How to draw QImage on QScrollArea? Did this, but have some minor problems QPainter::begin: Widget painting can only begin as a result of a paintEvent

Question

Ok, so here's what I want to accomplish: I want to draw QImage so the window would have scroll bars in case of the image turned out to be too big. For now one, I have sth like this:

#include "imagewidget.h"
#include <QImage>
#include <QPainter>
#include <QGridLayout>
#include <QLabel>

ImageWidget::ImageWidget(QWidget* parent)
    : QWidget(parent)
{
    m_image = QImage();

    scrollArea = new QScrollArea(this);

    QGridLayout *gridLayout = new QGridLayout(this);
    imgDisplayLabel = new QLabel(this);
    imgDisplayLabel->setPixmap(QPixmap::fromImage(m_image));
    imgDisplayLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
    imgDisplayLabel->setScaledContents(true);
    imgDisplayLabel->adjustSize();

    scrollArea->setWidget(imgDisplayLabel);

    gridLayout->addWidget(scrollArea,0,0);
    setLayout(gridLayout);

}

void ImageWidget::paintEvent(QPaintEvent* event)
{

    QPainter paint(this);
    if(!m_image.isNull())
        paint.drawImage(0,0, m_image);

    imgDisplayLabel->setPixmap(QPixmap::fromImage(m_image));
    imgDisplayLabel->adjustSize();
    imgDisplayLabel->setScaledContents(true);
}

void ImageWidget::setImage(QImage im)
{
    m_image = im;
    update();
}

void ImageWidget::removeImage()
{
    m_image = QImage();
    update();
}

However, it does not give me the effect that I want to have:

When I change QPainter paint(this); to QPainter paint(scrollArea); I have the error message (or, it's the warning I guess): QPainter::begin: Widget painting can only begin as a result of a paintEvent but I'm able to run the application, and open / close images. So, the program actually works with this, but it the error message bothers me and I would like to know how to get rid of it. With this only one changed line from the above src code app works and displays images as it should:

Answer 1

Question is, where do you want to paint: on ImageWidget , on imgDisplayLabel , or on scrollArea .

If I interpret correctly, the warning is basically saying that, if you want to begin a painter on a widget, you should do it in THE same widget's paint event.

In Qt 4.8 documentation

QPainter::QPainter(QPaintDevice * device)

Constructs a painter that begins painting the paint device immediately.

this means by calling the QPainter constructor with a target device, it begin s immediately.

So, try hijacking the scroll area's paint event.

FYI, whenever you hijack a event in Qt, I recommend calling the base class's implementation first in you new implementation like this so the base class's behavior is preserved.