To use this filter you must specify one or more contour values. A scalar tree is used to quickly locate cells that contain a contour surface. This is especially effective if multiple contours are being extracted. This calculation will be implemented in the future. In the mean time, use vtkPolyDataNormals to compute the surface normals.
Normal computation is failrly expensive in both time and storage.
If the output data will be processed by filters that modify topology or geometry, it may be wise to turn Normals and Gradients off. Gradient computation is fairly expensive in both time and storage. Note that if ComputeNormals is on, gradients will have to be calculated, but will not be stored in the output dataset. If the output data will be processed by filters that modify topology or geometry, it may be wise to turn Normals and Gradients off. Enable the use of a scalar tree to accelerate contour extraction.