GPU Ray casting with HIDDENCELLS

I am having difficulty in blanking cells from a vtkImageData because my experience shows that there seems to be an offset of one cell in each direction where [some] cells marked HIDDENCELL are rendered nonetheless. The attached Python script demonstrates that. In the extreme case of marking all cells hidden, I still see 3 planes of semi-transparent data corresponding to the Z-max,Y-max and X-max planes. (2.5 KB)