opencv4(五) VideoCapture获取摄像头图像

环境：ubuntu18.04        opencv4.4.0        

摄像头：usb摄像头，挺老的摄像头，还是usb2.0的

        csi摄像头也支持这种方法

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插入摄像头后 ls /dev可以看到 /dev/video0

video后面的数字就是后面需要的设备id

代码段摘自opencv手册，基本使用这段代码就够用了

#include <opencv2/core.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>
#include <stdio.h>
using namespace cv;
using namespace std;
int main(int, char**)
{
    Mat frame;
    //--- INITIALIZE VIDEOCAPTURE
    VideoCapture cap;
    // open the default camera using default API
    // cap.open(0);
    // OR advance usage: select any API backend
    int deviceID = 0;             // 0 = open default camera
    int apiID = cv::CAP_ANY;      // 0 = autodetect default API
    // open selected camera using selected API
    cap.open(deviceID, apiID);
    // check if we succeeded
    if (!cap.isOpened()) {
        cerr << "ERROR! Unable to open camera\n";
        return -1;
    }
    //--- GRAB AND WRITE LOOP
    cout << "Start grabbing" << endl
        << "Press any key to terminate" << endl;
    for (;;)
    {
        // wait for a new frame from camera and store it into 'frame'
        cap.read(frame);
        // check if we succeeded
        if (frame.empty()) {
            cerr << "ERROR! blank frame grabbed\n";
            break;
        }
        // show live and wait for a key with timeout long enough to show images
        imshow("Live", frame);
        if (waitKey(5) >= 0)
            break;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}

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API选择

在linux下自然选择v4l2，也可以用CAP_ANY让opencv自己选择，在linux下gstreamer也是基于V4L实现的，所以gstreamer和v4l2都可以。

需要注意的是，在编译opencv时就要把WITH_V4L打勾，好在在默认情况下WITH_V4L是勾选的

enum	cv::VideoCaptureAPIs {