OpenCV: passing an argument from a C main function to the OpenCV C++ API - opencv

Question

I have an application that is written in C, and I would like to use a few OpenCV functions to do some image processing. I see where the C API has been deprecated, so I am currently trying to interface with the C++ API from C. I am running into some compilation issues.
Just to reproduce the example, let's say that I have this C main function in a file grouping_test_main.c:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdarg.h>
#include <malloc.h>
extern "C"
{
#include "opencv_grouping.h"
}
uint8_t **allocate_2D_array(uint32_t SizeY, uint32_t SizeX);
int free_2D_array(uint8_t **image_array, uint32_t SizeY);
int main()
{
uint32_t SizeY=19;
uint32_t SizeX=20;
uint8_t **image_array;
FILE* fimage_array = fopen("./test_array.txt", "r");
double threshold_value=128;
ConnectedComponentsResults results;
int i;
int j;
image_array = allocate_2D_array(SizeY, SizeX);
for (i=0; i<SizeY; i++)
{
for (j=0; j<SizeX; j++)
{
fscanf(fimage_array, "%hhd", &image_array[i][j]);
}
}
fclose(fimage_array);
treshold_then_group(image_array, SizeY, SizeX, results);
printf("Number of connected components: %d\n", results.num_components);
free_2D_array(image_array, SizeY);
}
uint8_t **allocate_2D_array(uint32_t SizeY, uint32_t SizeX)
/*
* Allocates memory for a 2D image array.
*/
{
int i;
uint8_t **image_array;
image_array = malloc(SizeY * sizeof(uint8_t *));
for (i = 0; i < SizeY; i++)
{
image_array[i] = malloc(SizeX * sizeof(uint8_t ));
if(image_array[i] == NULL)
{
fprintf(stderr, "Memory allocation for image_array[%d] failed.\n", i);
exit(EXIT_FAILURE);
}
}
return image_array;
}
int free_2D_array(image_array, SizeY)
/*
* Frees memory allocated for 2D image array.
*/
uint8_t **image_array;
uint32_t SizeY;
{
int i;
for (i = 0; i < SizeY; i++)
{
free(image_array[i]);
}
return(1);
}
threshold_then_group() is the C++ function. test_array.txt is a text file with a 19X20 array:
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 255 255 255 0 0 0 0 0 0 255 255 255 255 255 0 0 0
0 0 0 255 255 255 0 0 0 0 0 0 0 255 255 255 0 0 0 0
0 0 255 255 255 255 0 0 0 0 0 0 0 0 255 0 0 0 0 0
0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 255 255 0 0 0 0 0 0 0 0 255 0 0 0 0 0
0 0 0 255 255 255 255 255 0 0 0 0 255 255 255 255 0 0 0 0
0 0 0 0 255 255 255 255 255 0 0 255 255 255 0 0 0 0 0 0
0 0 0 0 0 0 255 255 255 255 255 255 255 0 0 0 0 0 0 0
0 0 0 0 0 0 0 255 255 255 255 255 255 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 255 255 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
opencv_grouping.h is the header file where I declare the C++ functions.
/* Header file for opencv_grouping.cpp.
*/
#ifndef CONNECTED_COMPONENTS_H
#define CONNECTED_COMPONENTS_H
#include <stdint.h>
#include <opencv2/opencv.hpp>
struct ConnectedComponentsResults
{
int num_components;
cv::Mat labels;
cv::Mat stats;
cv::Mat centroids;
};
void threshold_then_group(uint8_t **image_array, uint32_t SizeY, uint32_t SizeX, double threshold_value, ConnectedComponentsResults &results);
#endif
Then, the C++ functions in opencv_grouping.cpp are:
/* Grouping and centroiding implementation using OpenCV's
* connectedComponentsWithStats().
*/
#include <stdint.h>
#include <iostream>
#include <fstream>
#include <cstdio>
#include <cstdlib>
#include <opencv2/opencv.hpp>
#include "opencv_grouping.h"
void threshold_then_group(uint8_t **image_array, uint32_t SizeY, uint32_t SizeX, double threshold_value, ConnectedComponentsResults &results)
{
uint8_t *image_data = new uint8_t[SizeY * SizeX];
double max_value=255.0;
int i;
int j;
for (i=0; i<SizeY; i++)
{
for (j=0; j<SizeX; j++)
{
image_data[i * SizeX + j] = image_array[i][j];
}
}
cv::Mat image(SizeY, SizeX, CV_8U, image_data);
cv::Mat binary;
threshold(image, binary, threshold_value, max_value, cv::THRESH_BINARY);
cv::Mat labels, stats, centroids;
int num_components = cv::connectedComponentsWithStats(binary, results.labels, results.stats, results.centroids, 8, CV_32S);
results.num_components = results.stats.rows - 1;
delete[] image_data;
}
I am attempting to compile the code like this:
g++ `pkg-config --cflags opencv4` -c opencv_grouping.cpp
gcc `pkg-config --cflags opencv4` -c grouping_test_main.c
gcc opencv_test.o opencv_grouping.o -lstdc++ `pkg-config --libs opencv4` -o opencv_test2
opencv_grouping.cpp compiles just fine, but grouping_test_main.c fails. This is because opencv_grouping.h includes opencv2/opencv.hpp, and the error messages indicate that the files included in opencv.hpp must be compiled using a C++ compiler. I haven't tried to compile the main function using g++, but even if this somehow works, this isn't really a solution. My actual main function is a lot more complicated than what I've shown here, and it uses a lot of C libraries that I don't think would compile with g++.