allow reading and writing rgb-alpha images in Python (#2925)

dlib/python/numpy_image.h

@@ -262,6 +262,7 @@ namespace dlib
         else if (is_image<float>(src))     assign_image(dest, numpy_image<float>(src));
         else if (is_image<double>(src))    assign_image(dest, numpy_image<double>(src));
         else if (is_image<rgb_pixel>(src)) assign_image(dest, numpy_image<rgb_pixel>(src));
+        else if (is_image<rgb_alpha_pixel>(src)) assign_image(dest, numpy_image<rgb_alpha_pixel>(src));
         else DLIB_CASSERT(false, "Unsupported pixel type used in assign_image().");
     }
 

tools/python/src/numpy_returns.cpp

@@ -20,6 +20,13 @@ numpy_image<rgb_pixel> load_rgb_image (const std::string &path)
     return img; 
 }
 
+numpy_image<rgb_pixel> load_rgb_alpha_image (const std::string &path)
+{
+    numpy_image<rgb_alpha_pixel> img;
+    load_image(img, path);
+    return img;
+}
+
 numpy_image<unsigned char> load_grayscale_image (const std::string &path)
 {
     numpy_image<unsigned char> img;
@@ -168,6 +175,10 @@ void bind_numpy_returns(py::module &m)
 	"Saves the given image to the specified path. Determines the file type from the file extension specified in the path",
 	py::arg("img"), py::arg("filename"), py::arg("quality") = 75
     );
+    m.def("save_image", &save_image<rgb_alpha_pixel>,
+	"Saves the given image to the specified path. Determines the file type from the file extension specified in the path",
+	py::arg("img"), py::arg("filename"), py::arg("quality") = 75
+    );
     m.def("save_image", &save_image<unsigned char>, 
 	"Saves the given image to the specified path. Determines the file type from the file extension specified in the path",
 	py::arg("img"), py::arg("filename"), py::arg("quality") = 75