pass Mat object C++ to Unity - c++

Question

I'd like to return a Mat object to Unity from c++ code. However i get access violation error at c++ part like that
Unity Editor [version: Unity 2017.3.0f3_a9f86dcd79df]
SaliencyCV.dll caused an Access Violation (0xc0000005)
in module SaliencyCV.dll at 0033:270027f0.
Error occurred at 2018-03-06_235212.
C:\Program Files\Unity\Editor\Unity.exe, run by Dilara.
43% memory in use.
16266 MB physical memory [9199 MB free].
18698 MB paging file [9861 MB free].
134217728 MB user address space [134185466 MB free].
Read from location 990d0000 caused an access violation.
Here is c++ code:
uchar* cppMethod(uchar* frameData, int WIDTH, int HEIGHT, int* rows, int* cols)
{
Mat img(HEIGHT, WIDTH, CV_8UC3);
img.data = frameData;
flip(img, img, 0);
Mat result = calculateSaliency(img);
*rows = result.rows;
*cols = result.cols;
int length = result.rows * result.cols * 3;
uchar* tmpArr = result.data;
uchar* resultArray = new uchar[length];
for (int i = 0; i < length; i++)
{
resultArray[i] = tmpArr[i];
}
return resultArray;
}
Can someone help me?

Answer 1

You should call the correct Mat constructor, which accepts external data pointer, to make the object not release/destruct the corresponding memory location data points to. You can read about this behaviour in Mat::release().
The problem with your code is that
Mat img(HEIGHT, WIDTH, CV_8UC3) allocates a memory block of type CV_8UC3 of size HEIGHT*WIDTH, which is not used (because you are changing the data member variable to point to a different memory location, anyways),
At function exit, img is destructed, which results in a call to release(), which in turn destructs frameData, which is not the intended behaviour.
Change your first two lines to read
Mat img(HEIGHT, WIDTH, CV_8UC3, frameData);
And if you are passing resultArray to C#, where you are most likely not managing the pointed-to-memory's lifetime, you would be most likely having memory leaks. #Programmer has already suggested in his answer to your previous question that you should allocate the memory in C#, pass it to C++, and write in-place in the C++ side.
In short, you should have something like:
#include <algorithm>
void cppMethod(uchar *frameData, uchar *out, const int WIDTH, const int HEIGHT,
int *rows, int *cols) {
/* this constructor will not manage frameData's lifetime */
Mat img(HEIGHT, WIDTH, CV_8UC3, frameData);
/* in-place operation */
flip(img, img, 0);
/* local variable --- it will be destructed properly */
Mat result = calculateSaliency(img);
/* well-defined if rows and cols are scalars passed by reference */
*rows = result.rows;
*cols = result.cols;
/* make sure length will not overflow */
int length = result.rows * result.cols * 3;
/* you don't need this */
// uchar *tmpArr = result.data;
/* you sholuld NOT do this */
// uchar *resultArray = new uchar[length];
// use std::copy from <algorithm>
// for (int i = 0; i < length; i++) {
// resultArray[i] = tmpArr[i];
// }
std::copy(result.data, result.data + length, out);
// return resultArray;
}