OpenCV - how to create Mat from uint8_t pointer

Question

I have the following C++ code:

void foo(const uint8_t* data, int height, int width) {
  // need to create a cv::Mat from *data, which is a pointer to grayscale image data

  // doesn't work correctly (compiles, but array access on the mat leads to a segmentation fault)
  auto img = cv::Mat(height, width, CV_8UC1, &data);

  // how can I fix the line above to create a proper cv::Mat?
}

// I'm calling foo like this
// img is a grayscale image
foo(img.ptr<uint8_t>(0), img.cols, img.rows);

Could anyone point me on what's wrong with my syntax on creating the matrix inside of foo?

Answer 1

On this page cv::Mat Class Reference , we can find the cv::Mat construction function as follows:

///! 2017.10.05 09:31:00 CST
/// cv::Mat public construction

Mat ()
Mat (int rows, int cols, int type)
Mat (Size size, int type)
Mat (int rows, int cols, int type, const Scalar &s)
Mat (Size size, int type, const Scalar &s)
Mat (int ndims, const int *sizes, int type)
Mat (int ndims, const int *sizes, int type, const Scalar &s)
Mat (const Mat &m)
Mat (int rows, int cols, int type, void *data, size_t step=AUTO_STEP)
Mat (Size size, int type, void *data, size_t step=AUTO_STEP)
Mat (int ndims, const int *sizes, int type, void *data, const size_t *steps=0)
Mat (const Mat &m, const Range &rowRange, const Range &colRange=Range::all())
Mat (const Mat &m, const Rect &roi)
Mat (const Mat &m, const Range *ranges)

---

To create cv::Mat from uint8_t pointer, we can use those two functions:

Mat (int rows, int cols, int type, void *data, size_t step=AUTO_STEP)
Mat (Size size, int type, void *data, size_t step=AUTO_STEP)

---

Here is my experiment:

///! 2017.10.05 09:40:33 CST
/// convert uint8_t array/pointer to cv::Mat

#include <opencv2/core.hpp>
#include <iostream>

int main(){        
    uint8_t uarr[] = {1,2,3,4,5,6,7,8,9,10,11,12};
    int rows = 2;
    int cols = 2;
    cv::Size sz(cols,rows);

    cv::Mat mat1(sz,CV_8UC3, uarr);
    cv::Mat mat2(rows, cols, CV_8UC3, uarr);

    std::cout<< "mat1: \n"<<mat1 << "\n\nmat2:\n" << mat2 << std::endl;
    return 0;
}

---

The result is excepted:

mat1: 
[  1,   2,   3,   4,   5,   6;
   7,   8,   9,  10,  11,  12]

mat2:
[  1,   2,   3,   4,   5,   6;
   7,   8,   9,  10,  11,  12]