Qt + OpenCV - Displaying images on QLabel

Question

I'm trying to capture live view from camera, and redirecting it to show up on QLabel. But only half view comes (see below): Screenshot

The left hand side window, is shown using cv::imshow(), which works perfectly. I'm capturing the Mat in a different thread, and then emitting a signal with a Qimage as a parameter, and then setting the image to the QLabel in the slot.

here's the code:

while(true){
    cam >> mat;
    cv::imshow("name",mat);
    emit send_UIupdate(mat2qimage(mat));
}

and in the slot setting the image to Qlabel:

void Dialog::updateUI(const QImage &img){
    label->setPixmap(QPixmap::fromImage(img));
}

using the below to convert Mat to QImage:

QImage camera::mat2qimage(const cv::Mat& mat) {
    cv::Mat rgb;
    cv::cvtColor(mat, rgb, CV_BGR2RGB);
    return QImage((const unsigned char*)(rgb.data), rgb.cols, rgb.rows, QImage::Format_RGB888);
}

Any suggestions, to solve this problem ??

CTZStef · Accepted Answer · 2012-08-08 23:15:17Z

you can try this :

QImage MainWindow::putImage(const Mat& mat)
{
    // 8-bits unsigned, NO. OF CHANNELS=1
    if(mat.type()==CV_8UC1)
    {
        // Set the color table (used to translate colour indexes to qRgb values)
        QVector<QRgb> colorTable;
        for (int i=0; i<256; i++)
            colorTable.push_back(qRgb(i,i,i));
        // Copy input Mat
        const uchar *qImageBuffer = (const uchar*)mat.data;
        // Create QImage with same dimensions as input Mat
        QImage img(qImageBuffer, mat.cols, mat.rows, mat.step, QImage::Format_Indexed8);
        img.setColorTable(colorTable);
        return img;
    }
    // 8-bits unsigned, NO. OF CHANNELS=3
    if(mat.type()==CV_8UC3)
    {
        // Copy input Mat
        const uchar *qImageBuffer = (const uchar*)mat.data;
        // Create QImage with same dimensions as input Mat
        QImage img(qImageBuffer, mat.cols, mat.rows, mat.step, QImage::Format_RGB888);
        return img.rgbSwapped();
    }
    else
    {
        qDebug() << "ERROR: Mat could not be converted to QImage.";
        return QImage();
    }
}

I call it with a QTimer. I got it from there : http://code.google.com/p/qt-opencv-multithreaded/

Hope this help.

Mohamed Bahrini · Accepted Answer · 2015-02-23 14:47:02Z

1

VideoCapture cap("video.avi");
Mat frame;
QImage img;
QPixmap pixel;
while(cap.isOpened())
{
    cap >> frame;
    img= QImage((uchar*) frame.data, frame.cols, frame.rows, frame2.step, QImage::Format_RGB888);
    pixel = QPixmap::fromImage(img);
    ui->label->setPixmap(pixel);
}

answered Feb 23, 2015 at 14:47

Mohamed Bahrini

112 bronze badges

Comments

Community · Accepted Answer · 2017-05-23 11:47:07Z

0

I think the problem is comes from the below codes:

while(true){
    cam >> mat;
    cv::imshow("name",mat);
    emit send_UIupdate(mat2qimage(mat));
}

Your application freezes because you are looping and never letting Qt do its necessary processing. ref-link: QT do while loop

You know, the Qt has it's mine event loop, so, if you use the while(true) loop in a function, the Qt may never receive the event.

Try to remove while loop, and use Qtimer as @CTZStef said.

The follow source code may be help to you: http://blog.csdn.net/robertkun/article/details/9030929

edited May 23, 2017 at 11:47

CommunityBot

11 silver badge

answered Dec 28, 2013 at 7:54

ajaxhe

6191 gold badge6 silver badges13 bronze badges

1 Comment

may5694 Over a year ago

I don't think the while loop causes the problem you state. He mentioned the Mat is captured in a separate thread.

Cengiz ALKAN · Accepted Answer · 2014-12-03 21:55:16Z

0

in mat2qimage function, You should return a copy.

return QImage((const unsigned char*)(rgb.data), rgb.cols, rgb.rows, Image::Format_RGB888).copy();

answered Dec 3, 2014 at 21:55

Cengiz ALKAN

1

1 Comment

Stewart_R Over a year ago

I'm struggling to see how this answers the original question? Could you explain?

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