Unable to paint on Qt Widget, shows error “paintEngine: Should no longer be called”

[beginMyCoding]

I have created a widget using Qt Creator such a way that it has two sub windows inside a main window and some push buttons to load, save images, set pen width and color to paint on the window. But when i start to paint it gives me error saying

 QWidget::paintEngine: Should no longer be called  
 QPainter::begin: Paint device returned engine == 0, type: 1  
 QPainter::setPen: Painter not active  
 QPainter::drawPoints: Painter not active

Does anyone know what mistake i am doing, i checked threads related to this topic but could not find suitable solution. I am also new to c++, so please help me to find a solution

This is my code below

widget.h

#define WIDGET_H

#include <QWidget>
#include <QtCore>
#include <QImage>
#include <QColor>
#include <QPoint>
#include <QtGui>
#include <QPainter>
#include <QMainWindow>
#include <QFileDialog>

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

    bool isModified() const { return modified; }
    QColor penColor() const { return newPenColor; }
    int penWidth() const { return newPenWidth; }


protected:
    void paintEvent(QPaintEvent *event);
    void mousePressEvent(QMouseEvent *event) Q_DECL_OVERRIDE;

private slots:
    void on_open_clicked();

    void on_save_clicked();

    void on_penWidth_clicked();

    void on_penColor_clicked();

private:
    Ui::Widget *ui;

    QImage image;
    QPixmap imageobject;
    int newPenWidth;
    QColor newPenColor;
    bool modified;
    bool scribbling;
    QPoint firstPoint, secondPoint;
    void drawFirstPoint(const QPoint);
    void drawSecondPoint(const QPoint);
};

#endif // WIDGET_H

main.cpp

#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    return a.exec();
}

widget.cpp

#include "widget.h"
#include "ui_widget.h"
#include <QtWidgets>
#ifndef QT_NO_PRINTER
#include <QtPrintSupport/QPrinter>
#include <QtPrintSupport/QPrintDialog>
#endif
#include <QLabel>
#include <QWidget>


Widget::Widget(QWidget *parent) : QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);

    setAttribute(Qt::WA_StaticContents);
    modified = false;
    scribbling = false;
    newPenWidth = 1;
    newPenColor = Qt::blue;
}

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

void Widget::on_open_clicked()
{
    QString filename = QFileDialog::getOpenFileName(this, tr("choose"), "", tr("Image(*.png *.jpg *.jpeg *.bmp *.gif)"));
       if (QString::compare(filename, QString())!=0)
       {
           QImage image;
           bool valid = image.load(filename);
           if (valid)
           {
               image = image.scaledToWidth(ui->inputWindow->width(), Qt::SmoothTransformation);
               ui->inputWindow->setPixmap(QPixmap::fromImage(image));

               image = image.scaledToWidth(ui->outputWindow->width(), Qt::SmoothTransformation);
               ui->outputWindow->setPixmap(QPixmap::fromImage(image));
           }
           else
           {
               //Error handling
           }
       }
}

void Widget::on_save_clicked()
{
    QString filename = QFileDialog::getSaveFileName(this,tr("choose"), "", tr("PNG (*.png);; JPEG (*.jpg *.jpeg);; BMP(*.bmp);; GIF(*.gif)"));

    QImage imageobject = image;
    imageobject.save(filename);

}

void Widget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    QRect dirtyRect = event->rect();
    painter.drawImage(dirtyRect, image, dirtyRect);
}

void Widget::mousePressEvent(QMouseEvent *event)
{
    scribbling = true;
    if (event->button() == Qt::LeftButton && scribbling)
    {
        firstPoint = event->pos();
        drawFirstPoint(firstPoint);
    }
    else if (event->button() == Qt::RightButton && scribbling) {
             secondPoint = event->pos();
             drawSecondPoint(secondPoint);

    }
}

void Widget::drawFirstPoint(const QPoint)
{
    QPainter painter(this);
    painter.setPen(QPen(newPenColor, newPenWidth, Qt::SolidLine, Qt::RoundCap,
                        Qt::RoundJoin));

    painter.drawPoint(firstPoint);
    modified = true;
    int rad = (newPenWidth / 2) + 2;
    update(QRect(firstPoint, firstPoint).normalized().adjusted(-rad, -rad, +rad, +rad));
}

void Widget::drawSecondPoint(const QPoint)
{
    QPainter painter(this);
    painter.setPen(QPen(newPenColor, newPenWidth, Qt::SolidLine, Qt::RoundCap,
                        Qt::RoundJoin));

    painter.drawPoint(secondPoint);
    modified = true;
    int rad = (newPenWidth / 2) + 2;
    update(QRect(secondPoint, secondPoint).normalized().adjusted(-rad, -rad, +rad, +rad));
}

void Widget::on_penWidth_clicked()
{
    bool ok;
    int newWidth = QInputDialog::getInt(this, tr("Scribble"),
                                        tr("Select pen width:"),
                                        this->penWidth(), 1, 50, 1, &ok);
    if (ok)
        newPenWidth = newWidth;
}

void Widget::on_penColor_clicked()
{
    QColor newColor = QColorDialog::getColor(this->penColor());
    if (newColor.isValid())
        newPenColor = newColor;
}

widget.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Widget</class>
 <widget class="QWidget" name="Widget">
  <property name="enabled">
   <bool>true</bool>
  </property>
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>1300</width>
    <height>700</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Test window</string>
  </property>
  <property name="layoutDirection">
   <enum>Qt::LeftToRight</enum>
  </property>
  <widget class="QLabel" name="inputWindow">
   <property name="geometry">
    <rect>
     <x>40</x>
     <y>120</y>
     <width>600</width>
     <height>500</height>
    </rect>
   </property>
   <property name="maximumSize">
    <size>
     <width>600</width>
     <height>16777215</height>
    </size>
   </property>
   <property name="cursor">
    <cursorShape>CrossCursor</cursorShape>
   </property>
   <property name="text">
    <string>TextLabel</string>
   </property>
  </widget>
  <widget class="QLabel" name="outputWindow">
   <property name="geometry">
    <rect>
     <x>660</x>
     <y>120</y>
     <width>600</width>
     <height>500</height>
    </rect>
   </property>
   <property name="maximumSize">
    <size>
     <width>600</width>
     <height>16777215</height>
    </size>
   </property>
   <property name="text">
    <string>TextLabel</string>
   </property>
  </widget>
  <widget class="QWidget" name="layoutWidget">
   <property name="geometry">
    <rect>
     <x>170</x>
     <y>10</y>
     <width>651</width>
     <height>31</height>
    </rect>
   </property>
   <layout class="QHBoxLayout" name="horizontalLayout">
    <item>
     <widget class="QPushButton" name="open">
      <property name="cursor">
       <cursorShape>PointingHandCursor</cursorShape>
      </property>
      <property name="text">
       <string>OPEN</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QPushButton" name="save">
      <property name="cursor">
       <cursorShape>PointingHandCursor</cursorShape>
      </property>
      <property name="text">
       <string>SAVE</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QPushButton" name="penWidth">
      <property name="text">
       <string>Pen Width</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QPushButton" name="penColor">
      <property name="text">
       <string>Pen Color</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QPushButton" name="pushButton_5">
      <property name="text">
       <string>PushButton</string>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

[Chris Kawa]

You are calling drawFirstPoint and drawSecondPoint from within mousePressEvent. Don't do that. The only place you should create and use a painter is inside paintEvent. Instead of drawing from mouse event store the data to draw (the position and which button was pressed) and call update(). This will schedule a paint event, inside of which you can check the data and paint what's needed.

Btw:

void paintEvent(QPaintEvent *event);
void mousePressEvent(QMouseEvent *event) Q_DECL_OVERRIDE;

Be consistent. If you use Q_DECL_OVERRIDE on one override (good!), use it on all of them.

[beginMyCoding]

@Chris-Kawa Thanks for the suggestion, i am inspired from the qt example scribble in doing my program, in that example the painter is created and used in drawLineTo and updated. Do you know how that works?

I also tried to modify my program accordingly as you said but its getting complicated for me, maybe i am doing mistakes.

One more question, what is the difference in these two below errors?

QPainter::begin: Paint device returned engine == 0, type: 3

and

QPainter::begin: Paint device returned engine == 0, type: 1

[mrjj]

hi
The Scribble example draws to a image

void ScribbleArea::drawLineTo(const QPoint &endPoint)
{
QPainter painter(&image); << to image
...
}

and then it paints that image in paint
void ScribbleArea::paintEvent(QPaintEvent *event)
{
QPainter painter(this);
QRect dirtyRect = event->rect();
painter.drawImage(dirtyRect, image, dirtyRect); << draw image
}

To avoid drawing whole picture each time, it uses the event->rect();
that tells which area need repainting.
This are is set by drawLineTo.

[beginMyCoding]

@mrjj thanks for your kind reply, I understood what you explained and wanted to try the scribble example. I made a test program to check the scribble example by using Qt Widget creator. I made widget using ui class and the inside code is completely from scribble example but modified. This time i am getting the error as

QPainter::begin: Paint device returned engine == 0, type: 3

Can you please find the problem. This is my code

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "inputwindow.h"
#include "outputwindow.h"
#include <QFileDialog>
#include <QInputDialog>
#include <QColorDialog>
#include <QtGui>
#include <QtCore>
#include <QPoint>
#include <QImage>

namespace Ui {
class MainWindow;
class Inputwindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    QImage image;
    QColor penColor() const { return newPenColor; }
    int penWidth() const { return newPenWidth; }

public slots:

private slots:
    void on_open_clicked();

    void on_penwidth_clicked();

    void on_pencolor_clicked();

private:
    Ui::MainWindow *ui;
    Inputwindow *inputwindow;
    int newPenWidth;
    QColor newPenColor;
};

#endif // MAINWINDOW_H

inputwindow.h

#ifndef INPUTWINDOW_H
#define INPUTWINDOW_H

#include <QWidget>
#include <QHBoxLayout>

namespace Ui {
class Inputwindow;
}

class Inputwindow : public QWidget
{
    Q_OBJECT

public:
    explicit Inputwindow(QWidget *parent = 0);
    ~Inputwindow();
    QImage image;
    bool open(const QString &fileName);
    bool isModified() const { return modified; }
    QColor penColor() const { return newPenColor; }
    int penWidth() const { return newPenWidth; }

protected:
    void mousePressEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseMoveEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseReleaseEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void paintEvent(QPaintEvent *event) Q_DECL_OVERRIDE;

private slots:

private:
    Ui::Inputwindow *ui;

    bool modified;
    bool scribbling;
    QPoint lastPoint;
    int newPenWidth;
    QColor newPenColor;
    void drawLineTo(const QPoint &endPoint);
};

#endif // INPUTWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "inputwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    //setCentralWidget(inputwindow);
}

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



void MainWindow::on_open_clicked()
{
    QString fileName = QFileDialog::getOpenFileName(this, tr("choose"), "", tr("Image(*.png *.jpg *.jpeg *.bmp *.gif)"));
       if (QString::compare(fileName, QString())!=0)
       {
               inputwindow = new Inputwindow(this);
               inputwindow->setGeometry(QRect(QPoint(10,50),QSize(690,400)));
               inputwindow->open(fileName);
               inputwindow->show();

       }
}

void MainWindow::on_penwidth_clicked()
{
    bool ok;
    int newWidth = QInputDialog::getInt(this, tr("Scribble"),
                                        tr("Select pen width:"),
                                        this->penWidth(), 1, 50, 1, &ok);
    if (ok)
        newPenWidth = newWidth;
}

void MainWindow::on_pencolor_clicked()
{
    QColor newColor = QColorDialog::getColor(this->penColor());
    if (newColor.isValid())
        newPenColor = newColor;
}

inputwindow.cpp

#include "inputwindow.h"
#include "ui_inputwindow.h"
#include <QtWidgets>

Inputwindow::Inputwindow(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Inputwindow)
{
    ui->setupUi(this);
    setAttribute(Qt::WA_StaticContents);
    modified = false;
    scribbling = false;
    newPenWidth = 1;
    newPenColor = Qt::blue;
}

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

bool Inputwindow::open(const QString &fileName)
{
    QImage image;
    bool valid = image.load(fileName);
    if (valid)
    {
    image = image.scaledToWidth(ui->label->width(), Qt::SmoothTransformation);
    ui->label->setPixmap(QPixmap::fromImage(image));
    update();
    return true;
    }
    else
    {
        //Error handling
    }
}



void Inputwindow::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    QRect dirtyRect = event->rect();
    painter.drawImage(dirtyRect, image, dirtyRect);
}

void Inputwindow::mousePressEvent(QMouseEvent *event)
{
    scribbling = true;
    if (event->button() == Qt::LeftButton) {
        lastPoint = event->pos();
        scribbling = true;
    }
}

void Inputwindow::mouseMoveEvent(QMouseEvent *event)
{
    if ((event->buttons() & Qt::LeftButton) && scribbling)
        drawLineTo(event->pos());
}

void Inputwindow::mouseReleaseEvent(QMouseEvent *event)
{
    if (event->button() == Qt::LeftButton && scribbling) {
        drawLineTo(event->pos());
        scribbling = false;
    }
}

void Inputwindow::drawLineTo(const QPoint &endPoint)
{
    QPainter painter(&image);
    painter.setPen(QPen(newPenColor, newPenWidth, Qt::SolidLine, Qt::RoundCap,
                        Qt::RoundJoin));
    painter.drawLine(lastPoint, endPoint);
    modified = true;

    int rad = (newPenWidth / 2) + 2;
    update(QRect(lastPoint, endPoint).normalized()
                                     .adjusted(-rad, -rad, +rad, +rad));
    lastPoint = endPoint;
}

[mrjj]

@beginMyCoding said:

drawLineTo

There you use image variable.
Where is image created/set a size for the image.
This error could be due to image having no size.
Im not sure its the case, but easy to test
could you try qDebug() <<"img:" << image.isNull() ;

[beginMyCoding]

@mrjj said:

qDebug() <<"img:" << image.isNull() ;

where should i test that function? In inputWindow?

[beginMyCoding]

@mrjj I created a widget window (inputwindow) to load image and use painter to draw on that image

[mrjj]

@beginMyCoding
Im still not 100% sure u set the image

you have defined in
class Inputwindow : public QWidget
{
public:
QImage image; <<<< -- this one is used in Inputwindow::drawLineTo
xxx

and you do in Inputwindow::open
ui->label->setPixmap(QPixmap::fromImage(image));
but this "image" is local variable.
so that is a Label having the image and the "other image " frm .h is not used. ?

or does the massive scrolling confuse my brain ?

--
qDebug() <<"img:" << image.isNull() ;
in paintevent

[beginMyCoding]

@mrjj said:

qDebug() <<"img:" << image.isNull() ;

This is what i am getting when i use

qDebug() <<"img:" << image.isNull() ;

in paintEvent

img: true
img: true
img: true
img: true
img: true
img: true
img: true
img: true
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setPen: Painter not active
img: true
img: true
img: true

[jsulm]

As @mrjj already said you load your image into a local variable in open():

bool Inputwindow::open(const QString &fileName)
{
    QImage image; <-- this one is not the one you use later!
    bool valid = image.load(fileName);
    if (valid)
    {
    image = image.scaledToWidth(ui->label->width(), Qt::SmoothTransformation);
    ui->label->setPixmap(QPixmap::fromImage(image));
    update();
    return true;
    }
    else
    {
        //Error handling
    }
}

That means your QImage image in InputWindow is not set! Remove this line in open():

QImage image;

[beginMyCoding]

@jsulm Hi
Thanks for you reply..
I removed the line QImage image as you mentioned. I got rid of the errors but its not painting if i try to paint on the Image. I wanted to paint on the image loaded in inputwindow.
Do you know what mistake i am making?

[mrjj]

Hi
Maybe the label is covering up the image.

ui->label->setPixmap(QPixmap::fromImage(image));

This shows a copy of the image on screen. (not the one u draw on)
Try to remove it and see if that makes difference.

[beginMyCoding]

@mrjj yeah!! its working bro if i remove label.. Thank you all for your suggestions