Question

我遇到了必须擦除Qt小部件上先前绘制的区域的问题。

基本思想是，用户通过单击并拖动鼠标来选择屏幕区域，并在所选区域上绘制一个矩形。

标题

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;
};

实施

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();
}

在具有纯色背景的小部件上，效果非常好，产生一个矩形。

enter image description here

但是，当背景设置为setAttribute(Qt::WA_TranslucentBackground);时，会发生以下情况。

enter image description here

之前绘制的矩形未被“删除”

有没有办法擦除半透明背景上先前绘制的矩形，若然，怎么办？

另外，对于“奖励积分”，为什么这种效果会发生在半透明的背景而不是固体背景上？

Answer 1

具有WA_TranslucentBackground属性的小部件不会自动清除其背景。你必须：

将构图模式从默认SourceOver更改为Source，
使用透明画笔明确清除旧矩形
绘制新矩形。

下面是一个工作示例，在Qt 5下测试。您必须按下鼠标才能绘制初始矩形并拖动它;释放鼠标时程序退出。

#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"

在Qt中擦除半透明小部件的绘制区域

1 个答案: