我尝试编写一个简单的程序,使用3个不同的线程读取三个视频文件(实际上是同一房间内的3个摄像机)。我使用的代码如下:
mainwindow.cpp
void MainWindow::init()
{
numCams = 3;
// Resize the video for displaying to the size of the widget
int WidgetHeight = ui->CVWidget1->height();
int WidgetWidth = ui->CVWidget1->width();
for (int i = 0; i < numCams; i++){
// Create threads
threads[i] = new QThread;
// Create workers
string Path = "/Users/alex/Desktop/PruebasHilos/Videos/" + to_string(i+1) + ".m2v";
workers[i] = new Worker(QString::fromStdString(Path), i, WidgetHeight, WidgetWidth);
workers[i]->moveToThread(threads[i]);
connectSignals2Slots(threads[i], workers[i]);
threads[i]->start();
qDebug() << "Thread from camera " << (i+1) << " started";
}
}
void MainWindow::connectSignals2Slots(QThread *thread, Worker *worker)
{
connect(thread, SIGNAL(started()), worker, SLOT(readVideo()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
connect(worker, SIGNAL(frameFinished(Mat, int)), this, SLOT(displayFrame(Mat,int)));
connect(worker, SIGNAL(finished(int)), thread, SLOT(quit()));
connect(worker, SIGNAL(finished(int)), worker, SLOT(deleteLater()));
}
void MainWindow::displayFrame(Mat frame, int index)
{
if (index == 0) {
// Camera 1
ui->CVWidget1->showImage(frame);
}
else if (index == 1) {
// Camera 2
ui->CVWidget2->showImage(frame);
}
else if (index == 2) {
// Camera 3
ui->CVWidget3->showImage(frame);
}
}
worker.cpp
Worker::Worker(QString path, int id, int WidgetHeight, int WidgetWidth) : filepath(path), index(id), WidgetHeight(WidgetHeight), WidgetWidth(WidgetWidth) {
}
Worker::~Worker(){
}
void Worker::readVideo()
{
VideoCapture cap(filepath.toStdString());
if (! cap.isOpened()) {
qDebug() << "Can't open video file " << filepath;
emit finished(index);
return;
}
Mat ActualFrame;
while (true) {
cap >> ActualFrame;
if (ActualFrame.empty()) {
// Empty frame to display when the video has finished
ActualFrame = Mat(Size(720, 576), CV_8UC3, Scalar(192, 0, 0));
emit frameFinished(ActualFrame, index);
qDebug() << "Video finished";
break;
}
// Background Subtraction
BackgroundSubtraction(ActualFrame, BackgroundMask);
emit frameFinished(ActualFrame.clone(), index);
QThread::msleep(35);
}
emit finished(index);
}
void Worker::BackgroundSubtraction(Mat ActualFrame, Mat &BackgroundMask)
{
pMOG2->apply(ActualFrame, BackgroundMask);
}
只需从VideoCapture中读取帧并通过另一个使用QWidgets的不同类将它们显示在UI中效果很好。
但是,当我包含BackgroundSubstraction方法时,UI不会为三个摄像头显示相同的帧编号,可能Camera1正在计算帧100,而Camera2和Camera3位于帧110中。
这是因为某些帧的计算速度比其他帧快,这会导致合成问题
我在QT中使用线程很新,所以我想在线程之间进行一些同步,所以我知道三个不同的帧何时处理以调用displayFrame方法,因此,显示三个相同的帧在同一时间。
修改
我认为最简单的方法是使用障碍
http://www.boost.org/doc/libs/1_55_0/doc/html/thread/synchronization.html#thread.synchronization.barriers。但我不知道如何做到这一点。
编辑2: 我已经实现了这个Syncronizacion using barriers,现在代码看起来像这样:
barrier.h
#ifndef BARRIER_H
#define BARRIER_H
#include <QMutex>
#include <QWaitCondition>
#include <QSharedPointer>
// Data "pimpl" class (not to be used directly)
class BarrierData
{
public:
BarrierData(int count) : count(count) {}
void wait() {
mutex.lock();
--count;
if (count > 0)
condition.wait(&mutex);
else
condition.wakeAll();
mutex.unlock();
}
private:
Q_DISABLE_COPY(BarrierData)
int count;
QMutex mutex;
QWaitCondition condition;
};
class Barrier {
public:
// Create a barrier that will wait for count threads
Barrier(int count) : d(new BarrierData(count)) {}
void wait() {
d->wait();
}
private:
QSharedPointer<BarrierData> d;
};
#endif // BARRIER_H
更新了worker.cpp
void Worker::readVideo()
{
VideoCapture cap(filepath.toStdString());
int framenumber = 0;
if (! cap.isOpened()) {
qDebug() << "Can't open video file " << filepath;
emit finished(index);
return;
}
Mat ActualFrame;
while (true) {
cap >> ActualFrame;
if (ActualFrame.empty()) {
// Empty frame to display when the video has finished
ActualFrame = Mat(Size(720, 576), CV_8UC3, Scalar(192, 0, 0));
emit frameFinished(ActualFrame, index);
qDebug() << "Video finished";
break;
}
// Background Subtraction
BackgroundSubtraction(ActualFrame, BackgroundMask);
QThread::msleep(5);
barrier.wait();
qDebug() << "Thread " << index << " processing frame " << framenumber ;
emit frameFinished(ActualFrame.clone(), index);
framenumber++;
}
emit finished(index);
}
void Worker::BackgroundSubtraction(Mat ActualFrame, Mat &BackgroundMask)
{
pMOG2->apply(ActualFrame, BackgroundMask);
}
它似乎完美无缺,但程序的输出如下:
Thread 1 processing frame 0
Thread 0 processing frame 0
Thread 2 processing frame 0
Thread 2 processing frame 1
Thread 1 processing frame 1
Thread 0 processing frame 1
Thread 2 processing frame 2
Thread 1 processing frame 2
Thread 0 processing frame 2
Thread 2 processing frame 3
Thread 1 processing frame 3
Thread 0 processing frame 3
Thread 2 processing frame 4
Thread 1 processing frame 4
Thread 0 processing frame 4
Thread 2 processing frame 5
Thread 0 processing frame 5
Thread 1 processing frame 5
Thread 2 processing frame 6
Thread 1 processing frame 6
Thread 2 processing frame 7
Thread 0 processing frame 6
Thread 1 processing frame 7
Thread 2 processing frame 8
Thread 0 processing frame 7
Thread 1 processing frame 8
Thread 2 processing frame 9
Thread 0 processing frame 8
Thread 1 processing frame 9
Thread 1 processing frame 10
Thread 2 processing frame 10
Thread 0 processing frame 9
Thread 1 processing frame 11
Thread 2 processing frame 11
Thread 0 processing frame 10
Thread 1 processing frame 12
一开始同步化完全正常,但似乎屏障不起作用,线程不等待彼此...
编辑3:已解决 似乎改变了
的价值QThread::msleep(5);
到
QThread::msleep(35);
解决了同步问题,虽然我不太明白原因。
答案 0 :(得分:1)
即使没有背景减法,您也需要进行一些同步以确保每个线程处理相同的帧编号。
在Qt中,最简单的(也是正确的)方法是删除无限循环,然后调用每个线程的一个槽来计算下一个图像,在所有线程发出它们的信号frameFinished之后。
您可以进一步使用一些缓冲来预先计算线程中的图像,并从该缓冲区加载它们。在这种情况下,您可以执行以下操作:
只要有可用的空闲缓冲区空间,每个线程就会在无限循环中填充缓冲区。如果缓冲区已满,则线程将等待,直到缓冲区空间被释放。
当gui显示并等待一段时间后,它会发送一个信号,该信号连接到每个线程的插槽,如sendMeANewImage。
每个线程从其缓冲区发送下一个可用图像,或者如果缓冲区为空则等待(无限循环或条件等待)图像。然后发出frameFinished信号并释放使用过的缓冲区空间。
当每个线程发出信号时,显示所有图像,等待一段时间再发出sendMeANewImage。
这还不是线程安全的,你将从缓冲区读取和写入关键部分。对于每个缓冲区,创建一个QMutex并在从该缓冲区读取或写入或询问大小等时调用mutex.lock()。之后立即调用mutex.unlock()。
当互斥锁被锁定而另一个线程(甚至同一个线程)试图再次锁定它时,线程将在那里等待,直到另一个线程解锁了互斥锁。这样,只有一个线程可以进入关键部分。