Signal/Slot Multithreaded With GUI Displaying OpenCV Mat

[VRonin]

You can try onImageChanged(0, tFrame.clone(), false);

[pistorinoj]

Thanks.

I tried something similar but no luck. Instead of passing an OpenCV Mat through the signal, I processed the Mat into a QImage and passed that as the parameter of the signal. I assume that that would make a local copy of the QImage on the GUI thread. But, again, the app crashes unless you delay the worker thread.

[J.Hilk]

@pistorinoj do you, by any Chance, pass your connect Statement, Qt::DirectConnection as 5th paarmeter?

[pistorinoj]

No. The Connect statement uses QueuedConnection.

[SGaist]

Depending on how you build your QImage it will have the same issue of needing the underlying data's existence to be guaranteed.

[pistorinoj]

I built it using the same code above. All I did was move everything before the pixmap call onto the worker thread and pass the img.

[SGaist]

Sorry, I understood that you changed your code to emit a QImage rather than a cv::Mat.

[pistorinoj]

Right. That is what I did.
My signal code now looks like what I say below. But I have to put in the msleep statements in the app crashes.

bool Channel::ImageDisp(int tChannelNumber, Mat tmat, bool MainWinCond)
{
   if (tmat.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*)tmat.data;
	// Create QImage with same dimensions as input Mat
	QImage img(qImageBuffer, tmat.cols, tmat.rows, tmat.step, QImage::Format_Indexed8);
	img.setColorTable(colorTable);
	emit ImageChanged(tChannelNumber, img , false, MainWinCond);
	QThread::msleep(15);
    }
    else if (tmat.type() == CV_8UC3)
    {
	// 8-bits unsigned, NO. OF CHANNELS=3
	// Copy input Mat
	const uchar *qImageBuffer = (const uchar*)tmat.data;
	// Create QImage with same dimensions as input Mat
	QImage img(qImageBuffer, tmat.cols, tmat.rows, tmat.step, QImage::Format_RGB888);
	emit ImageChanged(tChannelNumber, img , true, MainWinCond);
	QThread::msleep(15);
    }
    else
	return (false);

    return(true);
}

[jsulm]

@pistorinoj Please read http://doc.qt.io/qt-5/qimage.html#QImage-3
"The buffer must remain valid throughout the life of the QImage and all copies that have not been modified or otherwise detached from the original buffer."
Your buffer is tmat.data. tmat is a local variable and is destroyed when ImageDisp finishes. That's why QThread::msleep(15) helps. But this isn't a solution but a dirty work around.
You need to make sure the buffer is alive as long as you use the QImage. Maybe changing "Mat tmat" to "Mat &tmat" will already be anough.

[SGaist]

Then it's exactly the problem I described, you have to trigger a copy of your QImage so it will deep copy the data from your underlying buffer.

Or create a QImage of the right size and format and then do a memcpy of your buffer into the QImage internal buffer. Whatever you prefer.

[pistorinoj]

Thanks all.

I did not realize that even QImage was making a shallow copy.
I changed this so that the first thing the slot does is make a deep copy of the QImage and that seems to have addressed the issue.

Sorry for being so slow.

Thanks again.