Why imread is reallocating data buffer

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Hello!

I recently ask an other question similar to this one. Here I don't have any CUDA dependencies :

I create a Mat using a pre-allocated data buffer. Here's a code example :

#include <iostream>
#include <experimental/filesystem>
#include <opencv2/opencv.hpp>
#include <opencv2/imgcodecs.hpp>

namespace fs = std::experimental::filesystem;

int main(int argc, char const *argv[])
{
    // ---- get arguments ----
    if (argc < 2) {
        std::cout << "number of arguments invalid" << std::endl;
        std::cout << "1 - Input Folder" << std::endl;
        std::cout << "2 - Output Folder" << std::endl;
        exit(1);
    }
    std::string input_folder = argv[1];
    std::string output_folder = argv[2];

    int cols = 5344, rows = 4016;    //your image size

    cv::Vec3b *src_ptr = (cv::Vec3b *)malloc(rows*cols*sizeof(cv::Vec3b));
    cv::Mat src(rows, cols, CV_8UC3, src_ptr);

    std::cout << src_ptr << "    " << src.ptr<cv::Vec3b>() << "    " << src.size() << std::endl;

    for (const auto& entry : fs::directory_iterator(input_folder))
    {
        src = cv::imread(entry.path());
        std::cout << src_ptr << "    " << src.ptr<cv::Vec3b>() << "    " << src.size() << std::endl;
    }

    return 0;
}

I have an input folder with a set of images, and I am printing the address of the buffer alongside the current address of the data of Mat src. (Also with the size to prove that it is the same).

When running it, the address of src data change, and if you have more than one image, you can see that imread reallocate data at the same address every two images. Imread is supposed to call Mat::create : it should not reallocate if it is the same size!

I tried another test :

cv::Mat temp;

In the for loop :

    temp = cv::imread(entry.path());
    temp.copyTo(src);

Here the right pre-allocated address is used.

I'm looking to avoid this copy and directly use imread. Why does it behave like this?

Thanks in advance for any answers!!!