Thanks to ilusiondigital, Strannik, cosmic_fool, Zork, monstermaker and AztecSolja for comments.
Houdini: four pass rendering means that you render four different images (different shader and/or different Light)
and then compose these images in a composing package like Photoshop.
spasmodic_cheese: sorry, in the moment I have no time to make an indepth tutorial. But I keep it in mind.
NLightUK: You have right, I am a great fan from RM and I think I have all his books. And from time to time I take a look.
But during the work on the fungus I havenāt see any picture from RM. Maybe it was in my mind. I hope Iām far enough away
with my fungus, if not, itās only a fun work not for commercial use.
Ralf Stumpf