QPaintEvent in MainWindow

[kshegunov]

@Donn
Ah, I see. Well since this label of sorts is providing its own implementation of the painting, the parent class' painting method will not be invoked automatically. This means that if you want to see the text you set with label->setText("lalal") you'll have to paint it yourself in the void MyLabelWithOverlay::paintEvent(QPaintEvent * event) method. Does this clear it a bit? You could try setting a pixmap for the label (this should be displayed, although how well it's painted I can't vouch as I've written the example directly in the forum).

Kind regards.

[Donn]

I'm still confused... Sorry for my stupid questions but I want to make it clear. I assume MyLabelWithOverlay inherits most of the methods provided by QLabel and I don't get why it cannot directly display text with label->setText() ?

[kshegunov]

@Donn
It inherits all of the methods provided by QLabel, but it overrides the QLabel::paintEvent method, so no painting is done from QLabel::paintEvent and consequently you don't get the text painted (which is what QLabel::paintEvent actually does).

[Donn]

So QLabel::paintEvent is actually responsible for painting a simple text and if I override it doesn't provide the functionality for displaying text anymore and I need to do it myself in the body of overridden function, right? Can you provide some code how it could be in the body because I don't have any idea.

[kshegunov]

@Donn said:

So QLabel::paintEvent is actually responsible for painting a simple text and if I override it doesn't provide the functionality for displaying text anymore and I need to do it myself in the body of overridden function, right?

Correct.

Can you provide some code how it could be in the body because I don't have any idea.

void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    // This code draws the pixmap that was given with setPixmap(), and hopefully it does that correctly 
    QPixmap pix = pixmap();
    qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

    QTransform transform;
    transform.scale(ratio, ratio);

    QRect sourceRect = transform.mapRect(event->rect());

    QPainter painter(this);
    painter.drawPixmap(event->rect(), pix, sourceRect); //< Pixmap was drawn

    if (overlayEnabled)  {
        //< Here you can draw on top of that pixmap the lines (that's why it was left unfilled)
    }

    // Here you can draw the text (on top of the pixmap and the lines
    QRect widgetRect = rect(); //< This is the widget's rectangle
    QString labelText = text(); //< This is the text that was set
    // This following line draws the text centered horizontally and bound by the provided rectangle
    painter.drawText(widgetRect, labelText, Qt::AlignHCenter); 
}

[Donn]

void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    const QPixmap pix = *pixmap();
    qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

    QTransform transform;
      transform.scale(ratio, ratio);

    QRect sourceRect = transform.mapRect(event->rect());

    QPainter painter(this);
    painter.drawPixmap(event->rect(), pix, sourceRect);

    if (overlayEnabled)  {
        // Draw your lines here
    }
    // Here you can draw the text (on top of the pixmap and the lines
    QRectF widgetRect = rect(); //< This is the widget's rectangle
    QString labelText = text(); //< This is the text that was set
    // This following line draws the text centered horizontally and bound by the provided rectangle
    //painter.drawText(widgetRect, labelText, Qt::AlignHCenter);
}

My app crashes after this saying that

QWidget::repaint: Recursive repaint detected

If I uncomment the last line I get the following:

C2664: 'void QPainter::drawText(const QRectF &,const QString &,const QTextOption &)': cannot convert argument 1 from 'QRectF' to 'int'

[kshegunov]

const QPixmap pix = *pixmap();

Since you're not checking if the pointer returned is NULL, have you set a pixmap for the label with QLabel::setPixmap?
If you haven't this would cause a segfault, because dereferencing a NULL pointer isn't allowed.

Recursive repaint detected

This shouldn't be normally happening. Are you calling repaint() manually, or is there an explicit call to paintEvent() somewhere?

If I uncomment the last line I get the following: ... cannot convert argument 1 from 'QRectF' to 'int'

Have you included the header for QRectF?

[jsulm]

@kshegunov Wouldn't it be easier to first call QLabel::paintEvent(...) to get back the inherited functionality and then draw custom stuff?

[Donn]

@kshegunov said:

Since you're not checking if the pointer returned is NULL, have you set a pixmap for the label with QLabel::setPixmap?
If you haven't this would cause a segfault, because dereferencing a NULL pointer isn't allowed.

Could you please provide a solution for this because I really don't get it how it is done since it is a big mess for me and trying to get it step by step. I really appreatiate your help!

This shouldn't be normally happening. Are you calling repaint() manually, or is there an explicit call to paintEvent() somewhere?

I do not call any repaint() or paintEvent() explicitly I think. I just simply created a header file and a source file as mentioned above and have included header file to other header file and have promoted one QLabel widget to MyLabelWithOverlay.

Have you included the header for QRectF?

I have included the header but it is still the same problem.

[kshegunov]

@jsulm
Absolutely! Although, I was half way down writing the code when I realized I had a QLabel instead of QWidget (which would be usually the case) and was reluctant to rewrite it. :)

@Donn

Could you please provide a solution for this because I really don't get it how it is done since it is a big mess for me and trying to get it step by step.

Solution for what you mean? How to check whether a pointer is null? Simply check with if:

QPixmap * pixmapPointer = pixmap();
if (!pixmapPointer)
    return;

QPixmap & pix = *pixmapPointer; //< You have to make sure pixmapPointer points to a valid object before using the deref operator (the *)

Also something I noticed in your code is that you're passing const QPixmap * to QPainter::drawPixmap, which accepts const QPixmap &, you should fix that (look the snippet how conversion between the two is done).

I have included the header but it is still the same problem.

Usually this happens when the header is not present. Try passing the object directly:

painter.drawText(QRectF(rect()), labelText, Qt::AlignHCenter);

[Donn]

@kshegunov

Everything mentioned above compiles except the last problem:

Usually this happens when the header is not present. Try passing the object directly:

painter.drawText(QRectF(rect()), labelText, Qt::AlignHCenter);

I still get the same error message. Instead I tried

painter.drawText(20, 20, labelText);

which compiles but I don't get any text displayed in the promoted MyLabelWithOverlay label.

EDIT:

Maybe I'm doing something wrong in other place. If you can, please check it because I have no idea what could be wrong. Perhaps somewhere I don't completely get how things work...

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

MainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "mylabelwithoverlay.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->label_5->setText("hello world"); // label_5 is promoted to MyLabelWithOverlay
}

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

mylabelwithoverlay.h:

#ifndef MYLABELWITHOVERLAY_H
#define MYLABELWITHOVERLAY_H

#include "QtGui"
#include "QtCore"
#include "QLabel"


class MyLabelWithOverlay : public QLabel
{
    Q_OBJECT

public:
     MyLabelWithOverlay (QWidget * parent = NULL);

public slots:
     void enableOverlay(bool enabled);

protected:
     virtual void paintEvent(QPaintEvent * event);

private:
     bool overlayEnabled;
};


#endif // MYLABELWITHOVERLAY_H

MyLabelWithOverlay.cpp:

#include "mylabelwithoverlay.h"
#include <QRect>
#include <QRectF>


MyLabelWithOverlay::MyLabelWithOverlay (QWidget * parent): QLabel(parent), overlayEnabled(false)
{
}

void MyLabelWithOverlay::enableOverlay(bool enabled)
{
    overlayEnabled = enabled;
}


void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    const QPixmap * pixmapPointer = pixmap();
    if (!pixmapPointer)
        return;

    const QPixmap & pix = *pixmapPointer;

    qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

    QTransform transform;
      transform.scale(ratio, ratio);

    QRect sourceRect = transform.mapRect(event->rect());

    QPainter painter(this);
    painter.drawPixmap(event->rect(), pix, sourceRect);

    if (overlayEnabled)  {
        // Draw your lines here
    }
    // Here you can draw the text (on top of the pixmap and the lines
    QRect widgetRect = rect(); //< This is the widget's rectangle
    QString labelText = text(); //< This is the text that was set
    // This following line draws the text centered horizontally and bound by the provided rectangle
    //painter.drawText(QRectF(rect()), labelText, Qt::AlignHCenter);
    painter.drawText(20, 20, labelText);

}

[kshegunov]

@Donn
You're not setting a pixmap for the label, so when the paint events gets executed pixmapPointer will be NULL and the function will return. If you wish to support both/either text and pixmap you have to enclose each in its own if block.

void MyLabelWithOverlay::paintEvent(QPaintEvent * event)
{
    QPainter painter(this);

    const QPixmap * pixmapPointer = pixmap();
    if (pixmapPointer)  {
        const QPixmap & pix = *pixmapPointer;
        qreal ratio = qMin(static_cast<qreal>(width()) / pix.width(), static_cast<qreal>(height()) / pix.height());

        QTransform transform;
        transform.scale(ratio, ratio);

        QRect sourceRect = transform.mapRect(event->rect());
        painter.drawPixmap(event->rect(), pix, sourceRect);
    }

    if (overlayEnabled)  {
        // Draw your lines here
    }

    QString labelText = text();
    if (!labelText.empty())  {
        painter.drawText(0, 0, labelText);
    }
}

[Donn]

@kshegunov

I think I got it! I have included to the painterEvent the following:

QPixmap pm(200, 200);
pm.fill(Qt::white);
setPixmap(pm);

Now I can set the text from other class like:

ui->label->setText("lala");

However, I think I shouldn't set Pixmap inside paintEvent - better to do it in the other class so this type of label could be also used for other labels, right?

EDIT:

I liked the idea of @jsulm. Could you give me some hints how to achieve it? How to get the painting functionality from a standard QLabel and then add some additional my own painting?