diff --git a/lib/matplotlib/backends/backend_gtk3agg.py b/lib/matplotlib/backends/backend_gtk3agg.py
index f54849cf049c..c7b79c75a109 100644
--- a/lib/matplotlib/backends/backend_gtk3agg.py
+++ b/lib/matplotlib/backends/backend_gtk3agg.py
@@ -1,6 +1,8 @@
-import numpy as np
+import sys
import warnings
+import numpy as np
+
from . import backend_agg, backend_cairo, backend_gtk3
from ._gtk3_compat import gi
from .backend_cairo import cairo
@@ -31,7 +33,7 @@ def _render_figure(self, width, height):
backend_agg.FigureCanvasAgg.draw(self)
def on_draw_event(self, widget, ctx):
- """ GtkDrawable draw event, like expose_event in GTK 2.X
+ """GtkDrawable draw event, like expose_event in GTK 2.X.
"""
allocation = self.get_allocation()
w, h = allocation.width, allocation.height
@@ -45,17 +47,29 @@ def on_draw_event(self, widget, ctx):
ctx = backend_cairo._to_context(ctx)
for bbox in bbox_queue:
- area = self.copy_from_bbox(bbox)
- buf = np.fromstring(area.to_string_argb(), dtype='uint8')
-
x = int(bbox.x0)
y = h - int(bbox.y1)
width = int(bbox.x1) - int(bbox.x0)
height = int(bbox.y1) - int(bbox.y0)
+ buf = (np.fromstring(self.copy_from_bbox(bbox).to_string_argb(),
+ dtype='uint8')
+ .reshape((width, height, 4)))
+ # cairo wants premultiplied alpha. Only bother doing the
+ # conversion when the alpha channel is not fully opaque, as the
+ # cost is not negligible. (The unsafe cast is needed to do the
+ # multiplication in-place in an integer buffer.)
+ if sys.byteorder == "little":
+ rgb24 = buf[..., :-1]
+ alpha8 = buf[..., -1:]
+ else:
+ alpha8 = buf[..., :1]
+ rgb24 = buf[..., 1:]
+ if alpha8.min() != 0xff:
+ np.multiply(rgb24, alpha8 / 0xff, out=rgb24, casting="unsafe")
+
image = cairo.ImageSurface.create_for_data(
- buf.ravel().data, cairo.FORMAT_ARGB32,
- width, height, width * 4)
+ buf.ravel().data, cairo.FORMAT_ARGB32, width, height)
ctx.set_source_surface(image, x, y)
ctx.paint()
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