Paint to QWidget but not main window

[supersteve]

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

[raven-worx]

@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.

[supersteve]

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.
**
** $QT_BEGIN_LICENSE:BSD$
** 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>

#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]

[raven-worx]

@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.

[supersteve]

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.