A 4D interface constructed by Delaunay triangulation
<table border=1>
<tr>
<td> 3D block at time step = n </td> <td> 3D block at time step = n+1 </td>
</tr><tr align=center>
<td>  <img border="0" src="blk_3d_case_0.jpg" width=400>
     <br> <img border="0" src="xyz2.jpg"> </td>
<td>  <img border="0" src="blk_3d_case_7.jpg" width=400> 
     <br> <img border="0" src="xyz2.jpg"> </td>
</tr>
<tr> <td></td> </tr>
<br>

<tr>
<td> The projection of the 4D interface on x-y-z space</td><td>  on y-z-t space</td>
</tr><tr align=center>
<td>  <img border="0" src="test_blk4d_case_3_xyz.jpg" width=400>
     <br> <img border="0" src="xyz2.jpg"> </td>
<td>  <img border="0" src="test_blk4d_case_3_yzt.jpg" width=400>
     <br> <img border="0" src="yzt3.jpg"> </td>
</tr><tr>
<td> on x-z-t space</td><td> on x-y-t space</td>
</tr><tr align=center>
<td>  <img border="0" src="test_blk4d_case_3_ztx.jpg" width=400>
     <br> <img border="0" src="xzt2.jpg"> </td>
<td>  <img border="0" src="test_blk4d_case_3_txy.jpg" width=400> 
     <br> <img border="0" src="xyt.jpg"> </td>
</tr>
</table>
<BR>
Note: 
<br>	1. If the components of two corners are different and these two corner 
will be projected to one corner, we draw this corner in blue color.
<br>	2. For the last 4 pictures, all the faces are in blue color, since they 
are not the 'real' interface. They are only the projection of a 4D interface onto a 3D space.