Imread with preallocated data pointer (cuda unified memory) is reallocating with the exact same size, but shouldn't [closed]

asked 2020-09-23 02:46:37 -0600

Maxime gravatar image

updated 2020-09-24 11:27:38 -0600

I have a problem with processing a set of images using opencv and cuda Unified Memory.
I am trying to use a Mat created once using a pre-allocated data buffer with cudaMallocManaged. When reading the image, the data seem to be reallocated during the mat::create in the imread function, but create should not be re-allocating since my previous data pointer has the exact same size. Here's an example of my issue

#include <iostream>
#include <experimental/filesystem>
#include <cuda_runtime.h>
#include <opencv2/opencv.hpp>
#include <opencv2/cudawarping.hpp>

#define COLS 5344
#define ROWS 4016

namespace fs = std::experimental::filesystem;

int main(int argc, char const *argv[])
    int index = 0;
    //---- 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;
    std::string input_folder = argv[1];
    std::string output_folder = argv[2];

    // Unified pointers
    void *src_ptr, *dst_ptr;
    if (cudaSuccess != cudaMallocManaged(&src_ptr, ROWS*COLS*3)) return -1;
    cv::Mat src(ROWS, COLS, CV_8UC3, src_ptr);
    cv::cuda::GpuMat d_src(ROWS, COLS, CV_8UC3, src_ptr);
    if (cudaSuccess != cudaMallocManaged(&dst_ptr, ROWS*COLS*3)) return -2;
    cv::Mat dst(rows, cols, CV_8UC3, dst_ptr);
    cv::cuda::GpuMat d_dst(rows, cols, CV_8UC3, dst_ptr);

    for (const auto& entry : fs::directory_iterator(input_folder))
        std::cout << "Before imread : src total = " << << "    src elemsize = " << src.elemSize() << "    src_ptr : " << src_ptr << "    &(src 0 0)" << &<cv::Vec3b>(0, 0) << std::endl;

        // Reading image
        src = cv::imread(entry.path());

        std::cout << "After imread : src total = " << << "    src elemsize = " << src.elemSize() << "    src_ptr : " << src_ptr << "    &(src 0 0)" << &<cv::Vec3b>(0, 0) << std::endl;

        // Some action on cuda device
        cv::cuda::rotate(d_src, d_dst, d_dst.size(), 180.0);

        // write output file
        std::string out_name = output_folder + "preprocessed_image" + std::to_string(index) + ".jpg";
        cv::imwrite(out_name, dst);
        break; // only the first one is needed for the example
    return 0;

With this example, when the imread is perform (so a mat create), the data pointer is reallocated for the first loop. It seems weird since mat create should verify mat::total * mat::elemsize, and not reallocate if the result is the same as what is read in the image. And here this is the case. I'm printing the addresses of src_ptr and the data address of the src mat : before the imread, they are identical, after the imread, the second one changed.
My goal is to avoid explicit data transfer between host and cuda device. I really don't know why the data pointer is reallocated, i'm looking forward to some advice in order to correct this issu or avoid it. Thank you in advance.

[EDIT] The unique "solution" i found is to read to a temp Mat and then use copyto to the src one. With this solution the address doesn't change, but a copy is performed... I mean, the same ... (more)

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Closed for the following reason the question is answered, right answer was accepted by Maxime
close date 2020-09-24 11:28:13.896352