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Fix TkCairo bug when displaying images with partially transparent pixels #32315

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Merged
QuLogic merged 1 commit into from
Sep 8, 2026
+6 −6
Merged

Fix TkCairo bug when displaying images with partially transparent pixels #32315

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10 changes: 4 additions & 6 deletions lib/matplotlib/backends/backend_tkcairo.py
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Original file line number Diff line number Diff line change
@@ -1,7 +1,6 @@
import sys

import numpy as np

from .. import cbook
from . import _backend_tk
from .backend_cairo import cairo, FigureCanvasCairo
from ._backend_tk import _BackendTk, FigureCanvasTk
Expand All @@ -14,10 +13,9 @@ def draw(self):
self._renderer.set_context(cairo.Context(surface))
self._renderer.dpi = self.figure.dpi
self.figure.draw(self._renderer)
buf = np.reshape(surface.get_data(), (height, width, 4))
_backend_tk.blit(
self._tkphoto, buf,
(2, 1, 0, 3) if sys.byteorder == "little" else (1, 2, 3, 0))
premult_argb = np.reshape(surface.get_data(), (height, width, 4))
unmult_rgba = cbook._premultiplied_argb32_to_unmultiplied_rgba8888(premult_argb)
_backend_tk.blit(self._tkphoto, unmult_rgba, (0, 1, 2, 3))

@iccir iccir Sep 8, 2026

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The removal of byteorder confused me until I realized that _premultiplied_argb32_to_unmultiplied_rgba8888 was handling it.

I don't think we need a comment in the code about this, as it only applies to the diff.



@_BackendTk.export
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2 changes: 2 additions & 0 deletions lib/matplotlib/cbook.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2210,6 +2210,8 @@ def _premultiplied_argb32_to_unmultiplied_rgba8888(buf):
[2, 1, 0, 3] if sys.byteorder == "little" else [1, 2, 3, 0], axis=2)
rgb = rgba[..., :-1]
alpha = rgba[..., -1]
if alpha.min() == 0xff:
return rgba
# Un-premultiply alpha. The formula is the same as in cairo-png.c.
mask = alpha != 0
for channel in np.rollaxis(rgb, -1):
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