@catherineirkalla : “Like I said I don’t know for sure that Cycles is directly using it or is directly based on it. Its quite possible that there is an abstraction layer in between allowing both GLSL and other implementations under the hood, “
I do not know either (yet) but will get to the bottom of this later in 2019.
I’m planning to do a little C, C++ OpenGL coding myself, just to be able fully understand the stuff under the Blender hood.
