QT Update variable of worker thread from mainwindow - c++

Question

I am displaying opencv video using QT multithreading concept as suggested here
I also wish to update private "path" variable of worker thread class on button click. So I added
connect(this, SIGNAL(send_to_worker(QString)),workers[view], SLOT(get_from_main(QString)));
However, get_from_main(QString) function never gets called.
May I please know what is the safest method to update worker thread class variable from mainwindow ?
Here is full code..
class Worker : public QObject
{
Q_OBJECT
public:
Worker(QString path, int id);
~Worker();
public slots:
void readVideo(QString path = "");
void get_from_main(QString path);
signals:
// frame and index of label which frame will be displayed
void frameFinished(cv::Mat frame, int index);
void finished(int index);
private:
QString filepath;
int index;
};
//worker.cpp
#include "worker.h"
#include <QDebug>
#include <QThread>
#include <QTime>
Worker::Worker(QString path, int id) : filepath(path), index(id)
{
}
Worker::~Worker()
{
}
void Worker::get_from_main(QString path)
{
qDebug() << "updating";
}
void Worker::readVideo(QString path)
{
if (path.length() > 0)
filepath = path;
cv::VideoCapture cap(filepath.toStdString());
if (! cap.isOpened())
{
qDebug() << "Can't open video file " << filepath;
emit finished(index);
return;
}
cv::Mat frame;
while (true)
{
cap >> frame;
if (frame.empty())
{
frame = cv::Mat(cv::Size(720, 576), CV_8UC3, cv::Scalar(192, 0, 0));
emit frameFinished(frame, index);
break;
}
emit frameFinished(frame.clone(), index);
QThread::msleep(30);
}
emit finished(index);
}
//mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <opencv2/opencv.hpp>
#include "worker.h"
#define MAX_NUM_CAM 8
namespace Ui {
class MainWindow;
}
class QThread;
class QLabel;
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
void init();
private slots:
void displayFrame(cv::Mat frame, int index);
void file_open_clicked();
signals:
send_to_worker(QString path);
private:
Ui::MainWindow *ui;
int numCams;
QLabel *labels[MAX_NUM_CAM];
QThread* threads[MAX_NUM_CAM];
Worker* workers[MAX_NUM_CAM];
};
#endif // MAINWINDOW_H
//mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDebug>
#include <QThread>
#include <QLabel>
#include <QGridLayout>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
qRegisterMetaType< cv::Mat >("cv::Mat");
qDebug() << "Main thread " << QThread::currentThreadId();
init();
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::init()
{
QGridLayout *grid = new QGridLayout;
int numCols = 2;
numCams = 4;
int row = 0, col = 0;
for (int i = 0; i < numCams; i++)
{
labels[i] = new QLabel;
row = i / numCols;
col = i % numCols;
grid->addWidget(labels[i], row, col);
threads[i] = new QThread;
workers[i] = new Worker(QString("/home/shang/Videos/%1.mp4").arg(i+1), i);
workers[i]->moveToThread(threads[i]);
connect(workers[i], SIGNAL(frameFinished(cv::Mat, int)), this, SLOT(displayFrame(cv::Mat,int)));
connect(threads[i], SIGNAL(started()), workers[i], SLOT(readVideo()));
connect(workers[i], SIGNAL(finished(int)), threads[i], SLOT(quit()));
connect(workers[i], SIGNAL(finished(int)), workers[i], SLOT(deleteLater()));
connect(threads[i], SIGNAL(finished()), threads[i], SLOT(deleteLater()));
threads[i]->start();
}
this->centralWidget()->setLayout(grid);
}
void MainWindow::file_open_clicked(){
QString Path = QFileDialog::getSaveFileName( this,tr("OpenVideo"),"","Video (*.avi)");
if(Path.isEmpty())
return;
view =3;
connect(this, SIGNAL(send_to_worker(QString)),workers[view], SLOT(get_from_main(QString)));
emit this->send_to_worker(recorder_Path);
}
void MainWindow::displayFrame(cv::Mat frame, int index)
{
QPixmap p = QPixmap::fromImage(QImage(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888).rgbSwapped());
p = p.scaled(QSize(frame.cols/2, frame.rows/2));
labels[index]->setPixmap(p);
}
Qt + OpenCV play videos with std::thread

Answer 1

You're trying to perform an operation periodically whilst still processing events -- that's simply calling out for a QTimer.
(Note that the following code is untested.)
Change your Worker class to make use of a QTimer rather than a loop that blocks the event queue...
class Worker: public QObject {
Q_OBJECT;
public:
Worker (QString path, int id);
~Worker();
public slots:
void readVideo(QString path = "");
void get_from_main(QString path);
signals:
// frame and index of label which frame will be displayed
void frameFinished(cv::Mat frame, int index);
void finished(int index);
private:
QString filepath;
int index;
QTimer timer;
cv::VideoCapture cap;
};
Worker::Worker (QString path, int id)
: filepath(path)
, index(id)
, timer(this)
, cap(filepath.toStdString())
{
/*
* Connect QTimer::timeout to the readVideo slot that will read a
* single frame on each signal at 30ms intervals.
*/
connect(&timer, &QTimer::timeout, this, &Worker::readVideo);
timer.start(30);
}
Worker::~Worker ()
{
}
void Worker::get_from_main (QString path)
{
qDebug() << "updating";
filepath = path;
cap = cv::VideoCapture(filepath);
if (!cap.isOpened()) {
qDebug() << "Can't open video file " << filepath;
emit finished(index);
}
}
void Worker::readVideo ()
{
cv::Mat frame;
cap >> frame;
if (frame.empty())
{
frame = cv::Mat(cv::Size(720, 576), CV_8UC3, cv::Scalar(192, 0, 0));
emit frameFinished(frame, index);
break;
}
emit frameFinished(frame.clone(), index);
}
Now Worker::readVideo simply reads a single frame from the capture and then returns to the event loop.
Also remove the line...
connect(threads[i], SIGNAL(started()), workers[i], SLOT(readVideo()));
from MainWindow::init. As I stated above this is untested and probably needs a lot more error checking. But it should give you a good idea as to what's required.