Qt + OpenCV 使用 std::thread 播放视频

Question

这是我的图形用户界面，

我想要在这里做的是一起显示四个不同的视频。用户输入特定视频文件的路径并点击打开，应该会创建一个新线程来播放该视频。

我有一个名为 startThread() 的插槽，以及一个要在名为 openCamera() 的线程中调用的函数；

在mainwindow.hpp中有

public:
    static void openCamera(const std::string& address, QLabel* label);
private slots:
    void startThread();

在mainwindow.cpp中有

void MainWindow::startThread() {
    std::string address;
    QLabel* label;
    // Get the input from GUI and assign it to std::string address
    // and find the label corresponding to clicked button.
    // ...
    std::thread t(openCamera, address, label);
}
void MainWindow::openCamera(std::string address, QLabel* label) {
    cv::VideoCapture cap(address);
    cv::Mat frame;
    while (cap.isOpened()) {
        cap >> frame;
        // cv::imwrite("/Users/wking/Desktop/test.jpg", frame); // This function works!
        label->setPixmap(QPixmap::fromImage(QImage(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888).rgbSwapped()));
    }
}

我想通过使用 std::thread 来实现它。但是，它什么也没显示。在我单击输入正确的按钮后，程序不会更新标签。我在这里错过了什么吗？谢谢。

Answer 1

在 Qt 中，您只能从主线程访问小部件。您可以使用 Qt 信号槽机制将数据从工作线程传递到主线程：

class MainWindow
{
    ...

    static void openCamera(const std::string& address, QLabel* label, MainWindow *window);

signals:
    void updateLabelSig(QLabel *label, QPixmap pic);

private slots:
    void updateLabel(QLabel *label, QPixmap pic);

    ...
}

MainWindow::MainWindow()
{
    connect(this, SIGNAL(updateLabelSig(QLabel*, QPixmap), this, SLOT(QLabel*, QPixmap));
    ...
}

void MainWindow::startThread() 
{
    std::string address;
    QLabel* label;
    // Get the input from GUI and assign it to std::string address
    // and find the label corresponding to clicked button.
    // ...
    std::thread t(openCamera, address, label, this);
}

void MainWindow::openCamera(std::string address, QLabel* label, MainWindow *window) 
{
    cv::VideoCapture cap(address);
    cv::Mat frame;
    while (cap.isOpened()) {
        cap >> frame;
        // cv::imwrite("/Users/wking/Desktop/test.jpg", frame); // This function works!

        QPixmap pic = QPixmap::fromImage(QImage(frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888).rgbSwapped());

        window->updateLabelSig(label, pic);
    }
}

void MainWindow::updateLabel(QLabel *label, QPixmap pic)
{
     label->setPixmap(pic);
}

Answer 2

只有主线程（GUI 线程）可以更新 UI。
我有一个类似的项目。我使用 QThread 和 Worker 类。 Worker 类有一个插槽来读取视频文件。如果完成读取一帧，它将发出一个信号。
在 MainWindow 中，我创建了一些 QThread，将工作对象移动到线程。
将 Worker 的信号 frameFinished 与函数连接，以在标签上显示 cv::Mat。

class Worker : public QObject
{
    Q_OBJECT
public:
    Worker(QString path, int id);
    ~Worker();

public slots:
    void readVideo(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;
};

这里是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::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);
}

主窗口.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);


private:
    Ui::MainWindow *ui;

    int numCams;
    QLabel *labels[MAX_NUM_CAM];
    QThread* threads[MAX_NUM_CAM];
    Worker* workers[MAX_NUM_CAM];
};

#endif // MAINWINDOW_H

主窗口.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::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);
}

Answer 3

1.也许你想念t.detach();

2.更新ui：可以从子线程发出信号，然后在槽函数中更新ui。（槽函数将在主线程中运行）