CODEWITHSUNDEEP
c++opencv
MMM
MMM

Reputation: 79

Mat values are changed when I alter other Mat object

When I copy a Mat object, the values in other Mat object changes unexpectedly. The change occurs between the two print statement. Why is this?

Mat flow;

calcOpticalFlowFarneback(u_prev, u_curr, u_flow, 0.5, 2, 20, 3, 15, 1.2, OPTFLOW_FARNEBACK_GAUSSIAN);
flow = u_flow.getMat(ACCESS_READ);

cout << "1 " << flow.ptr<Pixel2>(680,192)->x << endl;


Mat out_img;
resized_frame.copyTo (out_img);

cout << "2 " << flow.ptr<Pixel2>(680,192)->x << endl;

Output as following

1 164.812
2 8.42217e-21

Upvotes: 0

Views: 76

Answers (1)

Apoorv
Apoorv

Reputation: 383

The parameters for Mat::ptr are Mat::ptr(int row, int col) as mentioned in the docs. Since flow is a 480x852 matrix, flow.ptr<Pixel2>(680, 192) accesses an out-of-bounds row and reads memory outside the matrix. Most likely that memory location is assigned to a different variable in your program (which might be out_img in this case) which gets changed over time. It might even segfault on some platforms depending on the memory layout used by the compiler.

Upvotes: 2

Related Questions