The right way to show a image in Qt

Question

I am new to Qt and almost every tutorial I find says to add the image to a QLabel using setPixmap(). Maybe this is the right way, but it doesn't feels to be, since using a label for a image seems to go beyond the purpose of a label.

Could anyone tell me if there is a "right way" or a special Class for this, or if the "Label way" is the right one, not just the simple one.

Answer 1

easiest way is to use a QLabel.in ImageViewer example http://qt-project.org/doc/qt-4.8/widgets-imageviewer.html they use QLabel ..

another way

QGraphicsView view(&scene);
    QGraphicsPixmapItem item(QPixmap("c:\\test.png"));
    scene.addItem(&item);

Answer 2

Using QLabel is the usual way to display an image in a QtWidgets-based UI. That might indeed feel a bit awkward, as QLabel's API is mostly concerned with text rendering. However, it does the job and there's no other class dedicated to only painting images. One could think of writing his own class (taking a QPixmap, reimplementing paintEvent(), sizeHint()), but that would only make sense to me if functionality is needed that QLabel lacks.

There are of course many other ways to paint an image, depending on the context, like images on buttons (QToolButton, QPushButton, ...), images in graphics scenes (QGraphicsScene/View) etc., but they all serve either more specialized or more complex use cases.

Answer 3

Here is a simple class that isn't Label based. I suppose it depends on what you personally feel is the proper way to do it and what you need to do with it. I prefer implementing my own class so you can add on to it later(maybe you want to manipulate the image).

imagewidget.h

#ifndef IMAGEWIDGET_H
#define IMAGEWIDGET_H

#include <QPainter>
#include <QImage>
#include <QWidget>

QT_BEGIN_NAMESPACE
class QPainter;
class QImage;
QT_END_NAMESPACE

class ImageWidget : public QWidget
{
    Q_OBJECT

public:
    ImageWidget(const QString &filename, QWidget* parent = 0);
    ~ImageWidget();

    bool load(const QString &fileName);
    bool save(const QString &fileName);

protected:
    void paintEvent(QPaintEvent* event);

private:
    QImage img;
};

#endif

imagewidget.cpp

#include <QDebug>
#include "imagewidget.h"

ImageWidget::ImageWidget(const QString &filename, QWidget* parent) : QWidget(parent)
{
    img.load(filename);

    setMinimumWidth(img.width());
    setMinimumHeight(img.height());
    setMaximumWidth(img.width());
    setMaximumHeight(img.height());

    this->show();
}

bool ImageWidget::load(const QString &fileName)
{
    img.load(fileName);
    return true;
}

bool ImageWidget::save(const QString &fileName)
{
    img.save(fileName, "PNG");
    return true;
}

ImageWidget::~ImageWidget()
{
}

void ImageWidget::paintEvent(QPaintEvent*)
{
    QPainter painter(this);
    painter.setViewport(0, 0, width(), height());
    painter.setWindow(0, 0, width(), height());
    painter.drawImage(0, 0, img);
}