Wow, that’s a blast from the past. Yes, I believe the filter you are looking for is vtkImplicitModeller (using append mode). It appears to be threaded. Yes, the algorithm can be quite slow as it is potentially accumulating a lot of data. Once the append process completes you should have a 3D volume that you can isocontour. I recommend that you use vtkFlyingEdges3D as it is fast. Make sure you are using multiple threads etc.