WPF 3D - 为什么我的ImageBrush不呈现？

I have a 3D plane which I'm trying to render with an ImageBrush as it's DiffuseMaterial.Brush. If i use a SolidColorBrush then the plane appears fine but I get nothing at all with either an ImageBrush or a VisualBrush.I have a 3D plane which I'm trying to render wi