Problem with drawing on top of a widget in Qt

Question

Qt

I did not have, in general, development experience on Qt, therefore, in order not to accidentally spoil anything, I started with an example project from the Qt website. Here is the Image Viewer project itself .

The technical essence of the project: the ImageViewer class inherits QMainWindow, sets its “central widget” to the QScrollArea scrolling area, in which the QLabel with the picture is located.

The source code is on the same page in the "Files" section at the top.

The problem is this: I wanted to do some drawing on top of the displayed image, but for the sake of example, let's reduce the task to just drawing a line.

Armed with Google, I came up with this option: in the imageviewer.h

file , we connect the library for drawing:

#include <QPainter>

and add a render event handler to the class:

protected:
    void paintEvent(QPaintEvent *);

Let's describe this handler in imageviewer.cpp :

void ImageViewer::paintEvent(QPaintEvent * e)
{
    QPainter p(this);

    p.drawLine(0,0,100,100);
} 

It was expected that a line would appear in the upper left corner, but ... no.

It seems to me that the problem is that the scrollArea overlaps the line, so for the experiment I commented out the line

setCentralWidget(scrollArea);

in the ImageViewer::ImageViewer() method. The line appeared, but the picture, of course, ceased to be displayed.

I tried to draw on scrollArea , also unsuccessfully (there is a picture, there is no line):

void ImageViewer::paintEvent(QPaintEvent * e)
{
    QPainter p(scrollArea);

    p.drawLine(0,0,100,100);
}

This code issued messages like this to the QCreator console

QPainter::begin: Paint device returned engine == 0, type: 1

Googling led to the idea of ​​drawing on scrollArea->viewport(), didn't help.

Attention, the question is: how to do the same, but correctly?)

Answer 1

Derivate from QLabel, override its paintEvent and in it first call QLabel::paintEvent(event); then add your own painting.

Answer 2

Sobsna, code:

#include <QMainWindow>
#include <QScrollArea>
#include <QLabel>
#include <QApplication>
#include <QPainter>

class MyLabel : public QLabel {
protected:
  virtual void paintEvent(QPaintEvent* e) {
    QLabel::paintEvent(e);
    
    QPainter p(this);
    
    p.setPen(Qt::green);
    p.drawLine(0, 0, 100, 100);
  }
};

class ImageView : public QMainWindow {
public:
  
  ImageView() : QMainWindow() {
    QScrollArea* scr = new QScrollArea();
    setCentralWidget( scr );
    
    QLabel* label = new MyLabel();
    label->setPixmap(QPixmap("./moon_from_andrey.jpg"));
    scr->setWidget(label);
  }
};


int main(int argc, char* argv[]) {
  QApplication app(argc, argv);
  
  ImageView view;
  view.setGeometry(100, 100, 500, 400);
  view.show();
  
  return app.exec();
}

Result:

Answer 3

I somehow drew on the button like this
void PaintButtonUp::paintEvent(QPaintEvent *event) { QPushButton::paintEvent(event); QPainter paint(this); static const QPointF points[3] = { QPointF(3.0, 25.0), QPointF(10.0, 15.0), QPointF(17.0, 25.0), }; paint.setPen(QPen(Qt::darkGray, 2, Qt::SolidLine, Qt::RoundCap)); paint.setRenderHint(QPainter::Antialiasing, true); paint.setBrush(QBrush(Qt::gray, Qt::SolidPattern)); paint.drawPolygon(points, 3); }
: first, the standard method was called,
QPushButton::paintEvent(event);
and then I drew what I needed on the button.
I don't know if this will help you or not.