Dear Paulo,
thank you for your helpful advices. In the end, I rotate slices using vtkTransform and vtkMatrix4x4. It change the cosines matrix by more comfortable way, But, your advice and link to Wolfram-Alpha are very helpful for me. I’ve learned many new things. Thank you once more!
Now, I need to solve another problem with the slice position. I’ve created separate thread: https://discourse.vtk.org/t/problem-with-the-vtkimagereslice-position-help-please/8202
Have a nice days,
Jirka