qt 实现窗口局部镂空，并截图显示。

主要实现2 个功能点：
1 截图
2 窗口镂空
实现截图主要使用QGuiApplication::primaryScreen() 获取到QScreen ，然后调用QGuiApplication::grabWindow函数进行截图，代码示例：

QScreen* screen = QGuiApplication::primaryScreen();if(const QWindow* window = windowHandle()){screen = window->screen();}if(!screen)return;originalPixmap = screen->grabWindow(0,rc.left(),rc.top(),rc.right() - rc.left(),rc.bottom() - rc.top());

此时originalPixmap 就是 截取的图片，类型为QPixmap，可直接展示。
实现窗口局部透明，即窗口局部镂空。首先将窗口 搞成全透明，然后在未截图情况下，
绘制窗口背景为半透明；（为什么要先 将窗口弄成全透明那？
因为 不将窗口搞成全透明的话，后面绘制的半透明，
其实 还存在一个窗口背景。做不到真正意义上的全透明）
在截图的情况下将 截图区域不绘制，即全透明，然后绘制非截图区域为半透明。代码如下：

  if(type == QEvent::Paint){qDebug() << "start paint";QRect rc = ComputeRect(ptStart,ptEnd);QPainter painter(screenshot_dlg_);if(bCapStart){//设置mask 使当前区域全透明QPolygon polygon1;QRect screenRc = ScreenRect();painter.setBrush(QBrush(QColor(0,0,0,100)));painter.setPen(QPen(Qt::NoPen));polygon1 << QPoint(screenRc.left(),screenRc.top())<< QPoint(screenRc.right(),screenRc.top())<< QPoint(rc.right(),rc.top())<< QPoint(rc.left(),rc.top());painter.drawPolygon(polygon1);QPolygon polygon2;polygon2 << QPoint(screenRc.left(),screenRc.top())<< QPoint(screenRc.left(),screenRc.bottom())<< QPoint(rc.left(),rc.bottom())<< QPoint(rc.left(),rc.top());painter.drawPolygon(polygon2);QPolygon polygon3;polygon3 << QPoint(screenRc.left(),screenRc.bottom())<< QPoint(screenRc.right(),screenRc.bottom())<< QPoint(rc.right(),rc.bottom())<< QPoint(rc.left(),rc.bottom());painter.drawPolygon(polygon3);QPolygon polygon4;polygon4 << QPoint(screenRc.right(),screenRc.top())<< QPoint(screenRc.right(),screenRc.bottom())<< QPoint(rc.right(),rc.bottom())<< QPoint(rc.right(),rc.top());painter.drawPolygon(polygon4);}else{painter.setBrush(QBrush(QColor(0,0,0,100)));painter.drawRect(ScreenRect());}}

以上4个区域polygon1、polygon2、polygon3、polygon4 即为非截图区域，
我们把它搞成半透明，未绘制的就是全透明了（因为窗口就是全透的）。
完整代码如下：
window.h

    #ifndef WINDOW_H
#define WINDOW_H#include <QWidget>
#include <QPushButton>
#include <QVBoxLayout>
#include <QLabel>
#include <QPixmap>
#include <QDialog>
#include <QRect>
#include <QSizeGrip>class SceenShot:public QWidget
{public:SceenShot(QWidget* parent = Q_NULLPTR);void updateShowScreenshot();bool eventFilter(QObject *watched, QEvent *event) override;
protected:void resizeEvent(QResizeEvent *event) override;QRect ScreenRect();
public slots:void StartShotScreen();void ShootScreen(QRect& rc);
private:QRect ComputeRect(QPoint& pt1,QPoint& pt2);QDialog*    screenshot_dlg_;QPushButton* shot_screen_;QVBoxLayout* vbox_;QLabel* show_label_;QPixmap originalPixmap;//绘制截图窗口用的QPixmap*    m_Cache;QSizeGrip   m_SizeGrip;
};#endif // WINDOW_H

window.cpp

#include "window.h"
#include <QGuiApplication>
#include <QWindow>
#include <QScreen>
#include <QMouseEvent>
#include <QPainter>
#include <QPen>
#include <QDebug>
#include <QImage>
#include <QBitmap>SceenShot::SceenShot(QWidget* parent):QWidget(parent),m_SizeGrip(nullptr)
{m_Cache = nullptr;vbox_ = new QVBoxLayout(this);shot_screen_ = new QPushButton(tr("shootScreen"),this);shot_screen_->setShortcut(tr("ctrl+a"));show_label_ = new QLabel(this);show_label_->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);show_label_->setAlignment(Qt::AlignCenter);show_label_->setMinimumSize(300,300);show_label_->setPixmap(originalPixmap);connect(shot_screen_,&QPushButton::clicked,this,&SceenShot::StartShotScreen);vbox_->addWidget(shot_screen_);vbox_->addWidget(show_label_);screenshot_dlg_ = new QDialog(this,Qt::FramelessWindowHint);screenshot_dlg_->setAttribute(Qt::WA_TranslucentBackground);screenshot_dlg_->hide();//给Dialog 安装事件过滤器screenshot_dlg_->installEventFilter(this);//this->setLayout(vbox_);
}void SceenShot::ShootScreen(QRect& rc)
{shot_screen_->setEnabled(false);QScreen* screen = QGuiApplication::primaryScreen();if(const QWindow* window = windowHandle()){screen = window->screen();}if(!screen)return;originalPixmap = screen->grabWindow(0,rc.left(),rc.top(),rc.right() - rc.left(),rc.bottom() - rc.top());updateShowScreenshot();shot_screen_->setEnabled(true);
}void SceenShot::updateShowScreenshot()
{show_label_->setPixmap(originalPixmap.scaled(show_label_->size(),Qt::KeepAspectRatio,Qt::SmoothTransformation));
}void SceenShot::resizeEvent(QResizeEvent *event)
{QSize scaledSize = originalPixmap.size();scaledSize.scale(show_label_->size(), Qt::KeepAspectRatio);if (!show_label_->pixmap() || scaledSize != show_label_->pixmap()->size())updateShowScreenshot();
}void SceenShot::StartShotScreen()
{//获取屏幕大小信息QScreen* primaryScreen = QGuiApplication::primaryScreen();QRect rect = primaryScreen->geometry();screenshot_dlg_->setGeometry(rect);screenshot_dlg_->show();
}bool SceenShot::eventFilter(QObject *watched, QEvent *event)
{static QPoint ptStart ={0,0};static QPoint ptEnd ={0,0};static bool bCapStart = false;QEvent::Type type =  event->type();QMouseEvent* mouse_event = static_cast<QMouseEvent*>(event);if(mouse_event && mouse_event->button() == Qt::LeftButton && type == QEvent::MouseButtonPress){//鼠标左键按下，记录按下位置bCapStart = true;ptStart = mouse_event->pos();}else if(mouse_event && mouse_event->button() == Qt::LeftButton && type == QEvent::MouseButtonRelease){//qDebug() << "MouseButtonRelease";bCapStart = false;QRect rc = ComputeRect(ptStart,ptEnd);qDebug() << "x:" << rc.left() << " y:" << rc.top() << " right:" << rc.right() << " bottom:" << rc.bottom();ShootScreen(rc);ptStart.setX(0);ptStart.setY(0);ptEnd.setX(0);ptEnd.setY(0);}else if(bCapStart && type == QEvent::MouseMove){ptEnd = mouse_event->pos();screenshot_dlg_->update();}//    //重写Dialog得绘制事件和改变大小事件
//    if(type == QEvent::Paint)
//    {//        if(m_Cache != NULL)
//          {//            QPainter painter(screenshot_dlg_);//            painter.drawPixmap(0, 0, *m_Cache);//        #if QT_VERSION >= 0x040500
//            if(!testAttribute(Qt::WA_TranslucentBackground)) screenshot_dlg_->setMask(m_Cache->mask());
//        #else
//            screenshot_dlg_->setMask(m_Cache->mask());
//        #endif
//          }
//    }
//    else if(type == QEvent::Resize)
//    {//        delete m_Cache;
//        m_Cache = new QPixmap(screenshot_dlg_->size());
//        m_Cache->fill(Qt::transparent);
//        QPainter painter(m_Cache);
//        QColor darkBlue ( 23,  23,  34);
//          QColor lightBlue(177, 177, 203);//          /********** Window's background **********/
//          QPolygon background;//          background << QPoint(           0,            16)
//                     << QPoint(          16,             0)
//                     << QPoint(width() -  1,             0)
//                     << QPoint(width() -  1, height() - 33)
//                     << QPoint(width() - 17, height() - 17)
//                     << QPoint(         272, height() - 17)
//                     << QPoint(         256, height() -  1)
//                     << QPoint(          16, height() -  1)
//                     << QPoint(          16,           272)
//                     << QPoint(           0,           256);//          painter.setPen  (QPen  (darkBlue));
//          painter.setBrush(QBrush(darkBlue));//          painter.drawPolygon(background);
//          /*****************************************///          /********** Window's frame **********/
//          QPolygon frame;//          frame << QPoint(           4,            20)
//                << QPoint(          20,             4)
//                << QPoint(width() -  4,             4)
//                << QPoint(width() -  4, height() - 37)
//                << QPoint(width() - 20, height() - 21)
//                << QPoint(         268, height() - 21)
//                << QPoint(         252, height() -  5)
//                << QPoint(          20, height() -  5)
//                << QPoint(          20,           268)
//                << QPoint(           4,           252);//          painter.setPen  (QPen(lightBlue));
//          painter.setBrush(Qt::NoBrush    );//          painter.drawPolygon(frame);
//          /*****************************************///          //m_SizeGrip.move  (width() - 32, height() - 49);
//          //m_SizeGrip.resize(          32,            32);
//    }if(type == QEvent::Paint){qDebug() << "start paint";QRect rc = ComputeRect(ptStart,ptEnd);QPainter painter(screenshot_dlg_);if(bCapStart){//设置mask 使当前区域全透明QPolygon polygon1;QRect screenRc = ScreenRect();painter.setBrush(QBrush(QColor(0,0,0,100)));painter.setPen(QPen(Qt::NoPen));polygon1 << QPoint(screenRc.left(),screenRc.top())<< QPoint(screenRc.right(),screenRc.top())<< QPoint(rc.right(),rc.top())<< QPoint(rc.left(),rc.top());painter.drawPolygon(polygon1);QPolygon polygon2;polygon2 << QPoint(screenRc.left(),screenRc.top())<< QPoint(screenRc.left(),screenRc.bottom())<< QPoint(rc.left(),rc.bottom())<< QPoint(rc.left(),rc.top());painter.drawPolygon(polygon2);QPolygon polygon3;polygon3 << QPoint(screenRc.left(),screenRc.bottom())<< QPoint(screenRc.right(),screenRc.bottom())<< QPoint(rc.right(),rc.bottom())<< QPoint(rc.left(),rc.bottom());painter.drawPolygon(polygon3);QPolygon polygon4;polygon4 << QPoint(screenRc.right(),screenRc.top())<< QPoint(screenRc.right(),screenRc.bottom())<< QPoint(rc.right(),rc.bottom())<< QPoint(rc.right(),rc.top());painter.drawPolygon(polygon4);}else{painter.setBrush(QBrush(QColor(0,0,0,100)));painter.drawRect(ScreenRect());}}return false;
}QRect SceenShot::ComputeRect(QPoint& pt1,QPoint& pt2)
{QRect rc;if(pt1.x() > pt2.x()){if(pt1.y() > pt2.y()){//pt1 为右下点rc.setTopLeft(pt2);rc.setBottomRight(pt1);}else{rc.setBottomLeft(pt2);rc.setTopRight(pt1);}}else{if(pt1.y() > pt2.y()){//pt1 为左下点rc.setBottomLeft(pt1);rc.setTopRight(pt2);}else{rc.setTopLeft(pt1);rc.setBottomRight(pt2);}}return rc;
}QRect SceenShot::ScreenRect()
{QScreen* screen = QGuiApplication::primaryScreen();return screen->geometry();
}

main.cpp

#include <QApplication>
#include "window.h"int main(int argc,char* argv[])
{QApplication app(argc,argv);SceenShot sc;sc.show();sc.resize(300,300);return app.exec();
}

以上就是简单的截图功能。使用鼠标拖拽截图。

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