Erasing painted areas from translucent widgets in Qt - c++

Question

I am faced with the problem of having to erase previously painted areas on a Qt widget.
The basic idea is, the user selects an area of the screen by clicking and dragging the mouse and a rectangle is drawn over the selected area.
The header
class ClearBack : public QWidget
{
Q_OBJECT
public:
explicit ClearBack(const QPoint &startingPos);
bool eventFilter(QObject *obj, QEvent *event);
void paintEvent(QPaintEvent *);
void mouseMoveEvent(QMouseEvent *event);
signals:
void regionSelected(const QRect &);
private:
QRect currentRegion;
};
The Implementation
ClearBack::ClearBack(const QPoint &startingPos)
{
setBackgroundRole(QPalette::Base);
installEventFilter(this);
currentRegion.setTopLeft(startingPos);
currentRegion.setBottomRight(startingPos);
this->setWindowFlags(Qt::Dialog | Qt::FramelessWindowHint);
this->showMaximized();
}
void ClearBack::paintEvent(QPaintEvent * event)
{
Q_UNUSED(event);
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
painter.setPen(Qt::black);
painter.drawRect(currentRegion);
}
void ClearBack::mouseMoveEvent(QMouseEvent *event)
{
QPoint currentPos(event->globalX(), event->globalY());
currentRegion.setBottomRight(currentPos);
this->repaint();
}
On a widget that has a solid background the effect works quite nicely, producing a single rectangle.
However, when the background is set to setAttribute(Qt::WA_TranslucentBackground); the following occurs.
The rectangles that were drawn previously are not "erased"
Is there a way to erase the previously painted rectangles on a translucent background, and if so, how?
Also for "bonus points" why does this effect occur on a translucent background and not on a solid one?

Answer 1

Widgets with WA_TranslucentBackground attribute do not clear their backgrounds automatically. You have to:
Change the composition mode from the default SourceOver to Source,
Explicitly clear the old rectangle with a transparent brush,
Paint the new rectangle.
Below is a working example, tested under Qt 5. You have to press the mouse to draw the initial rectangle and drag it around; the program exits when you release the mouse.
#include <QApplication>
#include <QWidget>
#include <QPainter>
#include <QMouseEvent>
class ClearBack : public QWidget
{
Q_OBJECT
QRect m_currentRegion, m_lastRegion;
public:
explicit ClearBack(const QPoint &startingPos) :
m_currentRegion(startingPos, startingPos)
{
setWindowFlags(Qt::Dialog | Qt::FramelessWindowHint);
setAttribute(Qt::WA_TranslucentBackground);
showMaximized();
}
Q_SIGNAL void regionSelected(const QRect &);
protected:
void paintEvent(QPaintEvent *) {
QPainter painter(this);
painter.setCompositionMode(QPainter::CompositionMode_Source);
painter.setRenderHint(QPainter::Antialiasing);
painter.setPen(QPen(Qt::transparent, 3));
painter.drawRect(m_lastRegion);
m_lastRegion = m_currentRegion;
painter.setPen(Qt::black);
painter.drawRect(m_currentRegion);
}
void mouseMoveEvent(QMouseEvent *event) {
m_currentRegion.setBottomRight(event->globalPos());
update();
}
void mouseReleaseEvent(QMouseEvent *) {
emit regionSelected(m_currentRegion);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
ClearBack back(QPoint(200,200));
a.connect(&back, SIGNAL(regionSelected(QRect)), SLOT(quit()));
return a.exec();
}
#include "main.moc"