Qt5 to stream two videos simultaneously - camera

Question

I created a GUI to play two videos from two cameras, one camera is a USB camera, and another one is integrated in my laptop. My code is simple, I have three files:
main.cpp
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MainWindow window;
window.show();
return app.exec();
}
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QCamera>
#include <QCameraInfo>
#include <QCameraImageCapture>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow {
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private:
Ui::MainWindow *ui;
QList <QCameraInfo> camList;
QCamera *camera1;
QCamera *camera2;
private slots:
void onCameraChanged(int);
};
#endif // MAINWINDOW_H
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent), ui(new Ui::MainWindow) {
ui->setupUi(this);
camera1 = NULL;
camera2 = NULL;
connect(ui->cameraComboBox,static_cast<void (QComboBox::*)(int)>(&QComboBox::currentIndexChanged),this,
&MainWindow::onCameraChanged);
camList = QCameraInfo::availableCameras();
for(QList <QCameraInfo>::iterator it = camList.begin();it!=camList.end();++it) {
ui->cameraComboBox->addItem(it->description());
}
}
MainWindow::~MainWindow() {
delete ui;
}
void MainWindow::onCameraChanged(int idx) {
if(camera1 != NULL) {
camera1->stop();
}
qDebug() << idx;
camera1 = new QCamera(camList.at(idx),this);
camera1->setViewfinder(ui->mainView);
camera1->setCaptureMode(QCamera::CaptureStillImage);
camera1->start();
if(camera2 != NULL) {
camera2->stop();
}
camera2 = new QCamera(camList.at(idx+1),this);
camera2->setViewfinder(ui->submainView);
camera2->setCaptureMode(QCamera::CaptureStillImage);
camera2->start();
}
I have deleted irrelevant code. Of course I have this in the *.pro file:
QT += multimedia multimediawidgets
My code compiles correctly, but it can only stream one video (camera1), and the other one shows nothing. The error I got is
CameraBin error: "Error starting streaming on device '/dev/video1'."
CameraBin error: "Could not negotiate format"
Any help will be appreciated.