Using QPainter to 'erase' an area of widget?

[altec42lansing]

I have a qpainter painting a certain image over an entire area. Now, i want to erase part of it completely so i can basically see the desktop behind it kind of thing. i am currently using:

painter.eraseRect(0, 0, width()/2, height()/2)

and it just makes a white background in that part of the ui. I have tried looking around to forums on setbackground or setattribute:

setBackgroundRole(QPalette::Base);
setAttribute(Qt::WA_NoSystemBackground);

but these seem to have no effect, and I can't seem to get that white 'erased' area to be completely gone or transparent. Might be a simple fix, any ideas?

[Santosh Reddy]

I would suggest to not paint at all, then no need to erase it. I mean don't paint the area which you want to erase. I hope you got it

[wysota]

Qt::WA_TranslucentBackground

[altec42lansing]

hmm, still cant seem to get this working, basically i have a screen that i draw, and i need to get ride of a portion of it completely, including that 'white box' that is left behind when i erase rect. I basically want to get ride of that white area and any remnants left over after erasing, so i can see whats behind the application, since my widgets are painted over top of everything else.
(here is my current code for erasing it

painter.eraseRect(0,0,width()/2,height()/2);
this->setBackgroundRole(QPalette::Base);
this->setAttribute(Qt::WA_TranslucentBackground);

[Rachol]

Try to test this:

#include <QtGui>
#include <QLabel>
 
//! [main function]
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    QLabel widget("TEST");
    QFont f = widget.font();
    f.setPointSize(100);
    widget.setFont(f);
    widget.setWindowFlags(Qt::WindowFlags(Qt::FramelessWindowHint));
    widget.setAttribute(Qt::WA_TranslucentBackground);
    widget.show();
 
    return app.exec();
}
//! [main function]

#include <QtGui>
#include <QLabel>

//! [main function]
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QLabel widget("TEST");
    QFont f = widget.font();
    f.setPointSize(100);
    widget.setFont(f);
    widget.setWindowFlags(Qt::WindowFlags(Qt::FramelessWindowHint));
    widget.setAttribute(Qt::WA_TranslucentBackground);
    widget.show();

    return app.exec();
}
//! [main function]

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[altec42lansing]

this worked and painted text with a transparent background over my windows, allowing me to see behind it where the text was not, but im not sure how to apply this to my problem

[Rachol]

Does this help? Use your widget as I have used MyWidget in code below. You don't have to erease anything in your paintEvent

#include <QtGui>
#include <QWidget>
 
class MyWidget : public QWidget
{
public:
    explicit MyWidget(QWidget *parent = 0):QWidget(parent){};
 
protected:
    void paintEvent ( QPaintEvent * );
 
};
 
void MyWidget::paintEvent ( QPaintEvent * )
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,5));
 
    p.drawRect(rect());
}
 
//! [main function]
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    MyWidget widget;
    widget.setFixedSize(300,200);
    widget.setWindowFlags(Qt::WindowFlags(Qt::FramelessWindowHint));
    widget.setAttribute(Qt::WA_TranslucentBackground);
    widget.show();
 
    return app.exec();
}
//! [main function]

#include <QtGui>
#include <QWidget>

class MyWidget : public QWidget
{
public:
    explicit MyWidget(QWidget *parent = 0):QWidget(parent){};

protected:
    void paintEvent ( QPaintEvent * );

};

void MyWidget::paintEvent ( QPaintEvent * )
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,5));

    p.drawRect(rect());
}

//! [main function]
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    MyWidget widget;
    widget.setFixedSize(300,200);
    widget.setWindowFlags(Qt::WindowFlags(Qt::FramelessWindowHint));
    widget.setAttribute(Qt::WA_TranslucentBackground);
    widget.show();

    return app.exec();
}
//! [main function]

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[altec42lansing]

I hate attached a small file which hopefully demonstrates what i am trying to do

[Rachol]

void MyWidget::paintEvent ( QPaintEvent * )
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,5));
 
    p.fillRect(rect(), Qt::white);
    p.drawText(0,0,width(), height(),Qt::AlignCenter || Qt::TextWordWrap, "THIS IS THE AREA WHERE I DRAW IMG");
 
    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.fillRect(rect().adjusted(0,0,-width()/2,-height()/2), Qt::transparent);
}

void MyWidget::paintEvent ( QPaintEvent * )
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,5));

    p.fillRect(rect(), Qt::white);
    p.drawText(0,0,width(), height(),Qt::AlignCenter || Qt::TextWordWrap, "THIS IS THE AREA WHERE I DRAW IMG");

    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.fillRect(rect().adjusted(0,0,-width()/2,-height()/2), Qt::transparent);
}

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[altec42lansing]

awesome, thank you!

---

Added after 56 minutes:

One more problem i am having though, now that i can see whatever is behind it on the desktop, i do not want to be able to click on that area, otherwise i'd lose focus of my current application, i just want to be able to see it, like a 'window' in the middle of my app. i tried changing the mode and redrawing a transparent rect on top but that didnt work:

painter.setCompositionMode(QPainter::CompositionMo de_SourceIn);
painter.fillRect(pipRect, Qt::transparent);
painter.setCompositionMode(QPainter::CompositionMo de_DestinationAtop);
painter.fillRect(pipRect, Qt::transparent);

[Rachol]

I don't know anything else but something like this:

void MyWidget::paintEvent ( QPaintEvent * )
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,5));
 
    p.fillRect(rect(), Qt::white);
    p.drawText(0,0,width(), height(),Qt::AlignCenter || Qt::TextWordWrap, "THIS IS THE AREA WHERE I DRAW IMG");
 
    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.fillRect(rect().adjusted(0,0,-width()/2,-height()/2), QColor(255,255,255,1));
}

void MyWidget::paintEvent ( QPaintEvent * )
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,5));

    p.fillRect(rect(), Qt::white);
    p.drawText(0,0,width(), height(),Qt::AlignCenter || Qt::TextWordWrap, "THIS IS THE AREA WHERE I DRAW IMG");

    p.setCompositionMode(QPainter::CompositionMode_Source);
    p.fillRect(rect().adjusted(0,0,-width()/2,-height()/2), QColor(255,255,255,1));
}

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instead of Qt:transparent use something with low alfa value, so the user can't even notice that there is something. QColor(255,255,255,1) works well for such purpose.

[wysota]

Maybe it will easier for you if you use QPixmap::grabWindow() or QPixmap::grabWidget() on your desktop and simply set that as the background of your widget.

[Rachol]

Well, as long as nothing in the background changes. Also positioning of such pixmap gets complicated.

[wysota]

You can't eat the cake and still have the cake.