send image opencv c++ to python

Question

I open the picture with python, send it to the C ++ function for processing, and return the picture. How to correctly get a picture from a c ++ function?

Segmentation error

Code example:

C++

// t_lib.cpp 
#include "opencv2/opencv.hpp"

extern "C" unsigned char*  getfg(int rows,int cols, unsigned char* fdata)
//extern "C" void getfg(int rows,int cols, unsigned char* fdata)
{
    cv::Mat frame = cv::Mat(rows, cols, CV_8UC3, (void *) fdata);
    // Work 
    //cv::imshow("Display window C++", frame);
    //cv::waitKey(0);

    // do something

    return frame.data; //fdata;
}

Python 2.7

# test.py
import cv2
import numpy as np
import ctypes
lib = ctypes.cdll.LoadLibrary('./lib.so')

def getfg(img):
    ptr = lib.getfg(img.shape[0], img.shape[1], img.ctypes.data_as(ctypes.POINTER(ctypes.c_ubyte)))
    buf = (ctypes.c_ubyte * img.shape[0] * img.shape[1]  * 3).from_address(ptr)
    res = np.ndarray(buffer=buf, dtype=np.uint8, shape=(img.shape[0], img.shape[1], 3), order="C")
    return res

f = cv2.imread("sample.jpg")
#gray_image = cv2.cvtColor(f, cv2.COLOR_BGR2GRAY)
AS = getfg(f)
print AS.shape
#np.array(AS)[0]
cv2.imshow("Window Python", AS)
cv2.waitKey(0)

Compilation:

g++ -c -fPIC t_lib.cpp -o lib -lX11 $(pkg-config opencv --cflags --libs) -I/usr/include/python2.7 -I/usr/lib/python2.7;

g++ -shared -Wl,-soname,lib.so -o lib.so lib -lopencv_core -lopencv_highgui -lopencv_imgproc

command for see opencv libs:

pkg-config opencv --cflags --libs

output:

-I/usr/include/opencv -lopencv_shape -lopencv_stitching 
-lopencv_superres -lopencv_videostab -lopencv_aruco 
-lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib
-lopencv_datasets -lopencv_dpm -lopencv_face 
-lopencv_freetype -lopencv_fuzzy -lopencv_hdf 
-lopencv_line_descriptor -lopencv_optflow -lopencv_video 
-lopencv_plot -lopencv_reg -lopencv_saliency -lopencv_stereo 
-lopencv_structured_light -lopencv_phase_unwrapping -lopencv_rgbd 
-lopencv_viz -lopencv_surface_matching -lopencv_text 
-lopencv_ximgproc -lopencv_calib3d -lopencv_features2d 
-lopencv_flann -lopencv_xobjdetect -lopencv_objdetect 
-lopencv_ml -lopencv_xphoto -lopencv_highgui 
-lopencv_videoio -lopencv_imgcodecs -lopencv_photo 
-lopencv_imgproc -lopencv_core

version c++ OpenCV 3.2.0

Answer 1

you have to compile this c++ code into a shared library, according to this and then write a wrapper