2dcontour.py

class PlotContour(PlotBase):
    
    ... #some other methods generating the simplices plotting them and calling the contour method
    
    @classmethod
    def contour(cls, simplices, ax):
        """
        :param simplices: the list of 3D simplices
        :param ax: the Axes3D object
        """
        
        # inner functionality
        def _compute_2D_outline(outlines):

            if outlines:
                for outline in outlines:
                    try:
                        outline.remove()
                    except ValueError:
                        pass

            proj = ax.get_proj()
            rot = np.vstack(tuple([proj[3, :3] * np.sign(proj[0, 3]),
                                   proj[0, :3] * np.sign(proj[1, 3]),
                                   proj[1, :3] * np.sign(proj[3, 3])]))

            inv_rot = np.linalg.inv(rot)

            poly = None
            for sim in simplices:
                sim2D = np.dot(rot, sim.T)[1:].T
                hull2D = sim2D[ConvexHull(sim2D, qhull_options="Qs").vertices]
                if poly is None:
                    poly = Polygon(hull2D)
                else:
                    poly = poly.union(Polygon(hull2D))

            if type(poly) is Polygon:
                poly = MultiPolygon([poly])
            for p in poly:
                if type(p) is Polygon:
                    contour2D = np.array(p.exterior.coords.xy)
                    contour3D = np.concatenate([[np.zeros(contour2D.shape[1])], contour2D])
                    contour3D_invrot = np.dot(inv_rot, contour3D)

                    outline = Line3DCollection([contour3D_invrot.T],
                                                     alpha=1, colors="b", linewidths=2, zorder=-5)
                    ax.add_collection3d(outline)
                    outlines.append(outline)
            return outlines

        # handler for figure moved
        def _on_move(event):
            if event.button and event.inaxes:
                cls.contour_outlines = _compute_2D_outline(cls.contour_outlines)

        # init
        try:
            cls.contour_outlines
        except AttributeError:
            cls.contour_outlines = []
            ax.get_figure().canvas.mpl_connect("motion_notify_event", _on_move)
        
        # outer 
        cls.contour_outlines = _compute_2D_outline(cls.contour_outlines)

This is the code allowing to find and draw the 2D contour of a 3D polygon.
It updates the display at each figure movement triggered by interactivity.

More context in
https://stackoverflow.com/questions/53652749/matplotlib-extract-2d-contour-of-a-3d-polygons-plot