QVideoFrame::map() only allows one call?

[csab6597]

Can QVideoFrame really do map() and unmap() only once?
Have a look at the following executable code

#include <QApplication>
#include <QMainWindow>
#include <QVBoxLayout>
#include <QVideoWidget>
#include <QVideoFrame>
#include <QVideoSink>
#include <QDebug>
#include <QThread>

int main(int argc, char** argv) {
	QApplication app(argc, argv);
	QWidget window;
	QVideoWidget* videoWidget = new QVideoWidget(&window);
	QImage image(200, 100, QImage::Format_RGBX8888);
	QVideoFrame frame = QVideoFrame(image);
	videoWidget->videoSink()->setVideoFrame(frame);

	QVBoxLayout* layout = new QVBoxLayout(&window);
	layout->addWidget(videoWidget);
	window.setLayout(layout);
	window.show();

	auto fcn = [&] {
		bool map = false;

		//first mapping
		map = frame.map(QVideoFrame::WriteOnly);
		qDebug() << "--- 1: map" << map << "isMapped" << frame.isMapped() << "isWritable" << frame.isWritable();
		frame.unmap();

		QThread::msleep(250);

		//second mapping
		map = frame.map(QVideoFrame::WriteOnly);
		qDebug() << "--- 2: map" << map << "isMapped" << frame.isMapped() << "isWritable" << frame.isWritable();
	};
	QThread* thread = QThread::create(fcn);
	thread->start();

	return app.exec();
}

I create a QVideoFrame, then I want to map() the frame. But the call to map() only succeeds first time, I cannot map the frame a second time. The relevant debug output is

--- 1: map true isMapped true isWritable true
--- 2: map false isMapped true isWritable false

Do I really have to create a new QVideoFrame every time I want to do a map() and unmap() cycle, or is there a step missing in between? I did not find anything in the documentation?

[ChrisW67]

It is worth mentioning that the code requires Qt 6.8.

The code behaves as expected if you take the cross-thread access of frame out of the equation and unmap() the second map.

Perhaps you could explain the problem you are trying to solve.

[csab6597]

Trying to post some more, I get "Post content was flagged as spam by Akismet.com"
Does this work?

[csab6597]

Well ok, I have a draft with more code, cannot post it, cannot see a problem in the draft...

Yes I am using Qt 6.8
What I am making is sort of a video player, I have a QVideoWidget and I repeatedly set new data into the frame via map() and unmap(). This already works quite nicely but only when I create a new QVideoFrame each time I want to show a new image. For now I can live with that. But after some more experimenting I feel there is something fishy around QVideoFrame...

[JonB]

@csab6597
Please put in extra copies of your qDebug() line. Put one immediately after the unmap() and another immediately before the second map(). You should not need the QThread::msleep(250);, but given that you have it put in both of these qDebug()s so we are sure what state your are in.

[csab6597]

Still cannot post code "Post content was flagged as spam by Akismet.com"

The code in the image gives me

--- 1.1: map true isMapped true isWritable true
--- 1.2: map true isMapped false isWritable false
--- 2.0: map true isMapped true isWritable false
--- 2.1: map false isMapped true isWritable false
--- 2:2 map false isMapped false isWritable false

Without any sleep() it does indeed work, but I mean in reality there is some delay between frames in a video, right? I feel there is a bug somewhere.