/ic/ - Artwork/Critique

I'm working through Robertson's HtD but I don't think he addresses this specific question. Let's say in the left drawing that lines A and B are the edges of a road on the ground and the viewer is standing in the middle of the road looking straight ahead. Then the line C that is perpendicular to A and B and cuts the road straight across is just parallel to the horizon line. But in the right diagram, where we're viewing the road from a different angle, how would I construct the equivalent of line C from the left diagram? Basically if I have a vanishing point and some parallel lines in the same plane converging to that point, how do I find a line in that same plane perpendicular to those parallel lines?