matplotlib · dstansby · Mar 1, 2019 · Sep 7, 2018 · Sep 7, 2018
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -486,16 +486,31 @@ def _make_image(self, A, in_bbox, out_bbox, clip_bbox, magnification=1.0,
                     raise ValueError("Invalid dimensions, got %s" % (A.shape,))
 
                 output = np.zeros((out_height, out_width, 4), dtype=A.dtype)
+                output_a = np.zeros((out_height, out_width), dtype=A.dtype)
 
                 alpha = self.get_alpha()
                 if alpha is None:
-                    alpha = 1.0
+                    alpha = 1
+
+                #resample alpha channel
+                alpha_channel = A[..., 3]
+                _image.resample(
+                    alpha_channel, output_a, t,
+                    _interpd_[self.get_interpolation()],
+                    self.get_resample(), alpha,
+                    self.get_filternorm(), self.get_filterrad())
 
+                #resample rgb channels
+                A = _rgb_to_rgba(A[..., :3])
                 _image.resample(
-                    A, output, t, _interpd_[self.get_interpolation()],
+                    A, output, t,
+                    _interpd_[self.get_interpolation()],
                     self.get_resample(), alpha,
                     self.get_filternorm(), self.get_filterrad())
 
+                #recombine rgb and alpha channels
+                output[..., 3] = output_a
+
             # at this point output is either a 2D array of normed data
             # (of int or float)
             # or an RGBA array of re-sampled input

diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.pdf b/lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.png b/lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.png
diff --git a/lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.svg b/lib/matplotlib/tests/baseline_images/test_image/image_composite_alpha.svg
diff --git a/lib/matplotlib/tests/baseline_images/test_image/interp_alpha.png b/lib/matplotlib/tests/baseline_images/test_image/interp_alpha.png
diff --git a/lib/matplotlib/tests/baseline_images/test_png/pngsuite.png b/lib/matplotlib/tests/baseline_images/test_png/pngsuite.png
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -43,6 +43,20 @@ def test_image_interps():
     ax3.set_ylabel('bicubic')
 
 
+@image_comparison(baseline_images=['interp_alpha'],
+                  extensions=['png'], remove_text=True)
+def test_alpha_interp():
+    'Test the interpolation of the alpha channel on RGBA images'
+    fig, (axl, axr) = plt.subplots(1, 2)
+    # full green image
+    img = np.zeros((5, 5, 4))
+    img[..., 1] = np.ones((5, 5))
+    # transparent under main diagonal
+    img[..., 3] = np.tril(np.ones((5, 5), dtype=np.uint8))
+    axl.imshow(img, interpolation="none")
+    axr.imshow(img, interpolation="bilinear")
+
+
 @image_comparison(baseline_images=['interp_nearest_vs_none'],
                   extensions=['pdf', 'svg'], remove_text=True)
 def test_interp_nearest_vs_none():