How to model N threads doing same thing with activity diagram?

Question

I want to draw uml-correct activity diagram representing process of my raytracer. I know I should use black rectangles to model fork/join. But in my application I spawn N threads doing the same thing (which is not simple and will be modeled via multiple activity elements). How can I draw such activity diagram without having the same thing without knowing number of threads?

My explanation is poor, image may help understand what I want to model with activity diagram

Answer 1

You can use the expansion region element.