Add support for imshow()

This PR adds support for the imshow() function to matplotlibcpp. imshow() displays an image in the current figure and allows for setting extra parameters such as the colormap used.

I've also added a version of the function which takes an OpenCV matrix as an input (in a separate commit), which provides a nicer interface. This requires OpenCV, but I've made this functionality optional: you have to explicitly define WITH_OPENCV if you want it.

matplotlibcpp.h

@@ -15,6 +15,10 @@
 #ifndef WITHOUT_NUMPY
 #  define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION
 #  include <numpy/arrayobject.h>
+
+#  ifdef WITH_OPENCV
+#  include <opencv2/opencv.hpp>
+#  endif // WITH_OPENCV
 #endif // WITHOUT_NUMPY
 
 #if PY_MAJOR_VERSION >= 3
@@ -45,6 +49,7 @@ struct _interpreter {
     PyObject *s_python_function_fill;
     PyObject *s_python_function_fill_between;
     PyObject *s_python_function_hist;
+    PyObject *s_python_function_imshow;
     PyObject *s_python_function_scatter;
     PyObject *s_python_function_subplot;
     PyObject *s_python_function_legend;
@@ -202,6 +207,9 @@ private:
         s_python_function_suptitle = safe_import(pymod, "suptitle");
         s_python_function_bar = safe_import(pymod,"bar");
         s_python_function_subplots_adjust = safe_import(pymod,"subplots_adjust");
+#ifndef WITHOUT_NUMPY
+        s_python_function_imshow = safe_import(pymod, "imshow");
+#endif
 
         s_python_empty_tuple = PyTuple_New(0);
     }
@@ -560,6 +568,78 @@ bool hist(const std::vector<Numeric>& y, long bins=10,std::string color="b",
     return res;
 }
 
+#ifndef WITHOUT_NUMPY
+    namespace internal {
+        void imshow(void *ptr, const NPY_TYPES type, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords)
+        {
+            assert(type == NPY_UINT8 || type == NPY_FLOAT);
+            assert(colors == 1 || colors == 3 || colors == 4);
+
+            detail::_interpreter::get();    //interpreter needs to be initialized for the numpy commands to work
+
+            // construct args
+            npy_intp dims[3] = { rows, columns, colors };
+            PyObject *args = PyTuple_New(1);
+            PyTuple_SetItem(args, 0, PyArray_SimpleNewFromData(colors == 1 ? 2 : 3, dims, type, ptr));
+
+            // construct keyword args
+            PyObject* kwargs = PyDict_New();
+            for(std::map<std::string, std::string>::const_iterator it = keywords.begin(); it != keywords.end(); ++it)
+            {
+                PyDict_SetItemString(kwargs, it->first.c_str(), PyUnicode_FromString(it->second.c_str()));
+            }
+
+            PyObject *res = PyObject_Call(detail::_interpreter::get().s_python_function_imshow, args, kwargs);
+            Py_DECREF(args);
+            Py_DECREF(kwargs);
+            if (!res)
+                throw std::runtime_error("Call to imshow() failed");
+            Py_DECREF(res);
+        }
+    }
+
+    void imshow(const unsigned char *ptr, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords = {})
+    {
+        internal::imshow((void *) ptr, NPY_UINT8, rows, columns, colors, keywords);
+    }
+
+    void imshow(const float *ptr, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords = {})
+    {
+        internal::imshow((void *) ptr, NPY_FLOAT, rows, columns, colors, keywords);
+    }
+
+#ifdef WITH_OPENCV
+    void imshow(const cv::Mat &image, const std::map<std::string, std::string> &keywords = {})
+    {
+        // Convert underlying type of matrix, if needed
+        cv::Mat image2;
+        NPY_TYPES npy_type = NPY_UINT8;
+        switch (image.type() & CV_MAT_DEPTH_MASK) {
+        case CV_8U:
+            image2 = image;
+            break;
+        case CV_32F:
+            image2 = image;
+            npy_type = NPY_FLOAT;
+            break;
+        default:
+            image.convertTo(image2, CV_MAKETYPE(CV_8U, image.channels()));
+        }
+
+        // If color image, convert from BGR to RGB
+        switch (image2.channels()) {
+        case 3:
+            cv::cvtColor(image2, image2, CV_BGR2RGB);
+            break;
+        case 4:
+            cv::cvtColor(image2, image2, CV_BGRA2RGBA);
+        }
+
+        internal::imshow(image2.data, npy_type, image2.rows, image2.cols, image2.channels(), keywords);
+    }
+#endif // WITH_OPENCV
+#endif // WITHOUT_NUMPY
+
 template<typename NumericX, typename NumericY>
 bool scatter(const std::vector<NumericX>& x,
              const std::vector<NumericY>& y,