Construct cv::Mat via C++ 1D array - c++

Question

I took over a program, which use 1d array to construct cv::Mat. I'm confuse about it, and I make a demo:
int main()
{
short data[] = {
11, 12, 13, 14,
21, 22, 23, 24,
31, 32, 33, 34,
43, 42, 43, 44
};
cv::Mat m{ 4, 4, CV_8U, data};
cv::imwrite("test.png", m);
return 0;
}
I expect the output is
11 12 13 14
21 22 23 24
31 32 33 34
43 42 43 44
But I open the img in MATLAB:
11 0 12 0
13 0 14 0
21 0 22 0
23 0 24 0

Answer 1

The 3rd parameter in the constructor you used for cv::Mat (CV_8U) specifies the format of the data buffer.
Your data is an array of short, i.e. each entry is 2 bytes. When you use values small values like you did, it means one of the bytes for each short will be 0.
When you use this buffer to initialized a cv::Mat of type CV_8U you tell opencv to treat the buffer as containing unsigned chars (1 byte elements). So each short is interpreted as 2 unsigned char bytes.
This is why you get the zeroes in the output.
Eaither change the type of the data buffer to unsigned char [], or change the cv::Mat data type to e.g. CV_16S. The 2 should usually match.
You can do it e.g. like this:
unsigned char data[] = {
11, 12, 13, 14,
21, 22, 23, 24,
31, 32, 33, 34,
43, 42, 43, 44
};
cv::Mat m{ 4, 4, CV_8U, data };
You can see the list of opencv data types here (in the Data types section):
https://docs.opencv.org/3.4/d1/d1b/group__core__hal__interface.html