从 Qt 中的半透明小部件擦除绘制区域 [英] Erasing painted areas from translucent widgets in Qt

问题描述

我面临着必须擦除 Qt 小部件上先前绘制的区域的问题.

I am faced with the problem of having to erase previously painted areas on a Qt widget.

基本思想是，用户通过单击并拖动鼠标来选择屏幕的一个区域，并在所选区域上绘制一个矩形.

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.

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，生成单个矩形.

On a widget that has a solid background the effect works quite nicely, producing a single rectangle.

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

However, when the background is set to setAttribute(Qt::WA_TranslucentBackground); the following occurs.

之前绘制的矩形没有被擦除"

The rectangles that were drawn previously are not "erased"

有没有办法在半透明背景上擦除之前绘制的矩形，如果有，如何擦除?

Is there a way to erase the previously painted rectangles on a translucent background, and if so, how?

同样是加分项"，为什么这种效果发生在半透明背景而不是纯色背景上?

Also for "bonus points" why does this effect occur on a translucent background and not on a solid one?

推荐答案

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

Widgets with WA_TranslucentBackground attribute do not clear their backgrounds automatically. You have to:

1. 将合成模式从默认的SourceOver改为Source,

用透明画笔明确清除旧矩形，

Explicitly clear the old rectangle with a transparent brush,

绘制新矩形.

下面是一个在 Qt 5 下测试的工作示例.您必须按下鼠标绘制初始矩形并拖动它；松开鼠标时程序退出.

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"

这篇关于从 Qt 中的半透明小部件擦除绘制区域的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！