problem on displaying live video in qt c++ application

Question

I want to show a live stream of the camera connected to raspberry in qt application (OS Linux). After googling it, I found out I must display the video inside QLabel. When displaying an image there's no problem and everything works fine, but when I want to display the live stream inside QLabel, the live stream window opens separately (not inside QLabel). would you tell me how to solve this problem? here's my code :

void Dialog::on_Preview_clicked()
{
    command = "raspistill";
    args<<"-o"<<"/home/pi/Pictures/Preview/"+Date1.currentDateTime().toString()+".jpg"<<"-t"<<QString::number(20000);
    Pic.start(command,args,QIODevice::ReadOnly);
    QPixmap pix("//home//pi//Pictures//Preview//test.jpg");
    ui->label_2->setPixmap(pix);
    ui->label_2->setScaledContents(true);
}

This code opens video capturing screen and captures an image after 20 seconds. the only problem is that the capture screen (which could be used as a live stream). isn't being displayed inside the "Lable_2". Is there anyway to do this without using OpenCV library? If not, tell me how to do it using OpenCV.

Thanks

Answer 1

It is pretty simple in opencv

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>

using namespace cv;
using namespace std;

 int main( int argc, char** argv )
 {
        VideoCapture cap(0); // open the default camera
    if(!cap.isOpened())  // check if we succeeded
        return -1;

    Mat edges;
    namedWindow("edges",1);
    for(;;)
    {
        Mat frame;

        cap >> frame; // get a new frame from camera

        imshow("edges", frame);
        if(waitKey(30) >= 0) break;
    }
return 0;
}

Answer 2

Stream the camera using OpenCV, and show it in QLabel is possible. When QCamera not working, and also use OpenCV in the project, could use VideoCapture to stream the video instead of QCamera.

The problem can be decomposed into several steps. Basically, We need:

1. Create a QThread for streaming(Don't let the GUI thread blocked).
2. In the sub-thread, using cv::VideoCapture to capture the frame into a cv::Mat.
3. Convert the cv::Mat to QImage(how to convert an opencv cv::Mat to qimage).
4. Pass QImage frame from sub-thread to the main GUI thread.
5. Paint the QImage on the QLabel.

I put the complete demo code in Github. it could paint the frame on the QLabel and QML VideoOutput.