CODEWITHSUNDEEP
c++opencvmemory-managementmat
Olivier Desenfans
Olivier Desenfans

Reputation: 91

How can I reserve a fixed buffer size in OpenCV Mat?

I am rewriting some legacy code that does matrix operations on doubles using a raw C-style array. Since the code already has a dependency on OpenCV somewhere else, I want to use the cv::Mat class instead.

The specific code that bothers me works on square matrixes from size 1*1 to NN. It does so by allocating an NN buffer and uses a subset of it for smaller matrixes.

double* buf = new double[NxN];
for (int i = 1; i < N; ++i) {
    // Reuse buf to create a i*i matrix and perform matrix operations
    ...

}
delete[] buf;

Basically, I want to replace that code to use cv::Mat objects in the loop instead. Problem is, the code requires a lot of loop iterations (there are nested loops and so on) and there are too many allocations/deallocations if I just use the naïve and clean approach. Therefore, I want to reserve the size of my matrix object beforehand and resize it for each iteration. This would ideally look like this:

cv::Mat m;
m.reserveBuffer(N * N * sizeof(double));
for (int i = 1; i < N; ++i) {
    m = cv::Mat(i, i, CV_64F);
    // Perform matrix operations on m
    ...

}

But in my understanding this would simply drop the previous instance of m and then allocate a i*i matrix. What would the right approach be?

Upvotes: 1

Views: 1465

Answers (1)

Oliort UA
Oliort UA

Reputation: 1647

You can create a submatix header for your buffer using cv::Mat::operator(). Pass a cv::Rect for ROI you want to process in current loop iteration ({0, 0, i, i} in your case) and it will return a view of your buffer region as another cv::Mat instance. It will not allocate new buffer but will refer to original buffer data instead.

cv::Mat m(N, N, CV_64FC1);
for (int i = 1; i < N; ++i) {
    cv::Mat subM = m({0, 0, i*i});
    // Perform matrix operations on "subM"
    // Modifying "subM" will modify "m" buffer region that "subM" represents
}

Note that subM will not be continious, so you will need to process it row by row if you do any raw buffer processing.

Upvotes: 1

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