VB.NET中屏幕坐标如何转换为世界坐标

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directx 9,左手坐系，VB2010,部分代码如下。
请问如何转换

Private Sub SetupCamera()
        '设置投影转换矩阵
        Dim fieldOfView As Single = Math.PI / 4
        Dim aspectRatio As Single = Me.Width / Me.Height
        Dim nearPlane As Single = 2.0F
        Dim farPlane As Single = 500.0F
        device.Transform.Projection = Matrix.PerspectiveFovLH(fieldOfView, aspectRatio, nearPlane, farPlane)
 
 
        '设置视图转换矩阵
        Select Case CameraDirect
            Case 0
                Dim cameraPosition As New Vector3(20, 20, -20.0F)
                Dim cameraTarget As New Vector3(0, 0, 0)
                Dim upDirection As New Vector3(0, 1, 0)
                device.Transform.View = Matrix.LookAtLH(cameraPosition, cameraTarget, upDirection)
            Case 1
                Dim cameraPosition As New Vector3(-20, -20, 20.0F)
                Dim cameraTarget As New Vector3(0, 0, 0)
                Dim upDirection As New Vector3(0, -1, 0)
                device.Transform.View = Matrix.LookAtLH(cameraPosition, cameraTarget, upDirection)
        End Select
        device.RenderState.Lighting = False '设置打开光照
        device.RenderState.CullMode = IIf(enableCullMode, Cull.CounterClockwise, Cull.None) '设置剔除背影
        device.RenderState.FillMode = IIf(enableSolidMode, FillMode.Solid, FillMode.WireFrame) '设置填充模式
        device.RenderState.Ambient = Color.White
        device.Lights(0).Range = 150.0F
    End Sub