3DXVECTOR3( 0.0f, 1.0f, 0.0f );
            vUpDir   = D3DXVECTOR3( 0.0f, 0.0f,-1.0f );
            break;
        case D3DCUBEMAP_FACE_NEGATIVE_Y:
            vLookDir = D3DXVECTOR3( 0.0f,-1.0f, 0.0f );
            vUpDir   = D3DXVECTOR3( 0.0f, 0.0f, 1.0f );
            break;
        case D3DCUBEMAP_FACE_POSITIVE_Z:
            vLookDir = D3DXVECTOR3( 0.0f, 0.0f, 1.0f );
            vUpDir   = D3DXVECTOR3( 0.0f, 1.0f, 0.0f );
            break;
        case D3DCUBEMAP_FACE_NEGATIVE_Z:
            vLookDir = D3DXVECTOR3( 0.0f, 0.0f,-1.0f );
            vUpDir   = D3DXVECTOR3( 0.0f, 1.0f, 0.0f );
            break;
    }

    // Set the view transform for this cubemap surface
    D3DXMATRIXA16 mView;
    D3DXMatrixLookAtLH( &mView, &vEyePt, &vLookDir, &vUpDir );
    return mView;
}

最后在：C/C++ code

D3DXMATRIXA16 matView;
D3DXMatrixMultiply( &matView, &matViewDir, &matView );

最后为什么是要把两个观察矩阵相乘？

D3DXMATRIXA16 matView;
D3DXMatrixMultiply( &matView, &matViewDir, &matView );
API所需 - -！