Replace Pixmap in GraphicsScene

I'd really like to attach to my web camera, process the frames I get out of it, and then display the processed frame in a window with some super-imposed markings.

The strategy I've been trying is to have a QGraphicsView with a QGraphicsScene that has a QGraphicsPixmap added. I then use a custom VideoSurface to grab frames. Right now I'm trying to just transfer the frames to the QGraphicsView (no extra markings or processing) but if I call setPixmap the program will crash just after the frame handler returns. If I skip the call to setPixmap, it doesn't crash. Could anyone point me in the right direction?

Code:

---

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    frame(640, 480)
{
    ui->setupUi(this);
    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();
    scene = new QGraphicsScene();
    pix = new QGraphicsPixmapItem(frame);
    scene->addItem(pix);
    ui->gv_viewfinder->setScene(scene);
}
 
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);
 
    bool good_pix = frame.convertFromImage(img);
    if (good_pix) pix->setPixmap(frame);
    this->update();
}

---

Code:

---

bool CameraFrameGrabber::present(const QVideoFrame &frame)
{
    if (frame.isValid()) {
        QVideoFrame cloneFrame(frame);
        cloneFrame.map(QAbstractVideoBuffer::ReadOnly);
        const QImage image(cloneFrame.bits(),
                           cloneFrame.width(),
                           cloneFrame.height(),
                           QVideoFrame::imageFormatFromPixelFormat(cloneFrame .pixelFormat()));
        emit frameAvailable(image);
        cloneFrame.unmap();
        return true;
    }
    return false;
}

---

The observed behavior would suggest that the "pix" pointer is not valid, thus accessing it crashes.

Can you try initializing it before starting the camera?

Cheers,
_

That was a good thought! But it doesn't seem to be the issue. I've also tried a slightly different approach in the handleFrame method, but it doesn't seem to make a difference either.

The thing is that it doesn't actually crash in handleFrame. I guess it crashes somewhere in the event handler. Certainly it is a null pointer, but the null pointer is way down the stack during a memcpy operation.

I've included the stack trace as well.


The current code:

Code:

---

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    frame(640, 480)
{
    ui->setupUi(this);
    scene = new QGraphicsScene();
    pix = new QGraphicsPixmapItem(frame);
    scene->addItem(pix);
    ui->gv_viewfinder->setScene(scene);
    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);
 
    scene->removeItem(pix);
    delete pix;
    frame = QPixmap::fromImage(img);
    pix = scene->addPixmap(frame);
    scene->invalidate();
 
    this->update();
}

---

Code:

---

1  memcpy                                                 VCRUNTIME140D                       0x7ffc229b1e10 
2  qt_blend_rgb32_on_rgb32                                qblendfunctions.cpp            399  0x7ffbfbf7bb85 
3  qt_blend_rgb32_on_rgb32_sse2                           qdrawhelper_sse2.cpp           139  0x7ffbfc32c17d 
4  QRasterPaintEnginePrivate::drawImage                   qpaintengine_raster.cpp        1021 0x7ffbfc03b9bd 
5  QRasterPaintEngine::drawImage                          qpaintengine_raster.cpp        2133 0x7ffbfc03235b 
6  QRasterPaintEngine::drawPixmap                         qpaintengine_raster.cpp        2019 0x7ffbfc03163b 
7  QPainter::drawPixmap                                   qpainter.cpp                   5070 0x7ffbfc05799c 
8  QGraphicsPixmapItem::paint                             qgraphicsitem.cpp              9769 0x5de350bf     
9  QGraphicsScenePrivate::draw                            qgraphicsscene.cpp             4961 0x5de7fe17     
10 QGraphicsScenePrivate::drawSubtreeRecursive            qgraphicsscene.cpp             4855 0x5de7f73b     
11 QGraphicsScenePrivate::drawItems                       qgraphicsscene.cpp             4714 0x5de7e9a0     
12 QGraphicsView::paintEvent                              qgraphicsview.cpp              3551 0x5dec8c82     
13 QWidget::event                                         qwidget.cpp                    8931 0x5d910fb1     
14 QFrame::event                                          qframe.cpp                     550  0x5db0c62f     
15 QAbstractScrollArea::viewportEvent                     qabstractscrollarea.cpp        1213 0x5dbfe81c     
16 QGraphicsView::viewportEvent                           qgraphicsview.cpp              2974 0x5dec66a4     
17 QAbstractScrollAreaPrivate::viewportEvent              qabstractscrollarea_p.h        111  0x5d899c55     
18 QAbstractScrollAreaFilter::eventFilter                 qabstractscrollarea_p.h        127  0x5dc0375d     
19 QCoreApplicationPrivate::sendThroughObjectEventFilters qcoreapplication.cpp           1099 0x5d0f8644     
20 QApplicationPrivate::notify_helper                     qapplication.cpp               3795 0x5d8a5e6e     
21 QApplication::notify                                   qapplication.cpp               3762 0x5d8a34c9     
22 QCoreApplication::notifyInternal2                      qcoreapplication.cpp           988  0x5d0f78c6     
23 QCoreApplication::sendSpontaneousEvent                 qcoreapplication.h             234  0x5d264c3b     
24 QWidgetPrivate::sendPaintEvent                         qwidget.cpp                    5699 0x5d91af5d     
25 QWidgetPrivate::drawWidget                             qwidget.cpp                    5640 0x5d91aa57     
26 QWidgetBackingStore::doSync                            qwidgetbackingstore.cpp        1357 0x5d8bf802     
27 QWidgetBackingStore::sync                              qwidgetbackingstore.cpp        1152 0x5d8bc8a3     
28 QWidgetPrivate::syncBackingStore                       qwidget.cpp                    1957 0x5d91d650     
29 QWidget::event                                         qwidget.cpp                    9084 0x5d911617     
30 QMainWindow::event                                     qmainwindow.cpp                1544 0x5db380b1     
31 QApplicationPrivate::notify_helper                     qapplication.cpp               3799 0x5d8a5e8e     
32 QApplication::notify                                   qapplication.cpp               3762 0x5d8a34c9     
33 QCoreApplication::notifyInternal2                      qcoreapplication.cpp           988  0x5d0f78c6     
34 QCoreApplication::sendEvent                            qcoreapplication.h             231  0x5d1027f2     
35 QCoreApplicationPrivate::sendPostedEvents              qcoreapplication.cpp           1649 0x5d0f9112     
36 QCoreApplication::sendPostedEvents                     qcoreapplication.cpp           1504 0x5d0f5c4c     
37 QGraphicsViewPrivate::dispatchPendingUpdateRequests    qgraphicsview_p.h              204  0x5d88ec31     
38 QGraphicsScenePrivate::_q_emitUpdated                  qgraphicsscene.cpp             383  0x5de7751b     
39 QGraphicsScene::qt_static_metacall                     moc_qgraphicsscene.cpp         180  0x5de6e856     
40 QMetaCallEvent::placeMetaCall                          qobject.cpp                    503  0x5d156c81     
41 QObject::event                                         qobject.cpp                    1263 0x5d14f6df     
42 QGraphicsScene::event                                  qgraphicsscene.cpp             3522 0x5de74608     
43 QApplicationPrivate::notify_helper                     qapplication.cpp               3799 0x5d8a5e8e     
44 QApplication::notify                                   qapplication.cpp               3159 0x5d8a0713     
45 QCoreApplication::notifyInternal2                      qcoreapplication.cpp           988  0x5d0f78c6     
46 QCoreApplication::sendEvent                            qcoreapplication.h             231  0x5d1027f2     
47 QCoreApplicationPrivate::sendPostedEvents              qcoreapplication.cpp           1649 0x5d0f9112     
48 QEventDispatcherWin32::sendPostedEvents                qeventdispatcher_win.cpp       1295 0x5d1b180a     
49 QWindowsGuiEventDispatcher::sendPostedEvents           qwindowsguieventdispatcher.cpp 82   0x7ffbfddfc204 
50 qt_internal_proc                                       qeventdispatcher_win.cpp       445  0x5d1af521     
51 CallWindowProcW                                        USER32                              0x7ffc29f41c24 
52 DispatchMessageW                                       USER32                              0x7ffc29f4156c 
53 QEventDispatcherWin32::processEvents                   qeventdispatcher_win.cpp       845  0x5d1aff2b     
54 QWindowsGuiEventDispatcher::processEvents              qwindowsguieventdispatcher.cpp 74   0x7ffbfddfc1c4 
55 QEventLoop::processEvents                              qeventloop.cpp                 135  0x5d0f24e8     
56 QEventLoop::exec                                       qeventloop.cpp                 210  0x5d0f272e     
57 QCoreApplication::exec                                 qcoreapplication.cpp           1261 0x5d0f55ff     
58 QGuiApplication::exec                                  qguiapplication.cpp            1640 0x7ffbfbc63208 
59 QApplication::exec                                     qapplication.cpp               2976 0x5d8a021a     
60 main                                                   main.cpp                       10   0x7ff6596b2759 
61 WinMain                                                qtmain_win.cpp                 123  0x7ff6596bb14d 
62 invoke_main                                            exe_common.inl                 109  0x7ff6596b8d7d 
63 __scrt_common_main_seh                                 exe_common.inl                 264  0x7ff6596b8c1e 
64 __scrt_common_main                                     exe_common.inl                 309  0x7ff6596b8ade 
65 WinMainCRTStartup                                      exe_winmain.cpp                17   0x7ff6596b8d99 
66 BaseThreadInitThunk                                    KERNEL32                            0x7ffc2a4a8364 
67 RtlUserThreadStart                                     ntdll                               0x7ffc2c1a5e91

---

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,
_

Quote:

---

Originally Posted by anda_skoa

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.

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.

Code:

---

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();
}

---

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

Code:

---

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

---

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

Can you try something like this?

Code:

---

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

---

Cheers,
_

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:

Code:

---

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

---

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.

Maybe but I expect more a threading problem.

Anyway, since it works,lets not dwell on that :)

Cheers,
_

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.