indeed, in this case it’s more related to transient vtkHDF.
Your UnstructuredGrid, with 20 timesteps, will have a NumberOfPoints with a size of 20. So for each timestep you can specify the number of points in this array (so in your case at frame 20, as there is no collision it will be equal to 0) same for NumberOfCells, you can check this data for example:
HDFWriter_transient_cube.hdf.vtkhdf (40.4 KB)