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SVG: simplify large filled paths e.g. fill_between (#22803) #31796

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SVG: simplify large filled paths e.g. fill_between (#22803) #31796

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12 changes: 10 additions & 2 deletions lib/matplotlib/backends/backend_svg.py
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Original file line number Diff line number Diff line change
Expand Up @@ -691,7 +691,13 @@ def draw_path(self, gc, path, transform, rgbFace=None):
# docstring inherited
trans_and_flip = self._make_flip_transform(transform)
clip = (rgbFace is None and gc.get_hatch_path() is None)
simplify = path.should_simplify and clip
# Also simplify large filled paths (e.g. fill_between); see #22803.
# Lines already simplify via should_simplify; honor the rcParam and
# leave hatched paths exact.
simplify = ((path.should_simplify
or (mpl.rcParams['path.simplify']
and len(path.vertices) >= 128))
and gc.get_hatch_path() is None)
path_data = self._convert_path(
path, trans_and_flip, clip=clip, simplify=simplify,
sketch=gc.get_sketch_params())
Expand All @@ -714,7 +720,9 @@ def draw_markers(
path_data = self._convert_path(
marker_path,
marker_trans + Affine2D().scale(1.0, -1.0),
simplify=False)
simplify=(marker_path.should_simplify
or (mpl.rcParams['path.simplify']
and len(marker_path.vertices) >= 128)))
style = self._get_style_dict(gc, rgbFace)
dictkey = (path_data, _generate_css(style))
oid = self._markers.get(dictkey)
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22 changes: 22 additions & 0 deletions lib/matplotlib/tests/test_backend_svg.py
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Original file line number Diff line number Diff line change
Expand Up @@ -705,3 +705,25 @@ def test_svgid():

assert plt.rcParams['svg.id'] == svg_id
assert tree.findall(f'.[@id="{svg_id}"]')


def test_fill_between_svg_simplified():
# Regression test for #22803: the polygon produced by fill_between is cached
# in the SVG backend like a reusable marker and was written without path
# simplification, producing very large files. It should be simplified like any
# other large path, while staying vector (not rasterized).
import re
rng = np.random.default_rng(424242)
n = 20000
x = np.arange(n)
y = rng.normal(size=(10, n))
fig, ax = plt.subplots()
ax.fill_between(x, y.min(axis=0), y.max(axis=0))
ax.set_xlim(0, n)
buf = BytesIO()
fig.savefig(buf, format="svg")
svg = buf.getvalue().decode("utf-8")
biggest = max(re.findall(r'\sd="([^"]*)"', svg, re.S), key=len)
n_verts = biggest.count("L") + biggest.count("M")
assert n_verts < n, f"fill path not simplified ({n_verts} vertices)"
assert "<image" not in svg
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