How to use v4l2 API to capture the V4L2_PIX_FMT_MJPEG stream and then decode it to show the image using OpenCV
井民全, Jing, mqjing@gmail.com
main.cpp
#include <iostream> #include <fcntl.h> #include <unistd.h> #include <sys/ioctl.h> #include <linux/videodev2.h> #include <sys/mman.h> #include <opencv2/opencv.hpp> // #include <opencv2/imgcodecs.hpp> #define DEVICE_PATH "/dev/video0" #define IMAGE_WIDTH 640 #define IMAGE_HEIGHT 480 int main() { int fd = open(DEVICE_PATH, O_RDWR); if (fd == -1) { perror("Failed to open device"); return 1; } struct v4l2_capability caps; if (ioctl(fd, VIDIOC_QUERYCAP, &caps) == -1) { perror("Failed to query capabilities"); close(fd); return 1; } if (!(caps.capabilities & V4L2_CAP_VIDEO_CAPTURE)) { std::cerr << "Device does not support video capture" << std::endl; close(fd); return 1; } struct v4l2_format format; format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; format.fmt.pix.width = IMAGE_WIDTH; format.fmt.pix.height = IMAGE_HEIGHT; format.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG; // V4L2_PIX_FMT_YUYV; // V4L2_PIX_FMT_RGB24; //V4L2_PIX_FMT_MJPEG; // MJPEG pixel format format.fmt.pix.field = V4L2_FIELD_NONE; if (ioctl(fd, VIDIOC_S_FMT, &format) == -1) { perror("Failed to set format"); close(fd); return 1; } struct v4l2_requestbuffers req; req.count = 1; req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; req.memory = V4L2_MEMORY_MMAP; if (ioctl(fd, VIDIOC_REQBUFS, &req) == -1) { perror("Failed to request buffer"); close(fd); return 1; } struct v4l2_buffer buffer; memset(&buffer, 0, sizeof(buffer)); buffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buffer.memory = V4L2_MEMORY_MMAP; buffer.index = 0; if (ioctl(fd, VIDIOC_QUERYBUF, &buffer) == -1) { perror("Failed to query buffer"); close(fd); return 1; } void *buffer_start = mmap(nullptr, buffer.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, buffer.m.offset); if (buffer_start == MAP_FAILED) { perror("Failed to map buffer"); close(fd); return 1; } if (ioctl(fd, VIDIOC_QBUF, &buffer) == -1) { perror("Failed to queue buffer"); close(fd); return 1; } enum v4l2_buf_type type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (ioctl(fd, VIDIOC_STREAMON, &type) == -1) { perror("Failed to start streaming"); close(fd); return 1; } if (ioctl(fd, VIDIOC_DQBUF, &buffer) == -1) { perror("Failed to dequeue buffer"); close(fd); return 1; }
// SAVE to JPEG file for test FILE* fout; fout = fopen("test.jpg", "wb"); if (!fout) { perror("Cannot open image"); exit(EXIT_FAILURE); } printf("(buffer).bytesused 2= %d\n", (buffer).bytesused); fwrite(buffer_start, (buffer).bytesused, 1, fout); fclose(fout); cv::Mat image2 = cv::imread("./test.jpg"); if (image2.empty() == true) { std::cout << "image2 empty"<< std::endl; return 0; }
cv::imshow("opencv_logo", image2); printf("image2 cols = %d\n", image2.cols); printf("image2 rows = %d\n", image2.rows); printf("image2 depth = %d\n", image2.depth()); printf("image2 type() = %d\n", image2.type()); // OPENCV Part /* unsigned char* newbuffer = (unsigned char*) malloc((buffer).bytesused); memcpy(newbuffer, buffer_start, (buffer).bytesused); // TEST FILE* fk; fk = fopen("test.jpg", "rb"); fread(newbuffer, (buffer).bytesused, 1, fk); fclose(fk); cv::Mat image = cv::imdecode(cv::Mat(1, (buffer).bytesused, CV_8UC1, (void*) newbuffer), cv::IMREAD_COLOR); // work */ /* // work cv::Mat raw(1, (buffer).bytesused, CV_8UC1, buffer_start); cv::Mat image = cv::imdecode(raw, cv::IMREAD_COLOR); */ /* // work cv::Mat raw(format.fmt.pix.height, format.fmt.pix.width, CV_8UC3 , buffer_start); cv::Mat image = cv::imdecode(raw, cv::IMREAD_COLOR); */ /* // work cv::Mat raw(format.fmt.pix.height, format.fmt.pix.width, CV_8UC3 ); raw.data = (unsigned char*) buffer_start; cv::Mat image = cv::imdecode(raw, cv::IMREAD_COLOR); */ // cv::Mat image = cv::imdecode(cv::Mat(1, (buffer).bytesused, CV_8UC1, buffer_start), cv::IMREAD_COLOR); // work cv::Mat image = cv::imdecode(cv::Mat(1, (buffer).bytesused, CV_8UC1, (void*) buffer_start), cv::IMREAD_COLOR); // work printf("cols = %d\n", image.cols); printf("rows = %d\n", image.rows); printf("depth = %d\n", image.depth()); printf("type() = %d\n", image.type()); if (image.empty() == true) { std::cout << "image empty"<< std::endl; return 0; } cv::imshow("Captured Image", image);
cv::waitKey(0);
if (ioctl(fd, VIDIOC_STREAMOFF, &type) == -1) { perror("Failed to stop streaming"); close(fd); return 1; } munmap(buffer_start, buffer.length); close(fd); return 0; } |
