c++ - 如何在 QT 中的自定义视频小部件 t 上绘制矩形？

Question

我想在自定义视频小部件上选择一个区域并在所选区域上绘制矩形。

到目前为止，我可以使用 QRubberband 选择一个区域，但我在释放左键单击后无法绘制矩形。

每当我单击拖动然后释放以绘制矩形时，它都会出现此错误:

QBackingStore::endPaint() called with active painter on backingstore paint device

The program has unexpectedly finished.

这是我的代码:

myvideoobject.h

#ifndef MYVIDEOOBJECT_H
#define MYVIDEOOBJECT_H

#include <QObject>
#include <QVideoWidget>
#include <QRubberBand>
#include <QPainter>
#include <QPen>
#include <QPaintEvent>
#include <QRect>
#include <QMouseEvent>
#include <QDebug>

class MyVideoObject : public QVideoWidget
{
    Q_OBJECT
public:
    explicit MyVideoObject(QWidget *parent = 0);

    void mouseMoveEvent(QMouseEvent *ev);
    void mousePressEvent(QMouseEvent *ev);
    void mouseReleaseEvent(QMouseEvent *ev);
    void paintEvent(QPaintEvent *ev);

private:
    QRubberBand* rubberBand;
    QPainter* painter;
    //QRect *rectangle;

    QPoint origin;
    QPoint endPoint;

};

#endif // MYVIDEOOBJECT_H

myvideoobject.cpp

#include "myvideoobject.h"

MyVideoObject::MyVideoObject(QWidget* parent) :
QVideoWidget(parent)
{
    rubberBand = new QRubberBand(QRubberBand::Rectangle, this);
    rubberBand->setGeometry(0,0,50,50);//ileride silebilrisin

}

void MyVideoObject::mouseMoveEvent(QMouseEvent *ev)
{
    rubberBand->setGeometry(QRect(origin,ev->pos()).normalized());
}

void MyVideoObject::mousePressEvent(QMouseEvent *ev)
{
    origin = ev->pos();
    if(!rubberBand)
         rubberBand = new QRubberBand(QRubberBand::Rectangle, this);
    rubberBand->setGeometry(QRect(origin,QSize()));
    rubberBand->show();
}

void MyVideoObject::mouseReleaseEvent(QMouseEvent *ev)
{
    rubberBand->hide();

    endPoint = ev->pos();

    painter->begin(this);
    painter->drawRect(QRect(origin,endPoint));

}

void MyVideoObject::paintEvent(QPaintEvent *ev)
{
    QRect rect = ev->rect();
    painter = new QPainter(this);
    painter->setRenderHint(QPainter::Antialiasing);
    painter->setPen(Qt::black);
    painter->drawText(rect,Qt::AlignCenter,"Data");
    painter->drawRect(rect);

//painter->setPen(Qt::red);
}

我没有添加 mainwindow.cpp 和 mainwindow.h，因为除了使用 openfiledialog 选择视频之外没有太多代码。

Answer 1

当您创建一个指针:QPainter *painter 时，它可以指向任何内存，因为它有垃圾。所以当您执行 painter->begin(this) 时，您正在访问未初始化的内存，这就是您收到该错误的原因。另一方面，在 QWidget 中，例如 QVideoWidget 应该只在方法 paintEvent 中绘制，策略是使用保存状态的变量您想要绘制的内容，例如 QRect，然后调用更新来绘制它。

myvideoobject.h

#ifndef MYVIDEOOBJECT_H
#define MYVIDEOOBJECT_H

#include <QVideoWidget>

class QRubberBand;

class MyVideoObject : public QVideoWidget
{
public:
    MyVideoObject(QWidget *parent = nullptr);
protected:
    void mouseMoveEvent(QMouseEvent *ev);
    void mousePressEvent(QMouseEvent *ev);
    void mouseReleaseEvent(QMouseEvent *ev);
    void paintEvent(QPaintEvent *ev);
private:
    QRubberBand *rubberBand;
    QPoint origin;
    QRect rect;
};

#endif // MYVIDEOOBJECT_H

myvideoobject.cpp

#include "myvideoobject.h"

#include <QMouseEvent>
#include <QPainter>
#include <QRubberBand>

MyVideoObject::MyVideoObject(QWidget *parent):
    QVideoWidget(parent),
    rubberBand(nullptr){}

void MyVideoObject::mousePressEvent(QMouseEvent *ev)
{
    origin = ev->pos();
    if(!rubberBand)
        rubberBand = new QRubberBand(QRubberBand::Rectangle, this);
    rubberBand->setGeometry(QRect(origin,QSize()));
    rubberBand->show();
    QVideoWidget::mousePressEvent(ev);
}

void MyVideoObject::mouseMoveEvent(QMouseEvent *ev)
{
    rubberBand->setGeometry(QRect(origin,ev->pos()).normalized());
    QVideoWidget::mouseMoveEvent(ev);
}

void MyVideoObject::mouseReleaseEvent(QMouseEvent *ev)
{
    rect = rubberBand->geometry();
    update();
    QVideoWidget::mouseReleaseEvent(ev);
}

void MyVideoObject::paintEvent(QPaintEvent *ev)
{
    QVideoWidget::paintEvent(ev);
    QPainter painter(this);
    painter.save();
    painter.setBrush(Qt::red);
    if(!rect.isNull())
        painter.drawRect(rect);
    painter.restore();
}