Editing Vtk/SimpleCubeTutorial (section)

=WarpScalar=
In an effort to figure out how to display the free-energy surface of self-gravitating fluid systems, I decided to see if I could combine what I had just learned about creating (and reading in) XML-formatted data structures with the capabilities of the ''vtkWarpScalar'' module.  The  capabilities of this "warping" module are illustrated by the VisTrails (and VTK) ''warpComb.vt'' example:
<div align="center">
.../VisTrails/examples/vtk_examples/VisualizationAlgorithms/warpComb.vt
</div>
(See also the ''imageWarp.vt'' vistrail in the same vtk_examples directory.)  

<div align="left">
<pre>
<?xml version="1.0"?>
<VTKFile type="PolyData" version="0.1" byte_order="LittleEndian">
  <PolyData>
    <Piece NumberOfPoints="36" NumberOfPolys="25">
      <CellData Scalars="cell_scalars" Normals="cell_normals">
        <DataArray type="Float32" Name="cell_scalars" format="ascii">
          0.2 0.2 0.2 0.2 0.2
          0.2 0.4 0.6 0.4 0.2
          0.2 0.6 0.8 0.6 0.2
          0.2 0.4 0.6 0.4 0.2
          0.2 0.2 0.2 0.2 0.2
        </DataArray>
        <DataArray type="Float32" Name="cell_normals" NumberOfComponents="3" format="ascii">
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1
        </DataArray>
      </CellData>
      <PointData Scalars="colorful" Normals="magnify">
        <DataArray type="Float32" Name="colorful" format="ascii">
          0.2 0.2 0.2 0.2 0.2 0.2
          0.2 0.4 0.6 0.6 0.4 0.2
          0.2 0.6 0.8 0.8 0.6 0.2
          0.2 0.6 0.8 0.8 0.6 0.2
          0.2 0.4 0.6 0.6 0.4 0.2
          0.2 0.2 0.2 0.2 0.2 0.2
        </DataArray>
        <DataArray type="Float32" Name="magnify" NumberOfComponents="3" format="ascii">
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1  0 0 1
          0 0 1   0 0 1   0 0 1   0 0 1  0 0 1  0 0 1
        </DataArray>
      </PointData>
      <Points>
        <DataArray type="Float32" NumberOfComponents="3" format="ascii">
         0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0
         0 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5 1 0
         0 2 0 1 2 0 2 2 0 3 2 0 4 2 0 5 2 0
         0 3 0 1 3 0 2 3 0 3 3 0 4 3 0 5 3 0
         0 4 0 1 4 0 2 4 0 3 4 0 4 4 0 5 4 0
         0 5 0 1 5 0 2 5 0 3 5 0 4 5 0 5 5 0
        </DataArray>
      </Points>
      <Polys>
        <DataArray type="Int32" Name="connectivity" format="ascii">
           0  1  7  6   1  2  8  7   2  3  9  8   3  4 10  9   4  5 11 10
           6  7 13 12   7  8 14 13   8  9 15 14   9 10 16 15  10 11 17 16
          12 13 19 18  13 14 20 19  14 15 21 20  15 16 22 21  16 17 23 22
          18 19 25 24  19 20 26 25  20 21 27 26  21 22 28 27  22 23 29 28
          24 25 31 30  25 26 32 31  26 27 33 32  27 28 34 33  28 29 35 34
        </DataArray>
        <DataArray type="Int32" Name="offsets" format="ascii">
          4  8 12 16 20 24 28 32 36 40
         44 48 52 56 60 64 68 72 76 80
         84 88 92 96 100
        </DataArray>
      </Polys>
    </Piece>
  </PolyData>
</VTKFile>
</pre>
</div>

1.  First, I constructed the new, XML-formatted input data file shown immediately above.  The configuration described by this data file has the following key features:
* As is detailed in the "Points" DataArray, the configuration has a structured grid containing 6 x 6 = 36 vertices. The ''z'' coordinate of all 36 points is zero, which means that the configuration is a plane of zero thickness that is lying in the <math>x-y</math> plane. 
* The scalar values assigned to these 36 points/vertices are provided in the "PointData" DataArray named "colorful."
* As is detailed in the pair of "Polys" DataArrays, the grid points are connected with one another, four at a time, to create 5 x 5 = 25 cells.
* The scalar values assigned to these 25 cells are provided in the "CellData" DataArray named "cell_scalars."

2.  Next, I read this XML_formatted data file into the VisTrails pipeline discussed above &#8212; specifically, the one displayed above as "Pipeline02" &#8212; to convince myself that the configuration would be rendered as a multicolored plane.

3.  Finally, as is illustrated below in the figure labeled "Pipeline03," I inserted the "vtkWarpScalar" module into the VisTrails pipeline between the XMLPolyData reader and the PolyData Mapper, and activated its ''SetScaleFactor" method with a value of 2.0.   Executing this pipeline generated the image in the upper-right whose caption reads, "Warp (Point Data) Image."

4.  The image in the lower-right whose caption reads, "Warp (Cell Data) Image," was generated by activating the ''SetScalarModeToUseCellData'' method in the "vtkPolyDataMapper" module.

<div align="center">
<table border="2" align="center">
<tr>
  <td align="center" rowspan="2">
[[Image:WarpScalarPipeline.png|right|thumb|500px|Pipeline03]]
  </td>
  <td align="center">
[[Image:WarpScalarImage.png|right|thumb|200px|Warp (Point Data) Image]]
  </td>
</tr>
<tr>
  <td align="center">
[[Image:WarpScalarCellDataImage.png|right|thumb|200px|Warp (Cell Data) Image]]
  </td>
</tr>
</table>
</div>


The following excerpt from &sect;5.1 (specifically, p. 77) of the [http://www.kitware.com/products/books/vtkguide.html ''VTK User's Guide''] explains why the two images shown on the right-hand side of the above figure have very different color patterns:  
<table border="0" cellpadding="8" align="center" width="80%">
<tr><td align="left">
"Another important feature of ''vtkMapper'' is controlling which attribute data (i.e., point or cell scalars, or a general data array) is used to color objects."  "Note that [different methods within the ''vtkMapper'' module] give strikingly different results:  point attribute data is interpolated across rendering primitives during the rendering process, whereas cell attribute data colors the cell a constant value."
</td></tr>
</table>