[vtkusers] problem when using the vtkImageGradient

[diaoxianfen]

Hello every one,
     I want to display an isoSurface with the gradient representing by arrows. I referred to the example of  "...\VTK-4.4-LatestRelease\VTK\Examples\ParallelProcessing\Generic\Cxx\task2.cxx". My image volume is a binary image. The voxel value is 1 or 0.  And I computed the gradient magnitude to see the range of the gradient magnitude.The maximum gradient magnitude is three. 
I only want to visualize those non-zero gradients. How can I achieve that?

Thanks!

Xianfen Diao
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