From 75858bd923ce31018b4437f849ece9eccaa6e597 Mon Sep 17 00:00:00 2001
From: Evans Castonguay <evans.castonguay@gmail.com>
Date: Sun, 31 May 2026 09:10:15 -0400
Subject: [PATCH] SVG: simplify large filled paths e.g. fill_between (#22803)

---
 lib/matplotlib/backends/backend_svg.py   | 12 ++++++++++--
 lib/matplotlib/tests/test_backend_svg.py | 22 ++++++++++++++++++++++
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/lib/matplotlib/backends/backend_svg.py b/lib/matplotlib/backends/backend_svg.py
index 24790356b9d7..eb61f5e89f4b 100644
--- a/lib/matplotlib/backends/backend_svg.py
+++ b/lib/matplotlib/backends/backend_svg.py
@@ -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())
@@ -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)
diff --git a/lib/matplotlib/tests/test_backend_svg.py b/lib/matplotlib/tests/test_backend_svg.py
index 6b63990f7620..242b880c25f1 100644
--- a/lib/matplotlib/tests/test_backend_svg.py
+++ b/lib/matplotlib/tests/test_backend_svg.py
@@ -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