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No update of my QGraphicsPixmapItem



  • I am making an application to display several picture with the ability to zoom in them
    I am using a QMainWindow in which I can open several picture so using QMdiSubWindow
    Each MdiSubWindow is made of an object ImageView which inherits QGraphicsView
    Each ImageView contains a scene ImageScene which inherits QGraphicsScene
    And in each scene there is only one object GraphicsPixmapItem which inherits QGraphicsPixmapItem to display the image in which I want to zoom in

    I make the zoom thanks to a wheelEvent on the mouse which is intercepted in the ImageView object. With that I change the scale of the GraphicsPixmapItem which is centered on the coordinate (0, 0). I don't forget to adapt the boundaries of my ImageScene.

    If I open a "little" image (let's said 1024x768 in 24 bits) everything is OK, I can zoom as much as I want in my picture.
    But if I open a bigger image (6000x4000 in 24 bits) when I try to zoom in it, I don't have any more update of my picture.
    When I zoom, I see the call to ImageView::paintEvent() but there is no call to GraphicsPixmapItem::paint() like if something said that there was no used to redraw the pixmap.
    I have checked to the rect() and region() of the ImageView and they seem to be correct. The boundary change of the ImageScene are also correct. I tried to call an update on a wheel event on the pixmap item but still without any success.
    I also tried a prepareGeometryChange() on my GraphicsPixmapItem but still nothing. I have a call to ImageView::paintEvent() but it doesn't call for GraphicsPixmapItem::paint().

    Does anyone have any idea what could prevent my object pixmap to be updated ? Apparently it is linked to the size of the picture but then what is the problem ? memory size ? time consumption ?

    I am using Qt 5.14.0 under Linux 4.19.0



  • I have a similar problem like wotan. I need always to write QApplication::process events();
    after i changed / set / loaded a pixmap. without nothing happens.

    I have this on both platforms Windows 10 and on MacOSX 10.14.6

    Could this be a bug? I am using Qt 5.15.0



  • Hi to all experts,

    i have googeln a whole day and tried out different solutions to get a graphics scene updated.

    1. Suggestion: removeold pixmap and add new one .
      scene->removeItem(&pixmap);
      scene->addItem(&pixmap);

    2. Suggestion: Do an update on the whole scene.
      scene->update();

    3. Suggestion: Do an update on the element which has changed.
      pixmap.update(ui->graphicsView->rect());

    None of this solution has been working.

    Pls can we get help on this problem - it is mandatory for us to actualize an element in graphics view?



  • Re: No update of my QGraphicsPixmapItem

    ademmler, is your problem also linked to the size of the pixmap ?
    Because I am thinking of an extraction of the part of the pixmap which is supposed to be seen in the view. That way I will have a littler pixmap (in size) and put it in the scene. That might solve my problem ...



  • @Wotan thx for answering. It does not matter if the pixmap is 32,32 or 500,500 in size.
    It is also just pure color ...



  • Nohing to do, I have tried an update in QGraphicsView, an update in QGraphicsScene and a QCoreApplication::processEvents() my QGraphicsPixmapItem is not drawn ...
    I feel a little despair ...



  • @Wotan I have the same issue - and I am also only qt beginner and user.

    I have also filed a bug in https://bugreports.qt.io

    Pls - does somebody of the pros look at this.



  • Can you pass the address of the bug ?





  • @Wotan I feel despair also. I am looking for a solution since months ...


  • Lifetime Qt Champion

    @ademmler you had an answer on the report which is pretty clear: you are blocking the event loop so no update will occur except if you call processEvent. Therefore fix your code to not block the event loop.



  • This post is deleted!


  • @SGaist Are you talking about the bug report I made.
    The solution described there I have got right now. I will test this.
    https://bugreports.qt.io/browse/QTBUG-86086



  • @SGaist

    Hi SGaist,

    I am still trying to solve the issue with my measurement loop and painting to graphics scene.
    But I get a "signal/slot" connect error. I can't see why it should not work.

    QObject::connect: No such signal MainWindow::&MainWindow::signalPaintColor(int i) in mainwindow.cpp:52
    QObject::connect: (sender name: 'MainWindow')
    QObject::connect: (receiver name: 'MainWindow')

    This is my modified minimal code example:

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H
    
    #include <QMainWindow>
    #include <QGraphicsScene>
    #include <QGraphicsPixmapItem>
    #include <QThread>
    #include <QDebug>
    
    
    //QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    //QT_END_NAMESPACE
    
    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
    
    
    signals:
        void signalPaintColor(int i);
    
    private slots:
        void on_startButton_released();
        void slotPaintColor(int i);
    
    private:
        Ui::MainWindow *ui;       
        QGraphicsScene *scene;
        QGraphicsPixmapItem pixmap;
    };
    #endif // MAINWINDOW_H
    
    #include "mainwindow.h"
    #include "ui_mainwindow.h"
    
    using namespace std;
    
    struct rgb {
        int R;
        int G;
        int B;
        char cn [10];
    } colors[5];
    
    
    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
        scene = new QGraphicsScene(this);
        ui->graphicsView->setScene(scene);
        scene->addItem(&pixmap);
    
        colors[0] = { 255, 255, 255, "white" };
        colors[1] = { 255, 0, 0, "red" };
        colors[2] = { 0, 255, 0, "green" };
        colors[3] = { 0, 0, 255, "blue" };
        colors[4] = { 0, 0, 0, "black" };
    
    }
    
    MainWindow::~MainWindow()
    {
        delete ui;
    }
    
    void MainWindow::slotPaintColor(int i){
    
        QPixmap pix(ui->graphicsView->size());
        pix.fill(QColor(colors[i].R, colors[i].G, colors[i].B));
        pixmap.setPixmap(pix);
    
        ui->statusbar->showMessage("Color: " + QString(colors[i].cn), 0);
    
        QApplication::processEvents();
    }
    
    void MainWindow::on_startButton_released()
    {
        ui->statusbar->showMessage("Start loop ...", 0);
        connect(this, SIGNAL(signalPaintColor(int i)), this, SLOT(slotPaintColor(int i)));
    
        for( int i = 0; i < 5; i++ ) {
           emit signalPaintColor(i);
        }
    }
    


  • @ademmler said in No update of my QGraphicsPixmapItem:

    QObject::connect: No such signal MainWindow::&MainWindow::signalPaintColor(int i) in mainwindow.cpp:52

    If you insist on using old-style signal/slot syntax, this is telling you you have got it wrong. Where did you get/copy connect(this, SIGNAL(signalPaintColor(int i)), this, SLOT(slotPaintColor(int i))); from? Please copy examples, should be:

    connect(this, SIGNAL(signalPaintColor(int)), this, SLOT(slotPaintColor(int)));
    

    Do yourself a favor and switch over now to new-style for everything:

    connect(this, &MainWindow::signalPaintColor, this, &MainWindow::slotPaintColor);
    

    Isn't that neater? :)

    Separately: why are you doing connect() signals/slots inside on_startButton_released()? That means a new, additional connection will be made each time the button is pressed --- put qDebug()s in slotPaintColor() to see. connect()s should be done during initialization only.



  • @JonB
    Dear Jon thx for this precise input to me.

    If you insist on using old-style signal/slot syntax, this is telling you you have got it wrong.

    I would not insist here ...

    Where did you get/copy connect(this, SIGNAL(signalPaintColor(int i)), this, SLOT(slotPaintColor(int i))); from?

    Honestly I do not remember.

    Please copy examples, should be:

    connect(this, SIGNAL(signalPaintColor(int)), this, SLOT(slotPaintColor(int)));
    

    Do yourself a favor and switch over now to new-style for everything:

    connect(this, &MainWindow::signalPaintColor, this, &MainWindow::slotPaintColor);
    

    Isn't that neater? :)

    Ofcourse it is neater. Doesn't the new syntax need to know about variables to be passed?

    Separately: why are you doing connect() signals/slots inside on_startButton_released()? That means a new, additional connection will be made each time the button is pressed --- put qDebug()s in slotPaintColor() to see. connect()s should be done during initialization only.

    Its a leftover from trying things out - in this minimal code ...
    Ofcourse I will move it back to MainWindow::MainWindow(QWidget *parent).

    What I am still wondering is how to get the loop from the measurement device together with painting the colors. Paint -> Measure -> Paint -> Measure ...
    Would I need a "signal" telling me, that the color has changed on the monitor?

    thx for your help - it is most appreciated!



  • I changed the minimal example to this using two signals and two slots.
    Please note " QThread::sleep(1);" is used only here - for simulating the measurement routine and the device - which is a black box routine eating some time ...

    The problem is still that with out QApplication::processEvents();
    in MainWindow::slotPaintColor(int i) the color does not change.

    How can I achieve a loop which first changes the color and than - when this color is shown on screen (not before) - the measurement takes place ... and again until the colorlist is at the end ... all without using QApplication::processEvents();

    #include "mainwindow.h"
    #include "ui_mainwindow.h"
    
    using namespace std;
    
    struct rgb {
        int R;
        int G;
        int B;
        char cn [10];
    } colors[5];
    
    
    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
        connect(this, &MainWindow::signalPaintColor, this, &MainWindow::slotPaintColor);
        connect(this, &MainWindow::signalMeasureColor, this, &MainWindow::slotMeasureColor);
    
        scene = new QGraphicsScene(this);
        ui->graphicsView->setScene(scene);
        scene->addItem(&pixmap);
    
        colors[0] = { 255, 255, 255, "white" };
        colors[1] = { 255, 0, 0, "red" };
        colors[2] = { 0, 255, 0, "green" };
        colors[3] = { 0, 0, 255, "blue" };
        colors[4] = { 0, 0, 0, "black" };
    
    }
    
    MainWindow::~MainWindow()
    {
        delete ui;
    }
    
    void MainWindow::slotPaintColor(int i){
    
        QPixmap pix(ui->graphicsView->size());
        pix.fill(QColor(colors[i].R, colors[i].G, colors[i].B));
        pixmap.setPixmap(pix);
    
        ui->statusbar->showMessage("Color: " + QString(colors[i].cn), 0);
    
        QApplication::processEvents();
    
        emit signalMeasureColor();
    }
    
    void MainWindow::slotMeasureColor() {
        //Sleep is for simulation of external task here ...
        QThread::sleep(1);
    }
    
    void MainWindow::on_startButton_released()
    {
        ui->statusbar->showMessage("Start loop ...", 0);    
    
        for( int i = 0; i < 5; i++ ) {
    
           emit signalPaintColor(i);
        }
    }
    

  • Lifetime Qt Champion

    Well: do not block the event loop.

    Since you are doing measures "on demand", use the worker object approach. Encapsulate the measurement part in its own class. Move the instance of that class to a dedicated QThread. And then use signals and slots to trigger the measure and pass back the data. Doing so you will decouple the controlling part and the UI.



  • @SGaist thx for the fast response.

    Is there a "Qt standard Example" using this technique ?
    That I can have a look how his works.

    Is this what you are talking about? https://wiki.qt.io/QThreads_general_usage

    Am I right: This examples uses still old signal/slots syntax?


  • Lifetime Qt Champion

    The wiki page is a bit outdated.

    Check the QThread documentation. It has vastly improved and shows both technique with modern syntax.



  • @SGaist thx so much - I try this out



  • Hi there,

    now i have modified my minimal app to give threads a try.
    I would need help with the new syntax for signal/slots across classes.

    I try to connect the signal from the thread with the slot in the main UI app like this:
    connect(mThread, &MyThread::signalPaintColor, this, &MainWindow::slotPaintColor);

    mainwindow.cpp:20:5: error: no matching member function for call to 'connect'
    qobject.h:242:43: note: candidate function [with Func1 = void (MyThread::)(int), Func2 = void (MainWindow::)(int)] not viable: cannot convert from base class pointer 'QThread ' to derived class pointer 'const typename QtPrivate::FunctionPointer<void (MyThread::)(int)>::Object ' (aka 'const MyThread ') for 1st argument
    qobject.h:222:36: note: candidate function not viable: no known conversion from 'void (MyThread::
    )(int)' to 'const char ' for 2nd argument
    qobject.h:225:36: note: candidate function not viable: no known conversion from 'void (MyThread::
    )(int)' to 'const QMetaMethod' for 2nd argument
    qobject.h:481:41: note: candidate function not viable: no known conversion from 'void (MyThread::
    )(int)' to 'const char ' for 2nd argument
    qobject.h:283:13: note: candidate template ignored: requirement '!QtPrivate::FunctionPointer<void (MainWindow::
    )(int)>::IsPointerToMemberFunction' was not satisfied [with Func1 = void (MyThread::)(int), Func2 = void (MainWindow::)(int)]
    qobject.h:322:13: note: candidate template ignored: requirement 'QtPrivate::FunctionPointer<void (MainWindow::)(int)>::ArgumentCount == -1' was not satisfied [with Func1 = void (MyThread::)(int), Func2 = void (MainWindow::*)(int)]
    qobject.h:274:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided
    qobject.h:314:13: note: candidate function template not viable: requires 3 arguments, but 4 were provided

    From mythread.h
    
    signals:
        void signalPaintColor(int i);
    
    public slots:
        void slotMeasureColor(int i);
    
    From mainwindow.h
    
    signals:        
    
    private slots:
        void on_startButton_released();
        void slotPaintColor(int i);
    


  • @ademmler
    Your error message is not on any of these. It states:

    mainwindow.cpp:20:5: error: no matching member function for call to 'connect'
    qobject.h:242:43: note: candidate function [with Func1 = void (MyThread::)(int), Func2 = void (MainWindow::)(int)] not viable: cannot convert from base class pointer 'QThread ' to derived class pointer 'const typename QtPrivate::FunctionPointer<void (MyThread::)(int)>::Object ' (aka 'const MyThread ') for 1st argument

    So show your corresponding code. It even gives you the line number of the call.



  • @JonB

    The code to connect at line 20:
    connect(mThread, &MyThread::signalPaintColor, this, &MainWindow::slotPaintColor);

    Pls be aware - this is a "minimal for testing and learning - not a rock solid solution.

    mainwindow.cpp

    #include "mainwindow.h"
    #include "ui_mainwindow.h"
    
    using namespace std;
    
    struct rgb {
        int R;
        int G;
        int B;
        char cn [10];
    } colors[6];
    
    
    MainWindow::MainWindow(QWidget *parent)
        : QMainWindow(parent)
        , ui(new Ui::MainWindow)
    {
        ui->setupUi(this);
    
        connect(mThread, &MyThread::signalPaintColor, this, &MainWindow::slotPaintColor);
    
        colors[0] = { 255, 255, 255, "white" };
        colors[1] = { 255, 0, 0, "red" };
        colors[2] = { 0, 255, 0, "green" };
        colors[3] = { 0, 0, 255, "blue" };
        colors[4] = { 0, 0, 0, "black" };
        colors[5] = { 128, 128, 128, "grey" };
    
        scene = new QGraphicsScene(this);
        ui->graphicsView->setScene(scene);
        scene->addItem(&pixmap);
        QPixmap pix(this->width(), this->height());
        pix.fill(QColor(colors[5].R, colors[5].G, colors[5].B));
        pixmap.setPixmap(pix);
    }
    
    MainWindow::~MainWindow()
    {
        delete ui;
    }
    
    void MainWindow::on_startButton_released() {
    
        mThread = new MyThread;
        mThread->start();
    }
    
    void MainWindow::slotPaintColor(int i){
    
        ui->statusbar->showMessage("Color: " + QString(colors[i].cn), 0);   
    
        QPixmap pix(ui->graphicsView->size());
        pix.fill(QColor(colors[i].R, colors[i].G, colors[i].B));
        pixmap.setPixmap(pix);
    }
    

    mythread.cpp

    #include "mythread.h"
    
    MyThread::MyThread(QObject *parent):
        QThread(parent)
    {
    }
    
    void MyThread::run()
    {
        QMutex mutex;
        //Prepare measurment device
        int err = 0;
        m_meas = new Measure(meas_i1d3, &err);
    
    
        if (!m_meas){
            qDebug() << "No connection to measurment device";
            return;
        }
    
        for( int i = 0; i < 5; i++ ) {
          emit signalPaintColor(i);
          slotMeasureColor(i);
        }
        mutex.unlock();
    }
    
    void MyThread::slotMeasureColor(int i)
    {
        if (int err = m_meas->measure())
        {
            qDebug() << "Could not get correct measurement!";
        }
    
        char buffer[128];
        sprintf(buffer, "%i, %4.1f %4.1f %4.1f", i, m_meas->meas.xyz.X, m_meas->meas.xyz.Y, m_meas->meas.xyz.Z);
        qDebug() << "Measurment: " << buffer;
    }
    

    mainwindow.h

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H
    
    #include <QMainWindow>
    #include <QGraphicsScene>
    #include <QGraphicsPixmapItem>
    #include <QMessageBox>
    #include <QThread>
    #include <QDebug>
    #include "color/measure.h"
    #include "mythread.h"
    
    
    QT_BEGIN_NAMESPACE
    namespace Ui { class MainWindow; }
    QT_END_NAMESPACE
    
    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = nullptr);
        ~MainWindow();
    
    signals:        
    
    private slots:
        void on_startButton_released();
        void slotPaintColor(int i);
    
    private:
        Ui::MainWindow *ui;       
        QGraphicsScene *scene;
        QGraphicsPixmapItem pixmap;
        Measure *m_meas;
        QThread *mThread;
    
    };
    #endif // MAINWINDOW_H
    

    mythread.h

    #ifndef MYTHREAD_H
    #define MYTHREAD_H
    #include <QtCore>
    #include <QDebug>
    #include "color/measure.h"
    
    class MyThread :public QThread
    {
          Q_OBJECT
    public:
    
        explicit MyThread(QObject *parent = 0);
        void run() override;
        QString name;
        bool Stop;
        Measure *m_meas;
    
    signals:
        void signalPaintColor(int i);
    
    public slots:
        void slotMeasureColor(int i);
    
    };
    
    #endif // MYTHREAD_H
    


  • I can't see what's wrong, but then I don't use threads :) While you await an expert who will spot it in 1 second, 10% chance that deleting all files in build output directory and rebuild will work, e.g. if you have added Q_OBJECTs recently.


  • Lifetime Qt Champion

    You are declaring a QThread and not a MyThread in your MainWindow class.

    Another issue you will have is that your are doing your connection on an initialized uninitialized pointer so you are in fact not connecting anything valid.

    You should just start your thread in on_startButton_released. The way it's working now is that it will create a new instance of your MyThread class each time you click on the button which is likely not what you want.

    [edit: Fixed typo]



  • @SGaist said in No update of my QGraphicsPixmapItem:

    You are declaring a QThread and not a MyThread in your MainWindow class.

    Dang!



  • @SGaist said in No update of my QGraphicsPixmapItem:

    MyThread

    Perfect HELP - thank You! The issue with the connect is gone!

    Another issue you will have is that your are doing your connection on an initialized pointer so you are in fact not connecting anything valid.

    What should I do instead. if I change MyThread *mThread; to MyThread mThread; I get again:
    /Users/ademmler/QtTutorials/QtMinimalMeasure/mainwindow.cpp:20: error: no matching member function for call to 'connect'

    When I run the code - the measurement works, but the pixmap does not get updated at all - again.


  • Lifetime Qt Champion

    @ademmler There was a (now fixed) typo in the sentence: it was "uninitialised" pointer. Which one is the no matching member function ?



  • @ademmler said in No update of my QGraphicsPixmapItem:

    What should I do instead. if I change MyThread *mThread; to MyThread mThread; I get again:
    /Users/ademmler/QtTutorials/QtMinimalMeasure/mainwindow.cpp:20: error: no matching member function for call to 'connect'

    If you change from MyThread *mThread to MyThread mThread you must change connect(mThread, ...) to connect(&mThread, ...).

    You should do this connect() before you invoke QThread::start(). So if you stick with on_startButton_released() doing a mThread = new MyThread the connect() should come on the next line.



  • Thank you very much to @SGaist and @JonB
    I can't set an answer to be the correct one ... missing user rights for that!

    In the mean time i did what you and @SGaist has told me to do.

    I have put all this to the to after ui->setupUi(this); and now it works as expected :-D

    mThread = new MyThread;
    connect(mThread, &MyThread::signalPaintColor, this, &MainWindow::slotPaintColor);
    

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