Replace Pixmap in GraphicsScene

[Royce]

Hmm, weird.

Have you tried saving the image? I.e. to see if it is somehow damaged?
Though I would assume that this would already cause a problem in QPixmap::fromImage().

Or maybe try showing the resulting pixmap in a separate QLabel, to verify it being OK.

Cheers,
_

Well setting to a label results in a crash in exactly the same place. Oddly, saving to a jpg file works fine (and not attempting to display onscreen). Its upside down, but that's is the only thing odd I see about the image.

I feel like there is something fundamental about what's wrong. Something I'm just not aware of in the Qt world. I don't think its threads, all the crashes and breakpoints show WinMain in the stack. But I have no idea what it could be.

Thanks for your help, anyway.

[Royce]

Perhaps it is something about the event handler. If I set the label to one of the stored pixmaps in the constructor, it works. I see the image. But the captured frame still crashes it.

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    frame(640, 480),
    pix_no(0)
{
    ui->setupUi(this);
 
    frame.load("campix1.jpg");
    ui->label->setPixmap(frame);
 
    QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
    cam = new QCamera(cameras.first(), this);
    cam->load();
    frame_grabber = new CameraFrameGrabber();
    cam->setViewfinder(frame_grabber);
    cam->setCaptureMode(QCamera::CaptureViewfinder);
    connect(frame_grabber, SIGNAL(frameAvailable(QImage)), this, SLOT(handleFrame(QImage)));
    cam->start();
    frame_timer.start();
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void MainWindow::handleFrame(const QImage& img)
{
    frame_times.push_back(frame_timer.elapsed());
    frame_timer.restart();
    if (frame_times.size() > 10) frame_times.pop_front();
    int frame_count = frame_times.size();
    int frame_total = 0;
    for(int i = 0; i < frame_count; ++i)
    {
        frame_total += frame_times[i];
    }
    double fps = 0;
    if (frame_count)
    {
        fps = (frame_total / frame_count);
    }
    ui->sb_fps->setValue(fps);
 
    frame = QPixmap::fromImage(img);
 
    QString fname("campix");
    fname += QString::number(++pix_no);
    fname += ".jpg";
    bool success = frame.save(fname);
 
    ui->label->setPixmap(frame);
 
    this->update();
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    frame(640, 480),
    pix_no(0)
{
    ui->setupUi(this);

    frame.load("campix1.jpg");
    ui->label->setPixmap(frame);

    QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
    cam = new QCamera(cameras.first(), this);
    cam->load();
    frame_grabber = new CameraFrameGrabber();
    cam->setViewfinder(frame_grabber);
    cam->setCaptureMode(QCamera::CaptureViewfinder);
    connect(frame_grabber, SIGNAL(frameAvailable(QImage)), this, SLOT(handleFrame(QImage)));
    cam->start();
    frame_timer.start();

}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::handleFrame(const QImage& img)
{
    frame_times.push_back(frame_timer.elapsed());
    frame_timer.restart();
    if (frame_times.size() > 10) frame_times.pop_front();
    int frame_count = frame_times.size();
    int frame_total = 0;
    for(int i = 0; i < frame_count; ++i)
    {
        frame_total += frame_times[i];
    }
    double fps = 0;
    if (frame_count)
    {
        fps = (frame_total / frame_count);
    }
    ui->sb_fps->setValue(fps);

    frame = QPixmap::fromImage(img);

    QString fname("campix");
    fname += QString::number(++pix_no);
    fname += ".jpg";
    bool success = frame.save(fname);

    ui->label->setPixmap(frame);

    this->update();
}

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This is the stack while stopped at a breakpoint in the handleFrame event.

1  MainWindow::handleFrame                      mainwindow.cpp                 62   0x7ff6d7082e44 
2  MainWindow::qt_static_metacall               moc_mainwindow.cpp             73   0x7ff6d7087bb5 
3  QMetaObject::activate                        qobject.cpp                    3742 0x5d14daf2     
4  QMetaObject::activate                        qobject.cpp                    3603 0x5d14d268     
5  CameraFrameGrabber::frameAvailable           moc_cameraframegrabber.cpp     128  0x7ff6d7087f1b 
6  CameraFrameGrabber::present                  cameraframegrabber.cpp         56   0x7ff6d7086e00 
7  DSCameraSession::presentFrame                dscamerasession.cpp            683  0x7ffc074ca264 
8  DSCameraSession::qt_static_metacall          moc_dscamerasession.cpp        111  0x7ffc074da2e6 
9  QMetaCallEvent::placeMetaCall                qobject.cpp                    503  0x5d156c81     
10 QObject::event                               qobject.cpp                    1263 0x5d14f6df     
11 QApplicationPrivate::notify_helper           qapplication.cpp               3799 0x5d8a5e8e     
12 QApplication::notify                         qapplication.cpp               3159 0x5d8a0713     
13 QCoreApplication::notifyInternal2            qcoreapplication.cpp           988  0x5d0f78c6     
14 QCoreApplication::sendEvent                  qcoreapplication.h             231  0x5d1027f2     
15 QCoreApplicationPrivate::sendPostedEvents    qcoreapplication.cpp           1649 0x5d0f9112     
16 QEventDispatcherWin32::sendPostedEvents      qeventdispatcher_win.cpp       1295 0x5d1b180a     
17 QWindowsGuiEventDispatcher::sendPostedEvents qwindowsguieventdispatcher.cpp 82   0x7ffbfc70c204 
18 qt_internal_proc                             qeventdispatcher_win.cpp       445  0x5d1af521     
19 CallWindowProcW                              USER32                              0x7ffc29f41c24 
20 DispatchMessageW                             USER32                              0x7ffc29f4156c 
21 QEventDispatcherWin32::processEvents         qeventdispatcher_win.cpp       845  0x5d1aff2b     
22 QWindowsGuiEventDispatcher::processEvents    qwindowsguieventdispatcher.cpp 74   0x7ffbfc70c1c4 
23 QEventLoop::processEvents                    qeventloop.cpp                 135  0x5d0f24e8     
24 QEventLoop::exec                             qeventloop.cpp                 210  0x5d0f272e     
25 QCoreApplication::exec                       qcoreapplication.cpp           1261 0x5d0f55ff     
26 QGuiApplication::exec                        qguiapplication.cpp            1640 0x7ffbfccf3208 
27 QApplication::exec                           qapplication.cpp               2976 0x5d8a021a     
28 main                                         main.cpp                       10   0x7ff6d7082749 
29 WinMain                                      qtmain_win.cpp                 123  0x7ff6d708afbd 
30 invoke_main                                  exe_common.inl                 109  0x7ff6d7088bed 
31 __scrt_common_main_seh                       exe_common.inl                 264  0x7ff6d7088a8e 
32 __scrt_common_main                           exe_common.inl                 309  0x7ff6d708894e 
33 WinMainCRTStartup                            exe_winmain.cpp                17   0x7ff6d7088c09 
34 BaseThreadInitThunk                          KERNEL32                            0x7ffc2a4a8364 
35 RtlUserThreadStart                           ntdll                               0x7ffc2c1a5e91

1  MainWindow::handleFrame                      mainwindow.cpp                 62   0x7ff6d7082e44 
2  MainWindow::qt_static_metacall               moc_mainwindow.cpp             73   0x7ff6d7087bb5 
3  QMetaObject::activate                        qobject.cpp                    3742 0x5d14daf2     
4  QMetaObject::activate                        qobject.cpp                    3603 0x5d14d268     
5  CameraFrameGrabber::frameAvailable           moc_cameraframegrabber.cpp     128  0x7ff6d7087f1b 
6  CameraFrameGrabber::present                  cameraframegrabber.cpp         56   0x7ff6d7086e00 
7  DSCameraSession::presentFrame                dscamerasession.cpp            683  0x7ffc074ca264 
8  DSCameraSession::qt_static_metacall          moc_dscamerasession.cpp        111  0x7ffc074da2e6 
9  QMetaCallEvent::placeMetaCall                qobject.cpp                    503  0x5d156c81     
10 QObject::event                               qobject.cpp                    1263 0x5d14f6df     
11 QApplicationPrivate::notify_helper           qapplication.cpp               3799 0x5d8a5e8e     
12 QApplication::notify                         qapplication.cpp               3159 0x5d8a0713     
13 QCoreApplication::notifyInternal2            qcoreapplication.cpp           988  0x5d0f78c6     
14 QCoreApplication::sendEvent                  qcoreapplication.h             231  0x5d1027f2     
15 QCoreApplicationPrivate::sendPostedEvents    qcoreapplication.cpp           1649 0x5d0f9112     
16 QEventDispatcherWin32::sendPostedEvents      qeventdispatcher_win.cpp       1295 0x5d1b180a     
17 QWindowsGuiEventDispatcher::sendPostedEvents qwindowsguieventdispatcher.cpp 82   0x7ffbfc70c204 
18 qt_internal_proc                             qeventdispatcher_win.cpp       445  0x5d1af521     
19 CallWindowProcW                              USER32                              0x7ffc29f41c24 
20 DispatchMessageW                             USER32                              0x7ffc29f4156c 
21 QEventDispatcherWin32::processEvents         qeventdispatcher_win.cpp       845  0x5d1aff2b     
22 QWindowsGuiEventDispatcher::processEvents    qwindowsguieventdispatcher.cpp 74   0x7ffbfc70c1c4 
23 QEventLoop::processEvents                    qeventloop.cpp                 135  0x5d0f24e8     
24 QEventLoop::exec                             qeventloop.cpp                 210  0x5d0f272e     
25 QCoreApplication::exec                       qcoreapplication.cpp           1261 0x5d0f55ff     
26 QGuiApplication::exec                        qguiapplication.cpp            1640 0x7ffbfccf3208 
27 QApplication::exec                           qapplication.cpp               2976 0x5d8a021a     
28 main                                         main.cpp                       10   0x7ff6d7082749 
29 WinMain                                      qtmain_win.cpp                 123  0x7ff6d708afbd 
30 invoke_main                                  exe_common.inl                 109  0x7ff6d7088bed 
31 __scrt_common_main_seh                       exe_common.inl                 264  0x7ff6d7088a8e 
32 __scrt_common_main                           exe_common.inl                 309  0x7ff6d708894e 
33 WinMainCRTStartup                            exe_winmain.cpp                17   0x7ff6d7088c09 
34 BaseThreadInitThunk                          KERNEL32                            0x7ffc2a4a8364 
35 RtlUserThreadStart                           ntdll                               0x7ffc2c1a5e91

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[anda_skoa]

Very strange, the conversion to QPixmap should have consumed the original pixel data, anything afterwards should be fine.

Can you try something like this?

QImage imgCopy = img;
imgCopy.detach();
QPixmap frame = QPixmap::fromImage(imgCopy);

QImage imgCopy = img;
imgCopy.detach();
QPixmap frame = QPixmap::fromImage(imgCopy);

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Cheers,
_

[Royce]

Woah! That worked!

In my code the QPixmap frame is a member variable. So it is not falling out of scope. The QImage however, as a stack variable, is falling out of scope. So I guess we have to proceed from the supposition that a reference to the image was being held.

Reviewing the documentation I see that the overload:

QPixmap QPixmap::fromImage(QImage &&image, Qt::ImageConversionFlags flags = Qt::AutoColor)

QPixmap QPixmap::fromImage(QImage &&image, Qt::ImageConversionFlags flags = Qt::AutoColor)

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says "...without copying if possible." I guess that actually applies to both overloads.

I suppose the lesson learned is to look for detach()? If I see it I need to be mindful of the possibility that there may be lingering references if I don't call it?

Anyway, thanks again. I can move forward with my project.

[anda_skoa]

Maybe but I expect more a threading problem.

Anyway, since it works,lets not dwell on that

Cheers,
_

[Royce]

Hmm. It'd have to be an implicit thread. I don't start one and all crashes and break points show WinMain in the stack trace.