I have a 2D square in a ViewPort3D that I want to do a tiling of an image (like a checkerboard or flooring with "tiles" effect).
I've created an image brush (the image is 50x50 pixels, the surface 250x550 pixels) and a viewport (trying to follow MS's site - though their example is for 2D), but only 1 of the colors in the "tile" image shows up and no tiling is seen. I can't find a single example on the Internet and MS's site has no info (that I can find) on 3D XAML anywhere, so I'm stumped as how to actually do this.

<Viewport3D>
        <Viewport3D.Camera>
            <PerspectiveCamera Position="125,790,120" LookDirection="0,-.7,-0.25"  UpDirection="0,0,1" />
        </Viewport3D.Camera>
        <ModelVisual3D>
            <ModelVisual3D.Content>
                <Model3DGroup>
                    <AmbientLight Color="white" />
                    <GeometryModel3D>
                        <GeometryModel3D.Geometry>
                            <MeshGeometry3D Positions="0,0,0 250,0,0 250,550,0 0,550,0 " TriangleIndices="0 1 3 1 2 3  "/>
                        </GeometryModel3D.Geometry>
                        <GeometryModel3D.Material>                               
                            <DiffuseMaterial>
                                <DiffuseMaterial.Brush>
                                    <ImageBrush ViewportUnits="Absolute" TileMode="Tile" ImageSource="testsquare.gif" Viewport="0,0,50,50" Stretch="None" ViewboxUnits="Absolute" />
                                </DiffuseMaterial.Brush> 
                            </DiffuseMaterial>
                        </GeometryModel3D.Material>
                    </GeometryModel3D>
                </Model3DGroup>
            </ModelVisual3D.Content>
        </ModelVisual3D>
    </Viewport3D>