C++ conversion from NumPy array to Mat (OpenCV)

Question

I am writing a thin wrapper around ArUco augmented reality library (which is based on OpenCV). An interface I am trying to build is very simple:

• Python passes

Answer 1

Optionally, if you don't like to use wrappers, and want to use native python extension module, you can do it like this.

python3:

my_image = cv.imread("my_image.jpg", 1)  # reads colorfull image in python
dims = my_image.shape  # get image shape (h, w, c)
my_image = my_image.ravel()  # flattens 3d array into 1d
cppextenionmodule.np_to_mat(dims, my_image)

c++:

static PyObject *np_to_mat(PyObject *self, PyObject *args){
    PyObject *size;
    PyArrayObject *image;

    if (!PyArg_ParseTuple(args, "O!O!", &PyTuple_Type, &size, &PyArray_Type, &image)) {
        return NULL;
    }
    int rows = PyLong_AsLong(PyTuple_GetItem(size ,0));
    int cols = PyLong_AsLong(PyTuple_GetItem(size ,1));
    int nchannels = PyLong_AsLong(PyTuple_GetItem(size ,2));
    char my_arr[rows * nchannels * cols];

    for(size_t length = 0; length<(rows * nchannels * cols); length++){
        my_arr[length] = (*(char *)PyArray_GETPTR1(image, length));
    }

    cv::Mat my_img = cv::Mat(cv::Size(cols, rows), CV_8UC3, &my_arr);

    ... whatever with the image
}