matplotlib · pelson · May 13, 2013 · Apr 25, 2013 · Apr 26, 2013 · Apr 27, 2013
diff --git a/lib/matplotlib/axes.py b/lib/matplotlib/axes.py
@@ -2054,7 +2054,7 @@ def draw(self, renderer=None, inframe=False):
             zorder_images.sort(key=lambda x: x[0])
 
             mag = renderer.get_image_magnification()
-            ims = [(im.make_image(mag), 0, 0) for z, im in zorder_images]
+            ims = [(im.make_image(mag), 0, 0, im.get_alpha()) for z, im in zorder_images]
 
             l, b, r, t = self.bbox.extents
             width = mag * ((round(r) + 0.5) - (round(l) - 0.5))

diff --git a/lib/matplotlib/backends/backend_cairo.py b/lib/matplotlib/backends/backend_cairo.py
@@ -163,8 +163,11 @@ def draw_image(self, gc, x, y, im):
                       buf, cairo.FORMAT_ARGB32, cols, rows, cols*4)
         ctx = gc.ctx
         y = self.height - y - rows
+
+        ctx.save()
         ctx.set_source_surface (surface, x, y)
         ctx.paint()
+        ctx.restore()
 
         im.flipud_out()
 

diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -990,7 +990,7 @@ def draw(self, renderer):
             # make a composite image blending alpha
             # list of (_image.Image, ox, oy)
             mag = renderer.get_image_magnification()
-            ims = [(im.make_image(mag), im.ox, im.oy)
+            ims = [(im.make_image(mag), im.ox, im.oy, im.get_alpha())
                    for im in self.images]
 
             im = _image.from_images(self.bbox.height * mag,

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/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -235,6 +235,32 @@ def test_image_composite_background():
     ax.set_axis_bgcolor((1, 0, 0, 0.5))
     ax.set_xlim([0, 12])
 
+@image_comparison(baseline_images=['image_composite_alpha'], remove_text=True)
+def test_image_composite_alpha():
+    """
+    Tests that the alpha value is recognized and correctly applied in the
+    process of compositing images together.
+    """
+    fig = plt.figure()
+    ax = fig.add_subplot(111)
+    arr = np.zeros((11, 21, 4))
+    arr[:, :, 0] = 1
+    arr[:, :, 3] = np.concatenate((np.arange(0, 1.1, 0.1), np.arange(0, 1, 0.1)[::-1]))
+    arr2 = np.zeros((21, 11, 4))
+    arr2[:, :, 0] = 1
+    arr2[:, :, 1] = 1
+    arr2[:, :, 3] = np.concatenate((np.arange(0, 1.1, 0.1), np.arange(0, 1, 0.1)[::-1]))[:, np.newaxis]
+    ax.imshow(arr, extent=[1, 2, 5, 0], alpha=0.3)
+    ax.imshow(arr, extent=[2, 3, 5, 0], alpha=0.6)
+    ax.imshow(arr, extent=[3, 4, 5, 0])
+    ax.imshow(arr2, extent=[0, 5, 1, 2])
+    ax.imshow(arr2, extent=[0, 5, 2, 3], alpha=0.6)
+    ax.imshow(arr2, extent=[0, 5, 3, 4], alpha=0.3)
+    ax.set_axis_bgcolor((0, 0.5, 0, 1))
+    ax.set_xlim([0, 5])
+    ax.set_ylim([5, 0])
+
+
 if __name__=='__main__':
     import nose
     nose.runmodule(argv=['-s','--with-doctest'], exit=False)
diff --git a/src/_image.cpp b/src/_image.cpp
@@ -796,6 +796,8 @@ _image_module::from_images(const Py::Tuple& args)
     Py::Tuple tup;
 
     size_t ox(0), oy(0), thisx(0), thisy(0);
+    float alpha;
+    bool apply_alpha;
 
     //copy image 0 output buffer into return images output buffer
     Image* imo = new Image;
@@ -823,6 +825,16 @@ _image_module::from_images(const Py::Tuple& args)
         Image* thisim = static_cast<Image*>(tup[0].ptr());
         ox = (long)Py::Int(tup[1]);
         oy = (long)Py::Int(tup[2]);
+        if (tup.size() <= 3 || tup[3].ptr() == Py_None)
+        {
+            apply_alpha = false;
+        }
+        else
+        {
+            apply_alpha = true;
+            alpha = Py::Float(tup[3]);
+        }
+
         bool isflip = (thisim->rbufOut->stride()) < 0;
         //std::cout << "from images " << isflip << "; stride=" << thisim->rbufOut->stride() << std::endl;
         size_t ind = 0;
@@ -851,7 +863,14 @@ _image_module::from_images(const Py::Tuple& args)
                 p.r = *(thisim->bufferOut + ind++);
                 p.g = *(thisim->bufferOut + ind++);
                 p.b = *(thisim->bufferOut + ind++);
-                p.a = *(thisim->bufferOut + ind++);
+                if (apply_alpha)
+                {
+                    p.a = (pixfmt::value_type) *(thisim->bufferOut + ind++) * alpha;
+                }
+                else
+                {
+                    p.a = *(thisim->bufferOut + ind++);
+                }
                 pixf.blend_pixel(thisx, thisy, p, 255);
             }
         }