The two pieces presented here are based on the same idea: Pick a closed 2-dimensional curve, project it into a paraboloid of revolution to obtain a 3-dimensional curve. Then join each point of the curve with another one with a straight segment to get a ruled surface.
It is a ruled surface: A 2D closed curve, a hypocycloid of 5 stars, is projected into a paraboloid of revolution to obtain a 3D curve. Each point of the curve is with another point of the same curve with a straight segment. This process generates the ruled surface.