How to put a function to a new thread? (Display video with OpenCv) Multiple Errors :-(

RolBri

Hello,

I am a beginner with basic C++ knowledge.
I really like Qt Creater and I want to learn the own Qt languge.
Sadly for this time is missing a the moment :-(

I would like to display a Webcam stream in a own thread.

Because of using QtSerialPort I had to instantiate a QApplication and therefore I now have problems creating threads.

Is there a possibility to create a QApplication just for the QtSerialPort functions that there is no special Qt stuff necessary in the rest of the program?

I get multiple errors:
QObject::moveToThread: Widgets cannot be moved to a new thread
QObject: Cannot create children for a parent that is in a different thread.
QSocketNotifier: Can only be used with threads started with QThread
QObject::startTimer: Timers can only be used with threads started with QThread

This is the minmal I used:
@#include "opencv2/core/core.hpp"
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

#include <iostream>
#include <thread>

#include <QCoreApplication>
#include <QApplication>
#include <QSerialPort>

using namespace std;
using namespace cv;

QSerialPort serial;

void InitialiseSerialPort()
{

serial.setPortName("/dev/ttyUSB0");
serial.open(QIODevice::WriteOnly);
serial.setBaudRate(QSerialPort::Baud19200);
serial.setDataBits(QSerialPort::Data8);
serial.setParity(QSerialPort::NoParity);
serial.setStopBits(QSerialPort::OneStop);
serial.setFlowControl(QSerialPort::NoFlowControl);
}

void DisplayWebCam ()
{
Mat frame;
VideoCapture cap (0);
namedWindow("Livebild");
while(char(waitKey(10)) != 'q' && cap.isOpened())
{
cap >> frame;
imshow("Livebild", frame);
}
}

int main (int argc, char *argv[])
{

QApplication app(argc, argv);
InitialiseSerialPort();

serial.write("Test");
serial.flush();
//DisplayWebCam();  //Works!
thread t1 (DisplayWebCam);  //Works not!

t1.join();

return 0;

}@

How can I solve my problem?

Thank you very much :-)

SGaist

Hi and welcome to devnet,

Short version: widget and GUI related stuff must be done in the GUI thread AKA main thread.

OpenCV also has a Qt backend for its GUI, hence the errors you are seeing.

Since your knowledge of C++ is currently limited, you should avoid starting directly with threads, it's one of the very complex part of programming.