To be honest, Baggins, this stuff is way above my head, but my layman way of imagining how it works makes me think that Vector Displacement might be working like a raytracer, but using different vector calculations instead.  When I look at what it does, especially the overshoots, it must be technically impossible for a light ray to come from above, pass an overshoot, and then still have a means of controlling what is below the overshoot.  In other words, it feels to me like it would be impossible for it to control whatever lies in the 'shadow' of the overshoot.

But it does, so ...

It might be drawing a virtual object around the main object, like a cube for example, and then firing off those vectors you speak of, basically just firing them off your object like reflections, and bouncing them off, say, the inside of an invisible cube, so that when they bounce, they are able to reach the undercuts that Standard Displacement can't reach.  Like shining a torch down a narrow maze, the Standard Displacement would fail due to the overshoots, but Vector Displacement, if it works sort of like a raytracer, might be bouncing vectors around like in a raytracer - just done different.

What you describe is possible if I had a full-blown shading language and I could write such algorithms. Poser doesn't allow that. I have to use the same set of nodes like all the rest of you, and if I want to do something more sophisticated, despite me knowing how to do it, there is no way for me to say it. The node system has dumbed the whole thing down. 

Working with nodes is like trying to bake a pie from scratch, and they gave you a chainsaw, an axe, a bathtub, a welding torch, and two screwdrivers as tools. Try to make a pie with that.

Speaking of that, I was looking at the latest Pixar renderman shading API (not RSL - the full blown API where I can add code in C or C++ or whatever I want) and I could do tons with that one.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)