Take Image object's alpha attribute into account when compositing multiple images together.

Author: Westacular

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))

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,

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[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);
             }
         }