Direction of view for two point perspective with DVP
Given a two-point perspective setup with an associated auxiliary vanishing point, this construction locates the station point, direction of view, viewpoint and circle of view using the inscribed angle theorem. The two VPs represent an angle α in world space. The auxiliary VP and VP1 represent an auxiliary angle in world space. If the auxiliary angle is α/2, the auxiliary VP is the diagonal VP (DVP) of the two-point perspective setup. DV is the direction of view or center of vision. Activate the check box if the angle at P in image space is identical to the angle in world space.