I managed to migrate this on VTK 7.1, but It is pretty slow, then I decided to update the solution to OpenGL2, which might increase dramatically the performance of the renderer.
However, I found that you are working on multiple volumes renderer with vtkGPUVolumeRayCastMapper using OpenGL2.
May I ask you if I can have more information about it:
Did you manage to do something relevant?
Did you encounter any difficulty?
Could you give a rough estimation of the release?