Hi,
I'm trying to port to QT, a program that sends webcam frames (cv :: Mat) through Qt socket, like a streaming video. I could do it successfully using C++ and pthreads only but I have issues using the QTcpSocket mechanism.

The program logic is really simple: at client side i capture webcam frames and I send to the server after connecting.

In a timer slot I capture the frames, resize to 640x480, convert to gray scale and write to a socket:

Qt Code: 
  1. // The main code of the timer slot
  2. QObject::connect(&captureLoopTimer, &QTimer::timeout, [&]()
  3.     {
  4.         mutex.lock();
  5.         capture.read(frame);
  6.         if(frame.empty())
  7.         {
  8.             qWarning() << "Empty frame";
  9.             mutex.unlock();
  10.             return;
  11.         }
  12.         else
  13.         {
  14.             qDebug() << "Frame received";
  15.         }
  16.         // I correctly got a webcam frame -> convert to grayscale
  17.         flip(frame, frame, 1);
  18.         cvtColor(frame, gray, CV_BGR2GRAY);
  19.  
  20.         // Frame is ready to be written to a tcp socket
  21.         isDataReady = 1;
  22.         mutex.unlock();
  23. });
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The frames are written to a QTcpSocket in an external thread launched using a QtConcurrent::run(writeFramesToSocket), following the main code

Qt Code: 
  1. void writeFramesToSocket()
  2. {
  3.     QTcpSocket socket;
  4.     socket.connectToHost("127.0.0.1", 2222);
  5.     if(socket.waitForConnected(5000))
  6.     {
  7.         qDebug() << "Connected...";
  8.     }
  9.     else
  10.     {
  11.         qWarning() << "Can't connect to host";
  12.         return;
  13.     }
  14.  
  15.     reshaped = (gray.reshape(0, 1)); // To make it continuous
  16.     unsigned int frameSize = reshaped.total() * reshaped.elemSize(); // Frame size is 307200 (640x480)
  17.     qint64 bytes;
  18.  
  19.     while(1)
  20.     {
  21.         if(aborted) //Aborted is a global variable
  22.         {
  23.             socket.disconnectFromHost();
  24.             break;
  25.         }
  26.  
  27.         QMutexLocker locker(&mutex);
  28.         if(isDataReady == 1)
  29.         {
  30.             qDebug() << "Writing frame to socket...";
  31.             bytes = socket.write((char*)reshaped.data, frameSize);
  32.             socket.waitForBytesWritten();
  33.             isDataReady = 0;
  34.         }
  35.         locker.unlock();
  36.         QThread::usleep(100);
  37.     }
  38. }
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The frames are written correctly to the socket, I can visualize them with a server written in pure C++ and pthreads but I can not do the same using Qt ( the reason of this post )

At the server side (a really basic implementation) I accept a client connect and I try to get the bytes sent from the client

Qt Code: 
  1. // FrameServer.cpp
  2. FramesServer::FramesServer(QObject *parent)
  3.     : QObject(parent)
  4. {
  5.     m_server = new QTcpServer(this);
  6.     connect(m_server, SIGNAL(newConnection()),
  7.             this, SLOT(newConnection()));
  8.  
  9.     if(!m_server->listen(QHostAddress::Any, 2222))
  10.     {
  11.         qDebug() << "Server could not start";
  13.     }
  14.     else
  15.     {
  16.         qDebug() << "Server listening on port 2222";
  17.     }
  18.  
  19.     m_frame = cv::Mat::zeros(480, 640, CV_8UC1);
  20.     m_frameSize = m_frame.total() * m_frame.elemSize(); // Frame size is always 307200
  21.  
  22.     m_bytesReceived = 0;
  23.     m_frameBuffer.clear();
  24. }
  25.  
  26. void FramesServer::newConnection()
  27. {
  28.     m_socket = m_server->nextPendingConnection();
  29.  
  30.     connect(m_socket, SIGNAL(disconnected()),
  31.             m_socket, SLOT(deleteLater()));
  32.  
  33.     connect(m_socket, SIGNAL(readyRead()),
  34.             this, SLOT(readFrame()));
  35. }
  36.  
  37. void FramesServer::readFrame()
  38. {
  39.    // Get the data
  40.  
  41.    while((m_bytesReceived = m_socket->bytesAvailable()) > 0)
  42.    {
  43.        m_frameBuffer.append(m_socket->readAll());
  44.        if(m_bytesReceived == m_frameSize) // I received a complete frame so I want to display it
  45.        {
  46.            qDebug() << m_frameBuffer.size();
  47.             for (int i = 0; i < img.rows; ++i)
  48.             {
  49.                 for (int j = 0; j < img.cols; ++j)
  50.                 {
  51.                     (img.row(i)).col(j) = (uchar)m_frameBuffer.at(((img.cols) * i) + j);
  52.                     cv::imshow("DISPLAY", img);
  53.                 }
  54.             }
  55.            m_frameBuffer.clear();
  56.         }
  57.     }
  58. }
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From the Qt doc I understood that the readyRead signal is fired every time that there is available data for reading so I connected to the slot readFrame thar read data and try to complete a frame ( 307200 bytes )
Unfortunately the m_frameBuffer not always shows 307200 value, most of time is a different value and I can not construct correctly the cv::Mat image so I can not display it.

I don't know if the logic is well implemented, I would receive a help on it

Thank you