WPF，3D图形纹理映射，为什么没有正确显示图片呢？

<Grid>
    <Viewport3D Name="myViewport" >
        <Viewport3D.Camera>
            <PerspectiveCamera Position="1,3,2" LookDirection="0,-3,-2" />
        </Viewport3D.Camera>
        <ModelVisual3D>
            <ModelVisual3D.Content>
                <DirectionalLight Color="White" Direction="0,2,0"/>
            </ModelVisual3D.Content>
        </ModelVisual3D>
        <ModelVisual3D>
            <ModelVisual3D.Content>
                <GeometryModel3D>
                    <GeometryModel3D.Geometry>
                        <MeshGeometry3D Positions="0,0,0 0,1,0 2,0,0 2,1,0" TriangleIndices="0,2,1 1,2,3" TextureCoordinates="1,1 0,1 1,0 0,0"/>
                    </GeometryModel3D.Geometry>
                    <GeometryModel3D.Material>
                        <DiffuseMaterial>
                            <DiffuseMaterial.Brush>
                                <ImageBrush ImageSource="6.jpg"/>
                            </DiffuseMaterial.Brush>
                        </DiffuseMaterial>
                    </GeometryModel3D.Material>
                </GeometryModel3D>
            </ModelVisual3D.Content>
        </ModelVisual3D>
    </Viewport3D>
</Grid>

上面代码中，3D矩形，设置了纹理映射，为什么没有显示图片呢？
[解决办法]
我用代码是这样写的
MaterialGroup mat = new MaterialGroup();
ImageBrush imgbrush = new ImageBrush(source);
imgbrush.ViewportUnits = BrushMappingMode.Absolute;
imgbrush.TileMode = TileMode.Tile;
SpecularMaterial specular = new SpecularMaterial(new SolidColorBrush(Colors.White), 60);
DiffuseMaterial diffuseMaterialSolid1 = new DiffuseMaterial(imgbrush);
mat.Children.Add(diffuseMaterialSolid1);
mat.Children.Add(specular);
this._model_solid.Material = mat;

应该差不多吧 仔细检查下吧 不然看看msdn