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Mat::create method checks Mat's size and reallocate data only if requested size or type differs from existed. So, if you call create twice with the same size and type it won't reallocate data. All OpenCV functions works in the same way. You can declare Mat object outside processing loop and it will be allocated only once, if all input frames have the same size:

VideoCapture cap;
Mat src;
mat gray;
Mat edges;
for(;;)
{
    // all following functions will allocate memory only once on first iteration
    cap >> src; 
    cvtColor(src, gray, COLOR_BGR2GRAY);
    Canny(gray, edges, 50, 100);
}

Mat::create method checks Mat's size and reallocate data only if requested size or type differs from existed. So, if you call create twice with the same size and type it won't reallocate data. All OpenCV functions works in the same way. You can declare Mat object outside of processing loop and it will be allocated only once, if all input frames have the same size:

VideoCapture cap;
Mat src;
mat >Mat gray;
Mat edges;
for(;;)
{
    // all following functions will allocate memory only once on first iteration
    cap >> src; 
    cvtColor(src, gray, COLOR_BGR2GRAY);
    Canny(gray, edges, 50, 100);
}

Mat::create method checks Mat's size and reallocate data only if requested size or type differs from existed. So, if you call create twice with the same size and type it won't reallocate data. All OpenCV functions works in the same way. You can declare Mat object outside of processing loop and it will be allocated only once, if all input frames have the same size:

VideoCapture cap;
Mat src;
>Mat Mat gray;
Mat edges;
for(;;)
{
    // all following functions will allocate memory only once on first iteration
    cap >> src; 
    cvtColor(src, gray, COLOR_BGR2GRAY);
    Canny(gray, edges, 50, 100);
}