v4l2 c++显示摄像头

要在 C++ 中显示摄像头，可以使用 V4L2（Video for Linux 2）库。以下是一个简单的示例代码，可以帮助你开始显示摄像头：

#include <iostream>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/videodev2.h>
#include <opencv2/opencv.hpp>

#define CAMERA_DEVICE "/dev/video0"
#define CAPTURE_WIDTH 640
#define CAPTURE_HEIGHT 480

using namespace std;
using namespace cv;

int main()
{
    int fd;
    struct v4l2_capability cap;
    struct v4l2_format fmt;
    struct v4l2_requestbuffers req;
    struct v4l2_buffer buf;
    void* buffer_start;
    unsigned int i;

    // 打开摄像头设备
    fd = open(CAMERA_DEVICE, O_RDWR);
    if (fd == -1)
    {
        cout << "无法打开摄像头设备" << endl;
        return -1;
    }

    // 查询摄像头设备信息
    if (ioctl(fd, VIDIOC_QUERYCAP, &cap) == -1)
    {
        cout << "无法查询摄像头设备信息" << endl;
        close(fd);
        return -1;
    }

    // 设置摄像头格式
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    fmt.fmt.pix.width = CAPTURE_WIDTH;
    fmt.fmt.pix.height = CAPTURE_HEIGHT;
    fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUV420;
    fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
    if (ioctl(fd, VIDIOC_S_FMT, &fmt) == -1)
    {
        cout << "无法设置摄像头格式" << endl;
        close(fd);
        return -1;
    }

    // 请求摄像头缓冲区
    req.count = 4;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;
    if (ioctl(fd, VIDIOC_REQBUFS, &req) == -1)
    {
        cout << "请求摄像头缓冲区失败" << endl;
        close(fd);
        return -1;
    }

    // 映射摄像头缓冲区
    buffer_start = malloc(req.count * sizeof(*buffer_start));
    if (!buffer_start)
    {
        cout << "映射摄像头缓冲区失败" << endl;
        close(fd);
        return -1;
    }

    for (i = 0; i < req.count; i++)
    {
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = i;
        if (ioctl(fd, VIDIOC_QUERYBUF, &buf) == -1)
        {
            cout << "无法查询摄像头缓冲区" << endl;
            close(fd);
            return -1;
        }

        *((void**)buffer_start + i) = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, buf.m.offset);
        if (*((void**)buffer_start + i) == MAP_FAILED)
        {
            cout << "无法映射摄像头缓冲区" << endl;
            close(fd);
            return -1;
        }
    }

    // 将缓冲区入队
    for (i = 0; i < req.count; i++)
    {
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = i;
        if (ioctl(fd, VIDIOC_QBUF, &buf) == -1)
        {
            cout << "无法将缓冲区入队" << endl;
            close(fd);
            return -1;
        }
    }

    // 开始采集
    enum v4l2_buf_type type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_STREAMON, &type) == -1)
    {
        cout << "无法开始采集" << endl;
        close(fd);
        return -1;
    }

    // 显示帧图像
    Mat frame(CAPTURE_HEIGHT, CAPTURE_WIDTH, CV_8UC3);
    while (true)
    {
        // 获取缓冲区
        if (ioctl(fd, VIDIOC_DQBUF, &buf) == -1)
        {
            cout << "无法获取缓冲区" << endl;
            close(fd);
            return -1;
        }

        // 处理图像
        memcpy(frame.data, *((void**)buffer_start + buf.index), buf.bytesused);

        // 将缓冲区重新入队
        if (ioctl(fd, VIDIOC_QBUF, &buf) == -1)
        {
            cout << "无法将缓冲区重新入队" << endl;
            close(fd);
            return -1;
        }

        // 显示图像
        imshow("camera", frame);
        waitKey(1);
    }

    // 停止采集
    if (ioctl(fd, VIDIOC_STREAMOFF, &type) == -1)
    {
        cout << "无法停止采集" << endl;
        close(fd);
        return -1;
    }

    // 释放缓冲区
    for (i = 0; i < req.count; i++)
    {
        munmap(*((void**)buffer_start + i), buf.length);
    }
    free(buffer_start);

    // 关闭摄像头设备
    close(fd);

    return 0;
}

这段代码使用了 OpenCV 库来显示图像，需要在编译时链接该库。你可以使用以下命令进行编译：

g++ -o camera camera.cpp `pkg-config opencv --cflags --libs`

请注意，这段代码可能需要根据你的摄像头设备和环境进行调整。你需要根据自己的需求修改它。