matplotlib · QuLogic · Sep 25, 2021 · Sep 25, 2021
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -148,18 +148,65 @@ def draw_path(self, gc, path, transform, rgbFace=None):
                     c = c[ii0:ii1]
                     c[0] = Path.MOVETO  # move to end of last chunk
                 p = Path(v, c)
+                p.simplify_threshold = path.simplify_threshold
                 try:
                     self._renderer.draw_path(gc, p, transform, rgbFace)
                 except OverflowError as err:
-                    raise OverflowError(
-                        "Exceeded cell block limit (set 'agg.path.chunksize' "
-                        "rcparam)") from err
+                    msg = (
+                        "Exceeded cell block limit in Agg.\n\n"
+                        "Please reduce the value of "
+                        f"rcParams['agg.path.chunksize'] (currently {nmax}) "
+                        "or increase the path simplification threshold"
+                        "(rcParams['path.simplify_threshold'] = "
+                        f"{mpl.rcParams['path.simplify_threshold']:.2f} by "
+                        "default and path.simplify_threshold = "
+                        f"{path.simplify_threshold:.2f} on the input)."
+                    )
+                    raise OverflowError(msg) from None
         else:
             try:
                 self._renderer.draw_path(gc, path, transform, rgbFace)
             except OverflowError as err:
-                raise OverflowError("Exceeded cell block limit (set "
-                                    "'agg.path.chunksize' rcparam)") from err
+                cant_chunk = ''
+                if rgbFace is not None:
+                    cant_chunk += "- can not split filled path\n"
+                if gc.get_hatch() is not None:
+                    cant_chunk += "- can not split hatched path\n"
+                if not path.should_simplify:
+                    cant_chunk += "- path.should_simplify is False\n"
+                if len(cant_chunk):
+                    msg = (
+                        "Exceeded cell block limit in Agg, however for the "
+                        "following reasons:\n\n"
+                        f"{cant_chunk}\n"
+                        "we can not automatically split up this path to draw."
+                        "\n\nPlease manually simplify your path."
+                    )
+
+                else:
+                    inc_threshold = (
+                        "or increase the path simplification threshold"
+                        "(rcParams['path.simplify_threshold'] = "
+                        f"{mpl.rcParams['path.simplify_threshold']} "
+                        "by default and path.simplify_threshold "
+                        f"= {path.simplify_threshold} "
+                        "on the input)."
+                        )
+                    if nmax > 100:
+                        msg = (
+                            "Exceeded cell block limit in Agg.  Please reduce "
+                            "the value of rcParams['agg.path.chunksize'] "
+                            f"(currently {nmax}) {inc_threshold}"
+                        )
+                    else:
+                        msg = (
+                            "Exceeded cell block limit in Agg.  Please set "
+                            "the value of rcParams['agg.path.chunksize'], "
+                            f"(currently {nmax}) to be greater than 100 "
+                            + inc_threshold
+                        )
+
+                raise OverflowError(msg) from None
 
     def draw_mathtext(self, gc, x, y, s, prop, angle):
         """Draw mathtext using :mod:`matplotlib.mathtext`."""

diff --git a/lib/matplotlib/tests/test_agg.py b/lib/matplotlib/tests/test_agg.py
@@ -8,9 +8,12 @@
 
 from matplotlib import (
     collections, path, pyplot as plt, transforms as mtransforms, rcParams)
-from matplotlib.image import imread
+from matplotlib.backends.backend_agg import RendererAgg
 from matplotlib.figure import Figure
+from matplotlib.image import imread
+from matplotlib.path import Path
 from matplotlib.testing.decorators import image_comparison
+from matplotlib.transforms import IdentityTransform
 
 
 def test_repeated_save_with_alpha():
@@ -72,10 +75,10 @@ def test_marker_with_nan():
 
 def test_long_path():
     buff = io.BytesIO()
-
-    fig, ax = plt.subplots()
-    np.random.seed(0)
-    points = np.random.rand(70000)
+    fig = Figure()
+    ax = fig.subplots()
+    points = np.ones(100_000)
+    points[::2] *= -1
     ax.plot(points)
     fig.savefig(buff, format='png')
 
@@ -251,3 +254,79 @@ def test_draw_path_collection_error_handling():
     ax.scatter([1], [1]).set_paths(path.Path([(0, 1), (2, 3)]))
     with pytest.raises(TypeError):
         fig.canvas.draw()
+
+
+@pytest.fixture
+def chunk_limit_setup():
+    N = 100_000
+    dpi = 500
+    w = 5*dpi
+    h = 6*dpi
+
+    # just fit in the width
+    x = np.linspace(0, w, N)
+    # and go top-to-bottom
+    y = np.ones(N) * h
+    y[::2] = 0
+
+    idt = IdentityTransform()
+    # make a renderer
+    ra = RendererAgg(w, h, dpi)
+    # setup the minimal gc to draw a line
+    gc = ra.new_gc()
+    gc.set_linewidth(1)
+    gc.set_foreground('r')
+    # make a Path
+    p = Path(np.vstack((x, y)).T)
+    # effectively disable path simplification (but leaving it "on")
+    p.simplify_threshold = 0
+
+    return ra, gc, p, idt
+
+
+def test_chunksize_hatch_fail(chunk_limit_setup):
+    ra, gc, p, idt = chunk_limit_setup
+
+    gc.set_hatch('/')
+
+    with pytest.raises(OverflowError, match='hatched path'):
+        ra.draw_path(gc, p, idt)
+
+
+def test_chunksize_rgbFace_fail(chunk_limit_setup):
+    ra, gc, p, idt = chunk_limit_setup
+
+    with pytest.raises(OverflowError, match='filled path'):
+        ra.draw_path(gc, p, idt, (1, 0, 0))
+
+
+def test_chunksize_no_simplify_fail(chunk_limit_setup):
+    ra, gc, p, idt = chunk_limit_setup
+    p.should_simplify = False
+    with pytest.raises(OverflowError, match="should_simplify is False"):
+        ra.draw_path(gc, p, idt)
+
+
+def test_chunksize_zero(chunk_limit_setup):
+    ra, gc, p, idt = chunk_limit_setup
+    # set to zero to disable, currently defaults to 0, but lets be sure
+    rcParams['agg.path.chunksize'] = 0
+    with pytest.raises(OverflowError, match='Please set'):
+        ra.draw_path(gc, p, idt)
+
+
+def test_chunksize_too_big_to_chunk(chunk_limit_setup):
+    ra, gc, p, idt = chunk_limit_setup
+    # set big enough that we do not try to chunk
+    rcParams['agg.path.chunksize'] = 1_000_000
+    with pytest.raises(OverflowError, match='Please reduce'):
+        ra.draw_path(gc, p, idt)
+
+
+def test_chunksize_toobig_chunks(chunk_limit_setup):
+    ra, gc, p, idt = chunk_limit_setup
+    # small enough we will try to chunk, but big enough we will fail
+    # to render
+    rcParams['agg.path.chunksize'] = 90_000
+    with pytest.raises(OverflowError, match='Please reduce'):
+        ra.draw_path(gc, p, idt)
diff --git a/lib/matplotlib/tests/test_simplification.py b/lib/matplotlib/tests/test_simplification.py
@@ -305,8 +305,8 @@ def test_start_with_moveto():
 
 def test_throw_rendering_complexity_exceeded():
     plt.rcParams['path.simplify'] = False
-    xx = np.arange(200000)
-    yy = np.random.rand(200000)
+    xx = np.arange(2_000_000)
+    yy = np.random.rand(2_000_000)
     yy[1000] = np.nan
 
     fig, ax = plt.subplots()

diff --git a/src/_backend_agg.cpp b/src/_backend_agg.cpp
@@ -45,7 +45,7 @@ RendererAgg::RendererAgg(unsigned int width, unsigned int height, double dpi)
       rendererBase(),
       rendererAA(),
       rendererBin(),
-      theRasterizer(8192),
+      theRasterizer(32768),
       lastclippath(NULL),
       _fill_color(agg::rgba(1, 1, 1, 0))
 {