3D Towards a Solid

This post starts with the last example from the 3D Rotation post. I want to add a second face of a cube, opposite the original surface. First add a Model3DGroup to contain all the GeometryModel3D elements that will form the cube.

<Grid Background="AliceBlue">
   <Viewport3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>

            <Model3DGroup>

            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, -0.5  0.5, -0.5, -0.5  0.5, 0.5, -0.5  -0.5, 0.5, -0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>0 1 2 2 3 0</MeshGeometry3D.TriangleIndices>
                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>

            </Model3DGroup>

         </ModelVisual3D.Content>
         <ModelVisual3D.Transform>
            <RotateTransform3D>
               <RotateTransform3D.Rotation>
                  <AxisAngleRotation3D  x:Name="Rotate">
                     <AxisAngleRotation3D.Axis>0,1,0</AxisAngleRotation3D.Axis>
                     <AxisAngleRotation3D.Angle>0</AxisAngleRotation3D.Angle>
                  </AxisAngleRotation3D>
               </RotateTransform3D.Rotation>
            </RotateTransform3D>
         </ModelVisual3D.Transform>
      </ModelVisual3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <AmbientLight Color="White" />
         </ModelVisual3D.Content>
      </ModelVisual3D>
      <Viewport3D.Camera>
         <PerspectiveCamera>
            <PerspectiveCamera.LookDirection>0,0,-1</PerspectiveCamera.LookDirection>
            <PerspectiveCamera.Position>0,0,2</PerspectiveCamera.Position>
            <PerspectiveCamera.FieldOfView>90</PerspectiveCamera.FieldOfView>
         </PerspectiveCamera>
      </Viewport3D.Camera>
   </Viewport3D>
   <Grid.Triggers>
      <EventTrigger RoutedEvent="Grid.MouseDown">
         <BeginStoryboard>
            <Storyboard>
               <DoubleAnimation From="0"
                                To="360"
                                BeginTime="0:0:0"
                                Duration="0:0:4"
                                Storyboard.TargetName="Rotate"
                                Storyboard.TargetProperty="Angle" />
            </Storyboard>
         </BeginStoryboard>
      </EventTrigger>
   </Grid.Triggers>
</Grid>

And add the second surface exactly the same as the first but with the Z coordinates moved from -0.5 to +0.5.

<Grid Background="AliceBlue">
   <Viewport3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <Model3DGroup>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, -0.5  0.5, -0.5, -0.5  0.5, 0.5, -0.5  -0.5, 0.5, -0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>0 1 2 2 3 0</MeshGeometry3D.TriangleIndices>
                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>

            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, +0.5  0.5, -0.5, +0.5  0.5, 0.5, +0.5  -0.5, 0.5, +0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>0 1 2 2 3 0</MeshGeometry3D.TriangleIndices>
                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>

            </Model3DGroup>
         </ModelVisual3D.Content>
         <ModelVisual3D.Transform>
            <RotateTransform3D>
               <RotateTransform3D.Rotation>
                  <AxisAngleRotation3D  x:Name="Rotate">
                     <AxisAngleRotation3D.Axis>0,1,0</AxisAngleRotation3D.Axis>
                     <AxisAngleRotation3D.Angle>0</AxisAngleRotation3D.Angle>
                  </AxisAngleRotation3D>
               </RotateTransform3D.Rotation>
            </RotateTransform3D>
         </ModelVisual3D.Transform>
      </ModelVisual3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <AmbientLight Color="White" />
         </ModelVisual3D.Content>
      </ModelVisual3D>
      <Viewport3D.Camera>
         <PerspectiveCamera>
            <PerspectiveCamera.LookDirection>0,0,-1</PerspectiveCamera.LookDirection>
            <PerspectiveCamera.Position>0,0,2</PerspectiveCamera.Position>
            <PerspectiveCamera.FieldOfView>90</PerspectiveCamera.FieldOfView>
         </PerspectiveCamera>
      </Viewport3D.Camera>
   </Viewport3D>
   <Grid.Triggers>
      <EventTrigger RoutedEvent="Grid.MouseDown">
         <BeginStoryboard>
            <Storyboard>
               <DoubleAnimation From="0"
                                To="360"
                                BeginTime="0:0:0"
                                Duration="0:0:4"
                                Storyboard.TargetName="Rotate"
                                Storyboard.TargetProperty="Angle" />
            </Storyboard>
         </BeginStoryboard>
      </EventTrigger>
   </Grid.Triggers>
</Grid>

We can now see two Maroon/Blue surfaces rotating but we always see the same colour for both surfaces. The original surface needs to point "out" from the centre of the cube. The easiest way would appear to be drawing the Triangles in a different order so their Normals are reversed. Swap 0 and 1, swap 2 and 3 in the TriangleIndices list.

<Grid Background="AliceBlue">
   <Viewport3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <Model3DGroup>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, -0.5  0.5, -0.5, -0.5  0.5, 0.5, -0.5  -0.5, 0.5, -0.5</MeshGeometry3D.Positions>

                     <MeshGeometry3D.TriangleIndices>1 0 3 3 2 1</MeshGeometry3D.TriangleIndices>

                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, +0.5  0.5, -0.5, +0.5  0.5, 0.5, +0.5  -0.5, 0.5, +0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>0 1 2 2 3 0</MeshGeometry3D.TriangleIndices>
                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>
            </Model3DGroup>
         </ModelVisual3D.Content>
         <ModelVisual3D.Transform>
            <RotateTransform3D>
               <RotateTransform3D.Rotation>
                  <AxisAngleRotation3D  x:Name="Rotate">
                     <AxisAngleRotation3D.Axis>0,1,0</AxisAngleRotation3D.Axis>
                     <AxisAngleRotation3D.Angle>0</AxisAngleRotation3D.Angle>
                  </AxisAngleRotation3D>
               </RotateTransform3D.Rotation>
            </RotateTransform3D>
         </ModelVisual3D.Transform>
      </ModelVisual3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <AmbientLight Color="White" />
         </ModelVisual3D.Content>
      </ModelVisual3D>
      <Viewport3D.Camera>
         <PerspectiveCamera>
            <PerspectiveCamera.LookDirection>0,0,-1</PerspectiveCamera.LookDirection>
            <PerspectiveCamera.Position>0,0,2</PerspectiveCamera.Position>
            <PerspectiveCamera.FieldOfView>90</PerspectiveCamera.FieldOfView>
         </PerspectiveCamera>
      </Viewport3D.Camera>
   </Viewport3D>
   <Grid.Triggers>
      <EventTrigger RoutedEvent="Grid.MouseDown">
         <BeginStoryboard>
            <Storyboard>
               <DoubleAnimation From="0"
                                To="360"
                                BeginTime="0:0:0"
                                Duration="0:0:4"
                                Storyboard.TargetName="Rotate"
                                Storyboard.TargetProperty="Angle" />
            </Storyboard>
         </BeginStoryboard>
      </EventTrigger>
   </Grid.Triggers>
</Grid>

Finally, add the remaining two vertical surfaces of the cube. The "left" surface uses the same TriangleIndices as the "front" whereas the "right" surface uses the same TriangleIndices as the "back".

<Grid Background="AliceBlue">
   <Viewport3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <Model3DGroup>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, -0.5  0.5, -0.5, -0.5  0.5, 0.5, -0.5  -0.5, 0.5, -0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>1 0 3 3 2 1</MeshGeometry3D.TriangleIndices>
                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>
                     <MeshGeometry3D.Positions>-0.5, -0.5, +0.5  0.5, -0.5, +0.5  0.5, 0.5, +0.5  -0.5, 0.5, +0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>0 1 2 2 3 0</MeshGeometry3D.TriangleIndices>
                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>

                     <MeshGeometry3D.Positions>-0.5, -0.5, -0.5  -0.5, -0.5, 0.5  -0.5, 0.5, 0.5  -0.5, 0.5, -0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>0 1 2 2 3 0</MeshGeometry3D.TriangleIndices>

                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>
            <GeometryModel3D>
               <GeometryModel3D.Geometry>
                  <MeshGeometry3D>

                     <MeshGeometry3D.Positions>0.5, -0.5, -0.5  0.5, -0.5, 0.5  0.5, 0.5, 0.5  0.5, 0.5, -0.5</MeshGeometry3D.Positions>
                     <MeshGeometry3D.TriangleIndices>1 0 3 3 2 1</MeshGeometry3D.TriangleIndices>

                  </MeshGeometry3D>
               </GeometryModel3D.Geometry>
               <GeometryModel3D.Material>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Maroon</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.Material>
               <GeometryModel3D.BackMaterial>
                  <DiffuseMaterial>
                     <DiffuseMaterial.Brush>Blue</DiffuseMaterial.Brush>
                  </DiffuseMaterial>
               </GeometryModel3D.BackMaterial>
            </GeometryModel3D>
            </Model3DGroup>
         </ModelVisual3D.Content>
         <ModelVisual3D.Transform>
            <RotateTransform3D>
               <RotateTransform3D.Rotation>
                  <AxisAngleRotation3D  x:Name="Rotate">
                     <AxisAngleRotation3D.Axis>0,1,0</AxisAngleRotation3D.Axis>
                     <AxisAngleRotation3D.Angle>0</AxisAngleRotation3D.Angle>
                  </AxisAngleRotation3D>
               </RotateTransform3D.Rotation>
            </RotateTransform3D>
         </ModelVisual3D.Transform>
      </ModelVisual3D>
      <ModelVisual3D>
         <ModelVisual3D.Content>
            <AmbientLight Color="White" />
         </ModelVisual3D.Content>
      </ModelVisual3D>
      <Viewport3D.Camera>
         <PerspectiveCamera>
            <PerspectiveCamera.LookDirection>0,0,-1</PerspectiveCamera.LookDirection>
            <PerspectiveCamera.Position>0,0,2</PerspectiveCamera.Position>
            <PerspectiveCamera.FieldOfView>90</PerspectiveCamera.FieldOfView>
         </PerspectiveCamera>
      </Viewport3D.Camera>
   </Viewport3D>
   <Grid.Triggers>
      <EventTrigger RoutedEvent="Grid.MouseDown">
         <BeginStoryboard>
            <Storyboard>
               <DoubleAnimation From="0"
                                To="360"
                                BeginTime="0:0:0"
                                Duration="0:0:4"
                                Storyboard.TargetName="Rotate"
                                Storyboard.TargetProperty="Angle" />
            </Storyboard>
         </BeginStoryboard>
      </EventTrigger>
   </Grid.Triggers>
</Grid>