2PP Quadrilateral From Viewpoint v2
What is going on here?
The quadrilateral in the right window represents the top-down projection of the perspective view, determined by the position of the viewpoint (EYE). The viewpoint is restricted to the semicircle passing through the two vanishing points (viewing semicircle) and tries to keep the the quadrilateral within the 60° cone of vision, producing an undistorted image (meaning the perspective image will resemble its top-down view, as shown in the right window).