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Use Path.arc() to interpolate polar arcs.
Currently, in polar plots, arcs are rendered by linearly interpolating in (theta, r) space before conversion to (x, y) space (if their _interpolation_steps attribute is >1 -- this attribute is described in the docs as "an implementation detail not intended for public use"). When _interpolation_steps == 1, this PR doesn't change anything -- there is an example (specialty_plots/radar_chart) that explicitly relies on being able to set _interpolation_steps to 1 and get a "polygonal" polar axes rather than a circular one. When _interpolation_steps > 1, however, this PR looks for paths segments that are either at a constant theta or a constant r. It transforms constant-theta segments to a single segment (because interpolation doesn't help there) and transforms constant-r segments (circular arcs) to a Bezier approximation of a circle which is already implemented in Path.arc()). This greatly decreases the number of control points for such plots in vector outputs (which are thus smaller), and also improves rasterization by mplcairo (cairo appears to be not so good at filling areas with a lot of close vertices). The many changes in baseline images are due to, well, the change of approximation for drawing circles.
1 parent a57c9a1 commit 73a763a

29 files changed

Lines changed: 2541 additions & 7335 deletions

‎lib/matplotlib/projections/polar.py‎

Lines changed: 52 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
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import matplotlib.axis as maxis
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import matplotlib.markers as mmarkers
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import matplotlib.patches as mpatches
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import matplotlib.path as mpath
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from matplotlib.path import Path
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import matplotlib.ticker as mticker
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import matplotlib.transforms as mtransforms
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import matplotlib.spines as mspines
@@ -50,11 +50,57 @@ def transform_non_affine(self, tr):
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def transform_path_non_affine(self, path):
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# docstring inherited
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vertices = path.vertices
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if len(vertices) == 2 and vertices[0, 0] == vertices[1, 0]:
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return mpath.Path(self.transform(vertices), path.codes)
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ipath = path.interpolated(path._interpolation_steps)
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return mpath.Path(self.transform(ipath.vertices), ipath.codes)
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if not len(path) or path._interpolation_steps == 1:
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return Path(self.transform_non_affine(path.vertices), path.codes)
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xys = []
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codes = []
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last_t = last_r = None
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for trs, c in path.iter_segments():
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trs = trs.reshape((-1, 2))
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if c == Path.LINETO:
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(t, r), = trs
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if t == last_t: # Same angle: draw a straight line.
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xys.extend(self.transform_non_affine(trs))
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codes.append(Path.LINETO)
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elif r == last_r: # Same radius: draw an arc.
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# The following is complicated by Path.arc() being
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# "helpful" and unwrapping the angles, but we don't want
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# that behavior here.
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last_td, td = np.rad2deg([last_t, t])
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if self._use_rmin and self._axis is not None:
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r = ((r - self._axis.get_rorigin())
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* self._axis.get_rsign())
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if last_td <= td:
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while td - last_td > 360:
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arc = Path.arc(last_td, last_td + 360)
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xys.extend(arc.vertices[1:] * r)
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codes.extend(arc.codes[1:])
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last_td += 360
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arc = Path.arc(last_td, td)
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xys.extend(arc.vertices[1:] * r)
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codes.extend(arc.codes[1:])
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else:
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# The reverse version also relies on the fact that all
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# codes but the first one are the same.
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while last_td - td > 360:
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arc = Path.arc(last_td - 360, last_td)
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xys.extend(arc.vertices[::-1][1:] * r)
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codes.extend(arc.codes[1:])
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last_td -= 360
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arc = Path.arc(td, last_td)
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xys.extend(arc.vertices[::-1][1:] * r)
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codes.extend(arc.codes[1:])
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else: # Interpolate.
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trs = cbook.simple_linear_interpolation(
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np.row_stack([(last_t, last_r), trs]),
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path._interpolation_steps)[1:]
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xys.extend(self.transform_non_affine(trs))
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codes.extend([Path.LINETO] * len(trs))
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else: # Not a straight line.
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xys.extend(self.transform_non_affine(trs))
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codes.extend([c] * len(trs))
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last_t, last_r = trs[-1]
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return Path(xys, codes)
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def inverted(self):
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# docstring inherited

‎lib/matplotlib/tests/baseline_images/test_axes/markevery_polar.pdf‎

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‎lib/matplotlib/tests/baseline_images/test_axes/markevery_polar.png‎

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‎lib/matplotlib/tests/baseline_images/test_axes/markevery_polar.svg‎

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‎lib/matplotlib/tests/baseline_images/test_axes/polar_alignment.png‎

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‎lib/matplotlib/tests/baseline_images/test_axes/polar_axes.pdf‎

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‎lib/matplotlib/tests/baseline_images/test_axes/polar_axes.png‎

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