It was produced with a script analysing a huge ASCII file, and which has drawn the histogram following the use of the transfer_function which made the empty bins transparent. Then I wrote the 3D histogram to a another root file, so I can open it directly, without reading the huge ASCII file again. But it had already used the transfer function method which I got from post https://root-forum.cern.ch/t/th3-and-glcolz-option/14604