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.

You'd have to create a pure C interface for the library. For C++, try frameworks that need a compiler such as Cython, SWIG, or Boost.Python. — Eryk Sun
– Eryk Sun, Commented May 6, 2016 at 19:43
Yes, the dll's export pure C functions (as far as I know, it's not possible to export non-pure-C functions from a dll). The problem is that these functions need a pointer to the OpenCV mat's data. I know how to access this in C++, but not in Python. — DKL
– DKL, Commented May 7, 2016 at 23:09
There are C++ DLLs (e.g. Microsoft's ulib.dll, a utility library used by console programs), but since there's no cross-platform, stable ABI, it's not a good idea in general, and such DLLs can't be easily used with ctypes. On Windows it's common for C++ devs to create COM DLLs, for which you can use comtypes or pythoncom. — Eryk Sun
– Eryk Sun, Commented May 7, 2016 at 23:42
As to getting a pointer, you can write a C stub function that returns the pointer. Generally this calls a C++ constructor to get a this instance for a Python proxy class. The proxy calls C wrappers for the C++ methods, manually supplying the this reference as the first argument. — Eryk Sun
– Eryk Sun, Commented May 7, 2016 at 23:45
Is it implicit in your suggested solution that OpenCV does not provide any way of directly accessing a Mat's data via Python as it does in C++? I was really hoping there would be a similarly-simple way to do custom image processing. I didn't quite grasp your suggestion, but it sounds like it would probably just be easier for me to stick with 100% C++. — DKL
– DKL, Commented May 8, 2016 at 1:15

DKL · Accepted Answer · 2017-04-28 16:04:25Z

3

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.

answered Apr 28, 2017 at 16:04

DKL

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2 Comments

Navid Over a year ago

Thanks for this, What is the ++ side? how to load the pointer into an opencv Mat?

DKL Over a year ago

I actually wasn't using opencv in the C++ code, rather I was using low-level CUDA code to do fast processing on the GPU. That said, IIRC with OpenCV C++ if you have a Mat with correct metadata, you can just change its pointer to wherever your data is.

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