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Fix bug with an isolated blend group in Cairo with certain group blend modes #32326

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Merged
ayshih merged 1 commit into from
Oct 9, 2026
Merged

Fix bug with an isolated blend group in Cairo with certain group blend modes #32326

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21 changes: 18 additions & 3 deletions lib/matplotlib/backends/backend_cairo.py
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Original file line number Diff line number Diff line change
Expand Up @@ -380,10 +380,25 @@ def close_blend_group(self):
ctx.save()
self.gc.set_blend_mode(group_state.blend_mode)
ctx.set_source(group)
if group_state.alpha != 1:
ctx.paint_with_alpha(group_state.alpha)

# For these two Porter-Duff compositing operators, we need to mask out fully
# transparent pixels, otherwise those pixels will override the backdrop
if group_state.blend_mode in {'knockout', 'clear'}:
# Extract the 8-bit alpha channel
mask_surface = ctx.get_target().create_similar_image(
cairo.FORMAT_A8, self.width, self.height)
mask_ctx = cairo.Context(mask_surface)
mask_ctx.set_source(group)
mask_ctx.paint()
mask_surface.flush()

# Pixels that are not fully transparent are painted with the group alpha
mask_buf = np.asarray(mask_surface.get_data())
mask_buf[mask_buf > 0] = 255 * group_state.alpha + 0.5 # round to int
ctx.mask_surface(mask_surface)
else:
ctx.paint()
ctx.paint_with_alpha(group_state.alpha)

ctx.restore()


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Binary file modified lib/matplotlib/tests/baseline_images/test_backends_rendering/blend_groups_agg.png
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Binary file modified ...matplotlib/tests/baseline_images/test_backends_rendering/blend_groups_cairo.png
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18 changes: 15 additions & 3 deletions lib/matplotlib/tests/test_backends_rendering.py
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Original file line number Diff line number Diff line change
Expand Up @@ -93,7 +93,7 @@ def draw(self, renderer):
renderer.close_blend_group()


def plot_blend_group_types(rasterize=False):
def plot_blend_group_types(rasterize=False, porterduff=False):
# Rows: top row is non-isolated, bottom row is isolated
# Columns: left column is non-knockout, right column is knockout
fig, axs = plt.subplots(2, 2, figsize=(3, 3), dpi=80, layout='constrained')
Expand Down Expand Up @@ -129,6 +129,18 @@ def plot_blend_group_types(rasterize=False):
gray = Circle((0, 0), 0.1, fc='gray', zorder=0)
axs[i, j].add_artist(gray)

# For backends that support the Porter-Duff compositing operators, additionally
# test 'knockout' and 'clear' as the blend mode for an isolated group
if porterduff:
for x, group_blend_mode, group_alpha in [(-0.6, 'knockout', 1),
(-0.2, 'knockout', 0.6),
(0.2, 'clear', 0.6),
(0.6, 'clear', 1)]:
circle = Circle((x, -0.8), 0.15, fc='k', alpha=0.75)
group = ArtistGroup([circle], group_blend_mode=group_blend_mode,
group_alpha=group_alpha)
axs[1, 0].add_artist(group)


@image_comparison(['blend_modes_agg.png'], style='mpl20')
def test_blend_modes_agg():
Expand Down Expand Up @@ -184,7 +196,7 @@ def test_blend_groups_agg():
# The top-right panel (knockout but not isolated) is not supported, so will be
# rendered like the top-left panel (neither knockout nor isolated)

plot_blend_group_types()
plot_blend_group_types(porterduff=True)


@pytest.mark.backend('cairo')
Expand All @@ -193,7 +205,7 @@ def test_blend_groups_cairo():
# The top-right panel (knockout but not isolated) is not supported, so will be
# rendered like the top-left panel (neither knockout nor isolated)

plot_blend_group_types()
plot_blend_group_types(porterduff=True)


@image_comparison(['blend_groups_svg.svg'], style='mpl20')
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