Python OpenCV pass mat ptr to c++ code

Question

Using Python 3 and OpenCV 3.

I have written some DLL's in C++ that export a few C-type functions and a uchar3 c-type struct. I have been using these with OpenCV in a C++ project, but I would like to be able to access them from Python as well. I am pretty new to Python, but I see that ctypes provides a fairly straightforward way to use DLL's.

What I can't sort out is how to get a pointer to the OpenCV Mat from within Python. Using C++, I do the following:

    myMatDataPointer = myMat.ptr<uchar>(0)

Ideally, I would like to find an easy way to use the uchar3 struct, but I see that a char* is supported out-of-the-box by ctypes, so I can make do with that if I can figure out how to get one pointing to the Mat's data.

Answer 1

I just realized that this never was answered. I can now provide one that I've been using. It supports bi-directional communication using a pure C interface.

import cv2
import ctypes
import numpy as np
import numpy.ctypeslib as npct

# Tell numpy what kind of pointer we want to use, load the dll
ucharPtr = npct.ndpointer(dtype=np.uint8, ndim=1, flags='CONTIGUOUS')
dll = ctypes.WinDLL('file.dll')

# the 'process_frame' function exported by the dll reads from
# the first pointer and writes to the second
process = dll['process_frame']
process.restype = None
process.argtypes = [ucharPtr, ucharPtr]

# Make two empty images for demonstration purposes
# OpenCV Python uses numpy arrays images, not Mat's
# This makes it super easy to work with images in python
in_image = np.zeros(shape,np.uint8,order='C').ravel()
out_image = np.zeros(self.shape,np.uint8,order='C').ravel()
process(in_image, out_image)

I could have used something more interesting for in_image of course, but we've successfully sent an image from OpenCV python to the dll, and can use whatever our dll gave back.