Editing Vtk/SimpleCubeTutorial (section)

===Comment Lines and Deleting XML Pieces===
Two major advantages of XML-formatted input data files is the ability to segment the data &#8212; and rendered elements of a scene &#8212; into separate ''Pieces'' and, then, the ability to "comment out" one or more of these pieces when you want to simplify the scene.  The following ASCII data file illustrates how the ''Simple Cube'' can be subdivided into a pair of 3-sided figures and how, by simply inserting or deleting a comment line, the two ''Pieces'' can be rendered together or separately. 

<div align="left">
<pre>
<?xml version="1.0"?>
<VTKFile type="PolyData" version="0.1" byte_order="LittleEndian" compressor="vtkZLibDataCompressor">
  <PolyData>
<!-- This is three of the sides of the cube -->
    <Piece NumberOfPoints="8" NumberOfPolys="3">
      <CellData Scalars="temperature" Normals="cell_normals">
        <DataArray type="Float32" Name="temperature" format="ascii">
          0.14286 0.28571 0.42857
        </DataArray>
        <DataArray type="Int32" Name="cell_normals" NumberOfComponents="3" format="ascii">
          0 0 1  0 0 1  0 -1 0
        </DataArray>
      </CellData>
      <Points>
        <DataArray type="Float32" NumberOfComponents="3" format="ascii">
          0 0 0 1 0 0
          1 1 0 0 1 0
          0 0 1 1 0 1
          1 1 1 0 1 1
        </DataArray>
      </Points>
      <Polys>
        <DataArray type="Int32" Name="connectivity" format="ascii">
          0 1 2 3   4 5 6 7   0 1 5 4
        </DataArray>
        <DataArray type="Int32" Name="offsets" format="ascii">
          4 8 12
        </DataArray>
      </Polys>
    </Piece>
<!-- This is the other three sides of the cube
    <Piece NumberOfPoints="8" NumberOfPolys="3">
      <CellData Scalars="temperature" Normals="cell_normals">
        <DataArray type="Float32" Name="temperature" format="ascii">
          0.57143 0.71429 0.85714
        </DataArray>
        <DataArray type="Int32" Name="cell_normals" NumberOfComponents="3" format="ascii">
          0 1 0  -1 0 0  1 0 0
        </DataArray>
      </CellData>
      <Points>
        <DataArray type="Float32" NumberOfComponents="3" format="ascii">
          0 0 0 1 0 0
          1 1 0 0 1 0
          0 0 1 1 0 1
          1 1 1 0 1 1
        </DataArray>
      </Points>
      <Polys>
        <DataArray type="Int32" Name="connectivity" format="ascii">
          2 3 7 6    0 4 7 3   1 2 6 5
        </DataArray>
        <DataArray type="Int32" Name="offsets" format="ascii">
          4 8 12
        </DataArray>
      </Polys>
    </Piece>
-->
  </PolyData>
</VTKFile>
</pre>
</div>

<font color="blue">'''General Explanation:''' </font>
* The fourth line of the above ASCII data file is a single, standard XML-formatted comment line &#8212; see the explanation provided, for example, on the [http://en.wikipedia.org/wiki/XML#Comments Wikipedia page that explains XML comment tags].  By XML convention, it begins with the string "<math>~< !--</math>" and ends with the string "<math>~--></math>".  The same beginning and ending strings also appear, respectively, as lines 31 and 58 in the data file.  In the first case, because the two strings appear on the same line of the file, they simply serve to encapsulate a short comment.  In the second case, all of the ASCII text that falls between lines 31 and 58 are considered part of the "comment" and therefore the second ''Piece'' of the cube is ignored by the ''XMLPolyDataReader''.  As presented here, this input data file will generate only three sides of the cube.  The entire cube will be rendered if line 58 is deleted and the string "<math>~--></math>" is added to the end of line 31.