Method draw

lib/matplotlib/patches.py:2173–2313

Draw the arc to the given *renderer*. Notes ----- Ellipses are normally drawn using an approximation that uses eight cubic Bezier splines. The error of this approximation is 1.89818e-6, according to this unverified source: Lancaster, Don.

(self, renderer)

    @artist.allow_rasterization
    def draw(self, renderer):
        """
        Draw the arc to the given *renderer*.

        Notes
        -----
        Ellipses are normally drawn using an approximation that uses
        eight cubic Bezier splines.  The error of this approximation
        is 1.89818e-6, according to this unverified source:

          Lancaster, Don.  *Approximating a Circle or an Ellipse Using
          Four Bezier Cubic Splines.*

          https://www.tinaja.com/glib/ellipse4.pdf

        There is a use case where very large ellipses must be drawn
        with very high accuracy, and it is too expensive to render the
        entire ellipse with enough segments (either splines or line
        segments).  Therefore, in the case where either radius of the
        ellipse is large enough that the error of the spline
        approximation will be visible (greater than one pixel offset
        from the ideal), a different technique is used.

        In that case, only the visible parts of the ellipse are drawn,
        with each visible arc using a fixed number of spline segments
        (8).  The algorithm proceeds as follows:

        1. The points where the ellipse intersects the axes (or figure)
           bounding box are located.  (This is done by performing an inverse
           transformation on the bbox such that it is relative to the unit
           circle -- this makes the intersection calculation much easier than
           doing rotated ellipse intersection directly.)

           This uses the "line intersecting a circle" algorithm from:

               Vince, John.  *Geometry for Computer Graphics: Formulae,
               Examples & Proofs.*  London: Springer-Verlag, 2005.

        2. The angles of each of the intersection points are calculated.

        3. Proceeding counterclockwise starting in the positive
           x-direction, each of the visible arc-segments between the
           pairs of vertices are drawn using the Bezier arc
           approximation technique implemented in `.Path.arc`.
        """
        if not self.get_visible():
            return

        self._recompute_transform()

        self._update_path()
        # Get width and height in pixels we need to use
        # `self.get_data_transform` rather than `self.get_transform`
        # because we want the transform from dataspace to the
        # screen space to estimate how big the arc will be in physical
        # units when rendered (the transform that we get via
        # `self.get_transform()` goes from an idealized unit-radius
        # space to screen space).