Improve autoscaling for high order Bezier curves

dstansby · dstansby · commit db06f62638f3 · 2021-01-01T21:11:18.000Z
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -2116,6 +2116,8 @@ def _update_patch_limits(self, patch):
                 ((not patch.get_width()) and (not patch.get_height()))):
             return
         p = patch.get_path()
+        p = p._replace_bezier_control_points_with_extrema()
+        # Don't include CLOSEPOLY and STOP vertices in the path
         vertices = p.vertices if p.codes is None else p.vertices[np.isin(
             p.codes, (mpath.Path.CLOSEPOLY, mpath.Path.STOP), invert=True)]
         if vertices.size > 0:
diff --git a/lib/matplotlib/path.py b/lib/matplotlib/path.py
@@ -461,6 +461,53 @@ def iter_bezier(self, **kwargs):
                 raise ValueError("Invalid Path.code_type: " + str(code))
             prev_vert = verts[-2:]
 
+    def _replace_bezier_control_points_with_extrema(self):
+        """
+        Return a new version of this path, with any control points for Bezier
+        segments replaced with any extrema for that segment.
+
+        This is helpful when autoscaling. We don't want to include the control
+        points, which are often far away from the actual curve, but in order to
+        calculate a bounding box for the whole curve we need to include any
+        extrema as well as start and end points for each Bezier segment.
+
+        Notes
+        -----
+        If there are no curved Bezier segments, the current object is returned.
+        """
+        # Shortcut case where there are no curved segments
+        if not np.any(np.isin(self.codes, (Path.CURVE3, Path.CURVE4))):
+            return self
+
+        vertices = np.array([[], []]).T
+        codes = []
+        for bez_segment, code in self.iter_bezier():
+            verts = bez_segment._cpoints
+
+            # For low order segments (straight lines or points), just add
+            # the vertices
+            if code in (Path.MOVETO, Path.LINETO, Path.CLOSEPOLY):
+                # Ignore the (already added) vertex at the start of this
+                # segment
+                if code in (Path.LINETO, Path.CLOSEPOLY):
+                    verts = verts[1]
+                codes += [code] * self.NUM_VERTICES_FOR_CODE[code]
+            # For higher order segments, calculate the extrema
+            elif code in (Path.CURVE3, Path.CURVE4):
+                _, t = bez_segment.axis_aligned_extrema()
+                if len(t):
+                    extrema = bez_segment(t)
+                    verts = np.vstack((verts[0], extrema, verts[-1]))
+                else:
+                    verts = np.vstack((verts[0], verts[-1]))
+
+                # Add extrema to the start and end points
+                codes += [Path.LINETO] * verts.shape[0]
+
+            vertices = np.vstack((vertices, verts))
+        # Construct path
+        return Path(vertices, codes)
+
     @_api.delete_parameter("3.3", "quantize")
     def cleaned(self, transform=None, remove_nans=False, clip=None,
                 quantize=False, simplify=False, curves=False,
