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To import a DMA-BUF file descriptor into an OpenCV cv::Mat, you can follow the below steps:

1. Get the file descriptor of the DMA-BUF buffer using functions like ioctl() or mmap().
2. Create a cv::Mat object with the desired size and pixel format.
3. Use the cv::Mat::ptr() method to get a pointer to the first pixel of the image data.
4. Map the DMA-BUF buffer to this pointer using mmap().
5. Set the cv::Mat data pointer to point to the mapped memory.

Here is a sample code snippet to import a DMA-BUF file descriptor into an OpenCV cv::Mat:

#include <opencv2/opencv.hpp>
#include <linux/dma-buf.h>
#include <fcntl.h>
#include <sys/mman.h>

// Get DMA-BUF buffer file descriptor
int fd = open("/dev/dri/card0", O_RDWR | O_CLOEXEC);
struct dma_buf_sync buf_sync = {0};
int ret = ioctl(fd, DRM_IOCTL_DMA_BUF_SYNCOBJ_CREATE, &buf_sync);
int dma_buf_fd = buf_sync.syncobjs[0];

// Set up cv::Mat object
cv::Mat image(480, 640, CV_8UC4);

// Map DMA-BUF buffer to cv::Mat data pointer
void* buffer_ptr = mmap(NULL, 480*640*4, PROT_READ | PROT_WRITE, MAP_SHARED, dma_buf_fd, 0);
image.data = reinterpret_cast<uchar*>(buffer_ptr);

// Access cv::Mat elements
uchar b = image.at<cv::Vec4b>(10, 20)[0]; // Get blue channel value at (10, 20)

Note that this is just an example and the details may vary depending on the specific use case. You should also check for errors and handle them appropriately.