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Fix PillowWriter GIF frame skew: frame_size must match the rendered RGBA buffer #32189

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Fix PillowWriter GIF frame skew: frame_size must match the rendered RGBA buffer #32189

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6 changes: 5 additions & 1 deletion lib/matplotlib/animation.py
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Original file line number Diff line number Diff line change
Expand Up @@ -178,7 +178,11 @@ def setup(self, fig, outfile, dpi=None):
def frame_size(self):
"""A tuple ``(width, height)`` in pixels of a movie frame."""
w, h = self.fig.get_size_inches()
return int(w * self.dpi), int(h * self.dpi)
# Match the rounding FigureCanvasBase.get_width_height(physical=True)
# applies when rendering: round up when within 1e-8 px of the integer,
# otherwise the frame size can be one pixel short of the RGBA buffer
# PillowWriter grabs via savefig(format="rgba"), skewing the frames.
return int(w * self.dpi + 1e-8), int(h * self.dpi + 1e-8)

def _supports_transparency(self):
"""
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20 changes: 20 additions & 0 deletions lib/matplotlib/tests/test_animation.py
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Original file line number Diff line number Diff line change
Expand Up @@ -558,3 +558,23 @@ def test_animation_with_transparency():
# Check that the alpha channel is not 255, so frame has transparency
assert frame.getextrema()[3][0] < 255
plt.close(fig)


def test_animation_frame_size_matches_rgba_buffer():
"""
The movie frame size must match the RGBA buffer that PillowWriter grabs via
savefig(format="rgba"); a one-pixel mismatch makes Image.frombuffer misread
the rows and skew the frames (3.11.1 regression, GH #32186).
"""
from io import BytesIO

# 8.2 in * 100 dpi = 819.9999999999999 px: plain int() truncates to 819,
# but the rendered RGBA buffer is rounded up to 820 (get_width_height).
fig, ax = plt.subplots(figsize=(8.2, 7), dpi=100)
writer = PillowWriter(fps=1)
writer.setup(fig, 'unused.gif', dpi=100)
buf = BytesIO()
fig.savefig(buf, format='rgba', dpi=100)
buffer_width = buf.getbuffer().nbytes // 4 // writer.frame_size[1]
assert writer.frame_size[0] == buffer_width
plt.close(fig)
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