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Fix PDF path collection culling for hexbin offsets #32000

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Fix PDF path collection culling for hexbin offsets #32000

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bb520f4
Fix to respect actual positions of offset paths
yuzie007 Jul 4, 2026
3e5af6f
Add `test_hexbin_negative_offsets`
yuzie007 Jul 5, 2026
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41 changes: 11 additions & 30 deletions lib/matplotlib/backends/backend_pdf.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2011,16 +2011,11 @@ def draw_path_collection(self, gc, master_transform, paths, all_transforms,
name = self.file.pathCollectionObject(
gc, path, transform, padding, filled, stroked)
path_codes.append(name)
# Compute the extent of each marker path to enable per-marker
# bounds checking. This allows us to skip markers that are
# completely outside the visible canvas while preserving markers
# that are partially visible.
if len(path.vertices):
bbox = path.get_extents(transform)
# Store half-width and half-height for efficient bounds checking
path_extents.append((bbox.width / 2, bbox.height / 2))
else:
path_extents.append((0, 0))
# Compute each transformed path's exact bounds for per-marker
# canvas checks. Offsets are not necessarily full canvas-space
# centers, e.g. for collections using AffineDeltaTransform, so
# cull based on the final path bounds translated by the offset.
path_extents.append(path.get_extents(transform).frozen())

# Create a mapping from path_id to extent for efficient lookup
path_extent_map = dict(zip(path_codes, path_extents))
Expand All @@ -2036,26 +2031,12 @@ def draw_path_collection(self, gc, master_transform, paths, all_transforms,
facecolors, edgecolors, linewidths, linestyles,
antialiaseds, urls, offset_position, hatchcolors=hatchcolors):

# Optimization: Fast path for markers with centers inside canvas.
# This avoids the dictionary lookup for the common case where
# markers are visible, improving performance for large scatter plots.
if 0 <= xo <= canvas_width and 0 <= yo <= canvas_height:
# Marker center is inside canvas - definitely render it
self.check_gc(gc0, rgbFace)
dx, dy = xo - lastx, yo - lasty
output(1, 0, 0, 1, dx, dy, Op.concat_matrix, path_id,
Op.use_xobject)
lastx, lasty = xo, yo
continue

# Marker center is outside canvas - check if partially visible.
# Skip markers completely outside visible canvas bounds to reduce
# PDF file size. Use per-marker extents to handle large markers
# correctly: only skip if the marker's bounding box doesn't
# intersect the canvas at all.
extent_x, extent_y = path_extent_map[path_id]
if not (-extent_x <= xo <= canvas_width + extent_x
and -extent_y <= yo <= canvas_height + extent_y):
# Skip markers completely outside the canvas to reduce PDF size.
# Use the translated path bounds, not the offset alone: the offset
# need not be the marker center in canvas coordinates.
bbox = path_extent_map[path_id]
if (bbox.x1 + xo < 0 or bbox.x0 + xo > canvas_width
or bbox.y1 + yo < 0 or bbox.y0 + yo > canvas_height):
continue

self.check_gc(gc0, rgbFace)
Expand Down
27 changes: 27 additions & 0 deletions lib/matplotlib/tests/test_backend_pdf.py
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Original file line number Diff line number Diff line change
Expand Up @@ -732,3 +732,30 @@ def test_scatter_polar(fig_test, fig_ref):
ax_ref = fig_ref.subplots(subplot_kw={'projection': 'polar'})
ax_ref.scatter(theta, r, c=c, s=50)
ax_ref.set_ylim(0, 2)


@check_figures_equal(extensions=["pdf"], tol=1.0)
def test_hexbin_negative_offsets(fig_test, fig_ref):
"""
Test path collection culling with non-canvas-space offsets.

Hexbin uses a repeated polygon path with offsets transformed by
AffineDeltaTransform. The transformed offsets are displacements, not the
final canvas-space marker centers, so PDF backend culling must account for
the transformed path bounds as well.
"""
rng = np.random.default_rng(19680801)
x = rng.normal(size=10_000)
y = rng.normal(size=10_000)

ax_test = fig_test.subplots()
ax_test.hexbin(x, y, gridsize=25, edgecolors="none")
ax_test.set_xlim(-2.0, 2.0)
ax_test.set_ylim(-2.0, 2.0)
ax_test.set_axis_off()

ax_ref = fig_ref.subplots()
ax_ref.hexbin(x + 2.0, y + 2.0, gridsize=25, edgecolors="none")
ax_ref.set_xlim(0.0, 4.0)
ax_ref.set_ylim(0.0, 4.0)
ax_ref.set_axis_off()
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