Paint to QWidget but not main window



  • Hi

    I am using the scribble example as a template and starting point. What i want to do is create a second window that will copy whatever is drawn on the scribble application.

    I have modified the paintEvent to copy the output to a second window.

    void ScribbleArea::paintEvent(QPaintEvent *event)
    //! [13] //! [14]
    {
        QPainter painter(this);
        QRect dirtyRect = event->rect();
        painter.drawImage(dirtyRect, image, dirtyRect);
    
        QPainter painter2(secondDisplay);
     //   QRect dirtyRect2 = event->rect();
     //   painter2.drawImage(dirtyRect, image, dirtyRect);
    }
    //! [14]
    

    The second blank window shows but i get the following error when i start drawing

    QWidget::paintEngine: Should no longer be called
    QPainter::begin: Paint device returned engine == 0, type: 1
    QWidget::paintEngine: Should no longer be called
    QPainter::begin: Paint device returned engine == 0, type: 1
    QWidget::paintEngine: Should no longer be called
    

    My second window is created in the scribblearea.cpp file

    ScribbleArea::ScribbleArea(QWidget *parent)
        : QWidget(parent)
    {
        setAttribute(Qt::WA_StaticContents);
        modified = false;
        scribbling = false;
        myPenWidth = 1;
        myPenColor = Qt::blue;
    
      //  projector = new ScribbleArea;   // create the display for the projector
       secondDisplay= new QWidget;
        secondDisplay->setWindowFlags(Qt::FramelessWindowHint);
        secondDisplay->resize(800,480);
        secondDisplay->show();
    
    }
    

    and a pointer created in the header

    private:
        void drawLineTo(const QPoint &endPoint);
        void resizeImage(QImage *image, const QSize &newSize);
    
        QWidget *secondDisplay;
    
        bool modified;
        bool scribbling;
        int myPenWidth;
        QColor myPenColor;
        QImage image;
        QPoint lastPoint;
    

    Steve


  • Moderators

    @supersteve
    every widget is only allowed to paint to itself in the paintEvent() handler
    You you need to change your design.

    When the data changes and a repaint is needed you should notify secondDisplay about the change and set the necessary data and let it repaint itself.



  • Thanks for the response, i have attempted what you suggested with not much luck.

    I created a function within the ScribbleArea class that returns the QImage image.

    I then used then used the existing Help menu button (just for testing) to update the Widget but i still get the same error.

    /****************************************************************************
    **
    ** Copyright (C) 2016 The Qt Company Ltd.
    ** Contact: https://www.qt.io/licensing/
    **
    ** This file is part of the examples of the Qt Toolkit.
    **
    ** $QT_BEGIN_LICENSE:BSD$
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    ** accordance with the commercial license agreement provided with the
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    ** and conditions see https://www.qt.io/terms-conditions. For further
    ** information use the contact form at https://www.qt.io/contact-us.
    **
    ** BSD License Usage
    ** Alternatively, you may use this file under the terms of the BSD license
    ** as follows:
    **
    ** "Redistribution and use in source and binary forms, with or without
    ** modification, are permitted provided that the following conditions are
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    **     the documentation and/or other materials provided with the
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    **   * Neither the name of The Qt Company Ltd nor the names of its
    **     contributors may be used to endorse or promote products derived
    **     from this software without specific prior written permission.
    **
    **
    ** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    ** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    ** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    ** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    ** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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    ** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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    ** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    ** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
    **
    ** $QT_END_LICENSE$
    **
    ****************************************************************************/
    
    #include <QtWidgets>
    
    #include "mainwindow.h"
    #include "scribblearea.h"
    
    //! [0]
    MainWindow::MainWindow()
    {
        screen = new ScribbleArea;      // create the screen
    
        setCentralWidget(screen);
    
        createActions();
        createMenus();
    
        setWindowTitle(tr("Scribble"));
        resize(800, 480);
    
          secondDisplay = new QWidget;
           secondDisplay->setWindowFlags(Qt::FramelessWindowHint);
           secondDisplay->resize(800,480);
           secondDisplay->show();
    
    
    }
    //! [0]
    
    //! [1]
    void MainWindow::closeEvent(QCloseEvent *event)
    //! [1] //! [2]
    {
        if (maybeSave()) {
            event->accept();
        } else {
            event->ignore();
        }
    }
    //! [2]
    
    //! [3]
    void MainWindow::open()
    //! [3] //! [4]
    {
        if (maybeSave()) {
            QString fileName = QFileDialog::getOpenFileName(this,
                                       tr("Open File"), QDir::currentPath());
            if (!fileName.isEmpty())
                screen->openImage(fileName);
        }
    }
    //! [4]
    
    //! [5]
    void MainWindow::save()
    //! [5] //! [6]
    {
        QAction *action = qobject_cast<QAction *>(sender());
        QByteArray fileFormat = action->data().toByteArray();
        saveFile(fileFormat);
    }
    //! [6]
    
    //! [7]
    void MainWindow::penColor()
    //! [7] //! [8]
    {
        QColor newColor = QColorDialog::getColor(screen->penColor());
        if (newColor.isValid())
            screen->setPenColor(newColor);
    }
    //! [8]
    
    //! [9]
    void MainWindow::penWidth()
    //! [9] //! [10]
    {
        bool ok;
        int newWidth = QInputDialog::getInt(this, tr("Scribble"),
                                            tr("Select pen width:"),
                                            screen->penWidth(),
                                            1, 50, 1, &ok);
        if (ok)
            screen->setPenWidth(newWidth);
    }
    //! [10]
    
    //! [11]
    void MainWindow::about()
    //! [11] //! [12]
    {
        QMessageBox::about(this, tr("About Scribble"),
                tr("<p>The <b>Scribble</b> example shows how to use QMainWindow as the "
                   "base widget for an application, and how to reimplement some of "
                   "QWidget's event handlers to receive the events generated for "
                   "the application's widgets:</p><p> We reimplement the mouse event "
                   "handlers to facilitate drawing, the paint event handler to "
                   "update the application and the resize event handler to optimize "
                   "the application's appearance. In addition we reimplement the "
                   "close event handler to intercept the close events before "
                   "terminating the application.</p><p> The example also demonstrates "
                   "how to use QPainter to draw an image in real time, as well as "
                   "to repaint widgets.</p>"));
    
            secondDisplay->update();
    }
    //! [12]
    
    void MainWindow::paintEvent(QPaintEvent *event){
        QPainter painter(secondDisplay);
        painter.drawImage(800,480, screen->updateProjector());
    }
    
    //! [13]
    void MainWindow::createActions()
    //! [13] //! [14]
    {
        openAct = new QAction(tr("&Open..."), this);
        openAct->setShortcuts(QKeySequence::Open);
        connect(openAct, SIGNAL(triggered()), this, SLOT(open()));
    
        foreach (QByteArray format, QImageWriter::supportedImageFormats()) {
            QString text = tr("%1...").arg(QString(format).toUpper());
    
            QAction *action = new QAction(text, this);
            action->setData(format);
            connect(action, SIGNAL(triggered()), this, SLOT(save()));
            saveAsActs.append(action);
        }
    
        printAct = new QAction(tr("&Print..."), this);
        connect(printAct, SIGNAL(triggered()), screen, SLOT(print()));
    
        exitAct = new QAction(tr("E&xit"), this);
        exitAct->setShortcuts(QKeySequence::Quit);
        connect(exitAct, SIGNAL(triggered()), this, SLOT(close()));
    
        penColorAct = new QAction(tr("&Pen Color..."), this);
        connect(penColorAct, SIGNAL(triggered()), this, SLOT(penColor()));
    
        penWidthAct = new QAction(tr("Pen &Width..."), this);
        connect(penWidthAct, SIGNAL(triggered()), this, SLOT(penWidth()));
    
        clearScreenAct = new QAction(tr("&Clear Screen"), this);
        clearScreenAct->setShortcut(tr("Ctrl+L"));
        connect(clearScreenAct, SIGNAL(triggered()),
                screen, SLOT(clearImage()));
    
        aboutAct = new QAction(tr("&About"), this);
        connect(aboutAct, SIGNAL(triggered()), this, SLOT(about()));
    
        aboutQtAct = new QAction(tr("About &Qt"), this);
        connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt()));
    }
    //! [14]
    
    //! [15]
    void MainWindow::createMenus()
    //! [15] //! [16]
    {
        saveAsMenu = new QMenu(tr("&Save As"), this);
        foreach (QAction *action, saveAsActs)
            saveAsMenu->addAction(action);
    
        fileMenu = new QMenu(tr("&File"), this);
        fileMenu->addAction(openAct);
        fileMenu->addMenu(saveAsMenu);
        fileMenu->addAction(printAct);
        fileMenu->addSeparator();
        fileMenu->addAction(exitAct);
    
        optionMenu = new QMenu(tr("&Options"), this);
        optionMenu->addAction(penColorAct);
        optionMenu->addAction(penWidthAct);
        optionMenu->addSeparator();
        optionMenu->addAction(clearScreenAct);
    
        helpMenu = new QMenu(tr("&Help"), this);
        helpMenu->addAction(aboutAct);
        helpMenu->addAction(aboutQtAct);
    
        menuBar()->addMenu(fileMenu);
        menuBar()->addMenu(optionMenu);
        menuBar()->addMenu(helpMenu);
    }
    //! [16]
    
    //! [17]
    bool MainWindow::maybeSave()
    //! [17] //! [18]
    {
        if (screen->isModified()) {
           QMessageBox::StandardButton ret;
           ret = QMessageBox::warning(this, tr("Scribble"),
                              tr("The image has been modified.\n"
                                 "Do you want to save your changes?"),
                              QMessageBox::Save | QMessageBox::Discard
                              | QMessageBox::Cancel);
            if (ret == QMessageBox::Save) {
                return saveFile("png");
            } else if (ret == QMessageBox::Cancel) {
                return false;
            }
        }
        return true;
    }
    //! [18]
    
    //! [19]
    bool MainWindow::saveFile(const QByteArray &fileFormat)
    //! [19] //! [20]
    {
        QString initialPath = QDir::currentPath() + "/untitled." + fileFormat;
    
        QString fileName = QFileDialog::getSaveFileName(this, tr("Save As"),
                                   initialPath,
                                   tr("%1 Files (*.%2);;All Files (*)")
                                   .arg(QString::fromLatin1(fileFormat.toUpper()))
                                   .arg(QString::fromLatin1(fileFormat)));
        if (fileName.isEmpty()) {
            return false;
        } else {
            return screen->saveImage(fileName, fileFormat.constData());
        }
    }
    //! [20]
    
    
    /****************************************************************************
    **
    ** Copyright (C) 2016 The Qt Company Ltd.
    ** Contact: https://www.qt.io/licensing/
    **
    ** This file is part of the examples of the Qt Toolkit.
    **
    ** $QT_BEGIN_LICENSE:BSD$
    ** Commercial License Usage
    ** Licensees holding valid commercial Qt licenses may use this file in
    ** accordance with the commercial license agreement provided with the
    ** Software or, alternatively, in accordance with the terms contained in
    ** a written agreement between you and The Qt Company. For licensing terms
    ** and conditions see https://www.qt.io/terms-conditions. For further
    ** information use the contact form at https://www.qt.io/contact-us.
    **
    ** BSD License Usage
    ** Alternatively, you may use this file under the terms of the BSD license
    ** as follows:
    **
    ** "Redistribution and use in source and binary forms, with or without
    ** modification, are permitted provided that the following conditions are
    ** met:
    **   * Redistributions of source code must retain the above copyright
    **     notice, this list of conditions and the following disclaimer.
    **   * Redistributions in binary form must reproduce the above copyright
    **     notice, this list of conditions and the following disclaimer in
    **     the documentation and/or other materials provided with the
    **     distribution.
    **   * Neither the name of The Qt Company Ltd nor the names of its
    **     contributors may be used to endorse or promote products derived
    **     from this software without specific prior written permission.
    **
    **
    ** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    ** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    ** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    ** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    ** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    ** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    ** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    ** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    ** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    ** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    ** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
    **
    ** $QT_END_LICENSE$
    **
    ****************************************************************************/
    
    #include <QtWidgets>
    #ifndef QT_NO_PRINTER
    #include <QPrinter>
    #include <QPrintDialog>
    #endif
    
    #include "scribblearea.h"
    #include "mainwindow.h"
    
    //! [0]
    ScribbleArea::ScribbleArea(QWidget *parent)
        : QWidget(parent)
    {
        setAttribute(Qt::WA_StaticContents);
        modified = false;
        scribbling = false;
        myPenWidth = 1;
        myPenColor = Qt::blue;
    
    }
    //! [0]
    
    //! [1]
    bool ScribbleArea::openImage(const QString &fileName)
    //! [1] //! [2]
    {
        QImage loadedImage;
        if (!loadedImage.load(fileName))
            return false;
    
        QSize newSize = loadedImage.size().expandedTo(size());
        resizeImage(&loadedImage, newSize);
        image = loadedImage;
        modified = false;
        update();
        return true;
    }
    //! [2]
    
    //! [3]
    bool ScribbleArea::saveImage(const QString &fileName, const char *fileFormat)
    //! [3] //! [4]
    {
        QImage visibleImage = image;
        resizeImage(&visibleImage, size());
    
        if (visibleImage.save(fileName, fileFormat)) {
            modified = false;
            return true;
        } else {
            return false;
        }
    }
    //! [4]
    //!
    //!
    QImage ScribbleArea::updateProjector(){
        QImage returnImage = image;
        return returnImage;
    }
    
    //! [5]
    void ScribbleArea::setPenColor(const QColor &newColor)
    //! [5] //! [6]
    {
        myPenColor = newColor;
    }
    //! [6]
    
    //! [7]
    void ScribbleArea::setPenWidth(int newWidth)
    //! [7] //! [8]
    {
        myPenWidth = newWidth;
    }
    //! [8]
    
    //! [9]
    void ScribbleArea::clearImage()
    //! [9] //! [10]
    {
        image.fill(qRgb(255, 255, 255));
        modified = true;
        update();
    }
    //! [10]
    
    //! [11]
    void ScribbleArea::mousePressEvent(QMouseEvent *event)
    //! [11] //! [12]
    {
        if (event->button() == Qt::LeftButton) {
            lastPoint = event->pos();
            scribbling = true;
        }
    }
    
    void ScribbleArea::mouseMoveEvent(QMouseEvent *event)
    {
        if ((event->buttons() & Qt::LeftButton) && scribbling)
            drawLineTo(event->pos());
    }
    
    void ScribbleArea::mouseReleaseEvent(QMouseEvent *event)
    {
        if (event->button() == Qt::LeftButton && scribbling) {
            drawLineTo(event->pos());
            scribbling = false;
        }
    }
    
    //! [12] //! [13]
    void ScribbleArea::paintEvent(QPaintEvent *event)
    //! [13] //! [14]
    {
        QPainter painter(this);
        QRect dirtyRect = event->rect();
        painter.drawImage(dirtyRect, image, dirtyRect);
    
     //   QPainter painter2(projector);
     //   QRect dirtyRect2 = event->rect();
     //   painter2.drawImage(dirtyRect, image, dirtyRect);
    }
    //! [14]
    
    //! [15]
    void ScribbleArea::resizeEvent(QResizeEvent *event)
    //! [15] //! [16]
    {
        if (width() > image.width() || height() > image.height()) {
            int newWidth = qMax(width() + 128, image.width());
            int newHeight = qMax(height() + 128, image.height());
            resizeImage(&image, QSize(newWidth, newHeight));
            update();
        }
        QWidget::resizeEvent(event);
    }
    //! [16]
    
    //! [17]
    void ScribbleArea::drawLineTo(const QPoint &endPoint)
    //! [17] //! [18]
    {
        QPainter painter(&image);
        painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
                            Qt::RoundJoin));
        painter.drawLine(lastPoint, endPoint);
        modified = true;
    
        int rad = (myPenWidth / 2) + 2;
        update(QRect(lastPoint, endPoint).normalized()
                                         .adjusted(-rad, -rad, +rad, +rad));
        lastPoint = endPoint;
    }
    //! [18]
    
    //! [19]
    void ScribbleArea::resizeImage(QImage *image, const QSize &newSize)
    //! [19] //! [20]
    {
        if (image->size() == newSize)
            return;
    
        QImage newImage(newSize, QImage::Format_RGB32);
        newImage.fill(qRgb(255, 255, 255));
        QPainter painter(&newImage);
        painter.drawImage(QPoint(0, 0), *image);
        *image = newImage;
    }
    //! [20]
    
    //! [21]
    void ScribbleArea::print()
    {
    #if !defined(QT_NO_PRINTER) && !defined(QT_NO_PRINTDIALOG)
        QPrinter printer(QPrinter::HighResolution);
    
        QPrintDialog printDialog(&printer, this);
    //! [21] //! [22]
        if (printDialog.exec() == QDialog::Accepted) {
            QPainter painter(&printer);
            QRect rect = painter.viewport();
            QSize size = image.size();
            size.scale(rect.size(), Qt::KeepAspectRatio);
            painter.setViewport(rect.x(), rect.y(), size.width(), size.height());
            painter.setWindow(image.rect());
            painter.drawImage(0, 0, image);
        }
    #endif // QT_NO_PRINTER
    }
    //! [22]
    
    

  • Moderators

    @supersteve
    because you keep doing the same mistake!

    void MainWindow::paintEvent(QPaintEvent *event){
        QPainter painter(secondDisplay);
        painter.drawImage(800,480, screen->updateProjector());
    }
    

    As i said: A widget is only allowed to paint to itself in it's paintEvent() handler.
    You need a custom class and reimplement the paintEvnet() handler and do the painting there. Or you simply use a QLabel and set a pixmap on it.



  • Thanks for your help, using your advice i used QLabel and converted the first screen to pixmap and now i can get the image on my second screen.


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