如何使用 c++ 使用 v4l2 API 连续捕获 h264 视频？

Question

下面的修改代码 (original code saves a frame into the image) 捕获一个帧并以 mp4 文件的形式保存到磁盘中。我正在尝试更改代码以从网络摄像头（Logitech c920）捕获 h264 视频帧。网络摄像头支持 h264 视频流。

如何修改代码以实时捕获和存储流式视频，直到用户退出程序？

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <linux/ioctl.h>
#include <linux/types.h>
#include <linux/v4l2-common.h>
#include <linux/v4l2-controls.h>
#include <linux/videodev2.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <string.h>
#include <fstream>
#include <string>

#include <iostream>
#include <vector>
#include <cstring>
#include <cassert>

using namespace std;

// Sepecfic to WEB Cam
short unsigned int CAM_WIDTH=1920;
short unsigned int CAM_HEIGHT=1080;

int process() {
    // 1.  Open the device
    int fd; // A file descriptor to the video device
    fd = open("/dev/video0",O_RDWR);
    if(fd < 0){
        perror("Failed to open device, OPEN");
        return 1;
    }

    // 2. Ask the device if it can capture frames

    v4l2_capability capability;
    if(ioctl(fd, VIDIOC_QUERYCAP, &capability) < 0){
        // something went wrong... exit
        perror("Failed to get device capabilities, VIDIOC_QUERYCAP");
        return 1;
    }

    // 3. Set Image format
    v4l2_format imageFormat;
    imageFormat.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    imageFormat.fmt.pix.width = CAM_WIDTH;
    imageFormat.fmt.pix.height = CAM_HEIGHT;
    imageFormat.fmt.pix.pixelformat = V4L2_PIX_FMT_H264; // V4L2_PIX_FMT_MJPEG;
    imageFormat.fmt.pix.field = V4L2_FIELD_NONE;
    // tell the device you are using this format
    if(ioctl(fd, VIDIOC_S_FMT, &imageFormat) < 0){
        perror("Device could not set format, VIDIOC_S_FMT");
        return 1;
    }

    // 4. Request Buffers from the device
    v4l2_requestbuffers requestBuffer = {0};
    requestBuffer.count = 10; // one request buffer
    requestBuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; // request a buffer wich we an use for capturing frames
    requestBuffer.memory = V4L2_MEMORY_MMAP;

    if(ioctl(fd, VIDIOC_REQBUFS, &requestBuffer) < 0){
        perror("Could not request buffer from device, VIDIOC_REQBUFS");
        return 1;
    }

    // 5. Quety the buffer to get raw data ie. ask for the you requested buffer
    // and allocate memory for it
    v4l2_buffer queryBuffer = {0};
    queryBuffer.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    queryBuffer.memory = V4L2_MEMORY_MMAP;
    queryBuffer.index = 5;

    if(ioctl(fd, VIDIOC_QUERYBUF, &queryBuffer) < 0){
        perror("Device did not return the buffer information, VIDIOC_QUERYBUF");
        return 1;
    }
    // use a pointer to point to the newly created buffer
    // mmap() will map the memory address of the device to
    // an address in memory
    char* buffer = (char*)mmap(NULL, queryBuffer.length, PROT_READ | PROT_WRITE, MAP_SHARED,
                        fd, queryBuffer.m.offset);
    memset(buffer, 0, queryBuffer.length);

    // 6. Get a frame
    // Create a new buffer type so the device knows whichbuffer we are talking about
    v4l2_buffer bufferinfo;
    memset(&bufferinfo, 0, sizeof(bufferinfo));
    bufferinfo.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    bufferinfo.memory = V4L2_MEMORY_MMAP;
    bufferinfo.index = 0;

    // Activate streaming
    int type = bufferinfo.type;
    if(ioctl(fd, VIDIOC_STREAMON, &type) < 0){
        perror("Could not start streaming, VIDIOC_STREAMON");
        return 1;
    }

/***************************** Begin looping here *********************/
    // Queue the buffer
    if(ioctl(fd, VIDIOC_QBUF, &bufferinfo) < 0){
        perror("Could not queue buffer, VIDIOC_QBUF");
        return 1;
    }

    // Dequeue the buffer
    if(ioctl(fd, VIDIOC_DQBUF, &bufferinfo) < 0){
        perror("Could not dequeue the buffer, VIDIOC_DQBUF");
        return 1;
    }
    // Frames get written after dequeuing the buffer
    cout << "Buffer has: " << (double)bufferinfo.bytesused / 1024
            << " KBytes of data" << endl;

    // Write the data out to file
    ofstream outFile;
    //    outFile.open("webcam_output.jpeg", ios::binary| ios::app);
    outFile.open("webcam_output.mp4", ios::binary| ios::app);
    int bufPos = 0, outFileMemBlockSize = 0;  // the position in the buffer and the amoun to copy from
                                        // the buffer

    int remainingBufferSize = bufferinfo.bytesused; // the remaining buffer size, is decremented by
                                                    // memBlockSize amount on each loop so we do not overwrite the buffer

    uint8_t* outFileMemBlock = NULL;  // a pointer to a new memory block
    int itr = 0; // counts thenumber of iterations
    while(remainingBufferSize > 0) {
        bufPos += outFileMemBlockSize;  // increment the buffer pointer on each loop
                                        // initialise bufPos before outFileMemBlockSize so we can start
                                        // at the begining of the buffer

        outFileMemBlockSize = 1024;    // set the output block size to a preferable size. 1024 :)
        outFileMemBlock = new uint8_t[sizeof(uint8_t) * outFileMemBlockSize];

        // copy 1024 bytes of data starting from buffer+bufPos
        memcpy(outFileMemBlock, buffer+bufPos, outFileMemBlockSize);
        outFile.write(outFileMemBlock,outFileMemBlockSize);

        // calculate the amount of memory left to read
        // if the memory block size is greater than the remaining
        // amount of data we have to copy
        if(outFileMemBlockSize > remainingBufferSize)
            outFileMemBlockSize = remainingBufferSize;

        // subtract the amount of data we have to copy
        // from the remaining buffer size
        remainingBufferSize -= outFileMemBlockSize;
        // display the remaining buffer size
        cout << itr++ << " Remaining bytes: "<< remainingBufferSize << endl;
    }

    // Close the file
    outFile.close();

/******************************** end looping here **********************/
    // end streaming
    if(ioctl(fd, VIDIOC_STREAMOFF, &type) < 0){
        perror("Could not end streaming, VIDIOC_STREAMOFF");
        return 1;
    }
    close(fd);
    return 0;
}

int main() {
  process();
}

更新 我通过this example from v4l docs，但我无法运行它？

#include "../libv4l/include/libv4l2.h"

由于上述头文件丢失，我无法找到如何包含这些头文件。

Answer 1

编写低级代码来捕获网络摄像头看起来非常复杂。但是我们可以使用更强大的 OpenCV，它的功能与问题所要求的相同。如果你不使用 opencv 网络摄像头功能，那么你看看 OpenCV 视频捕获是如何实现的。

试试这个代码 -

#include "opencv2/opencv.hpp"
using namespace cv;
int main(int, char**)
{
    VideoCapture cap(0); // open the default camera
    if(!cap.isOpened())  // check if we succeeded
        return -1;
    Mat edges;
    namedWindow("frame",1);
    for(;;)
    {
        Mat frame;
        cap >> frame; 
        imshow("frame", frame);
        if(waitKey(30) >= 0) break;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}

opencv docs 中提供了有关如何操作的更多信息