How to draw rectangle on custom video widget t in QT? - c++

Question

I want to select an area on a custom video widget and draw rectangle on selected area.
So far I can select an area with QRubberband but I am having trouble with drawing the rectangle after releasing left click.
Whenever I click-drag then release to draw rectangle it gives this error:
QBackingStore::endPaint() called with active painter on backingstore paint device
The program has unexpectedly finished.
Here is my code:
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);
}
I didn't add mainwindow.cpp and mainwindow.h cuz there isn't much code in those other than selecting video with openfiledialog.

Answer 1

When you create a pointer: QPainter *painter, this can point to any memory since it has garbage. so when you do painter->begin(this) you are accessing uninitialized memory, that's why you get that error. On the other hand in a QWidget such as QVideoWidget should only be painted in the method paintEvent, the strategy is to have variables that save the state of what you want to paint, for example the QRect, and call update to paint it.
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();
}