Display image in Qt to fit label size

Keep a copy of your original pixmap around. Then connect the resized signal to a slot (or override the resizeEvent() function) that implements this :

lblImage->setPixmap(pixmap.scaled(lblImage->size(), Qt::KeepAspectRatio, Qt::SmoothTransformation));

Does QLabel::setScaledContents(bool) help? There may also be some useful information in the image viewer example too.

I will also answer my own question, but won't mark it as solution, because I requested a simple one which was given above. I ended up using a not too simple solution after all, so anyone who also needs to do something similar and has the time to play with it here's my final working code. The idea is to expand the QLabel and overload the setPixmap and the drawEvent methods.

QPictureLabel.hpp (header file)

#include "QImage.h"
#include "QPixmap.h"
#include "QLabel.h"

class QPictureLabel : public QLabel
{
private:
    QPixmap _qpSource; //preserve the original, so multiple resize events won't break the quality
    QPixmap _qpCurrent;

    void _displayImage();

public:
    QPictureLabel(QWidget *aParent) : QLabel(aParent) { }
    void setPixmap(QPixmap aPicture);
    void paintEvent(QPaintEvent *aEvent);
};

QPictureLabel.cpp (implementation)

#include "QPainter.h"

#include "QPictureLabel.hpp"

void QPictureLabel::paintEvent(QPaintEvent *aEvent)
{
    QLabel::paintEvent(aEvent);
    _displayImage();
}

void QPictureLabel::setPixmap(QPixmap aPicture)
{
    _qpSource = _qpCurrent = aPicture;
    repaint();
}

void QPictureLabel::_displayImage()
{
    if (_qpSource.isNull()) //no image was set, don't draw anything
        return;

    float cw = width(), ch = height();
    float pw = _qpCurrent.width(), ph = _qpCurrent.height();

    if (pw > cw && ph > ch && pw/cw > ph/ch || //both width and high are bigger, ratio at high is bigger or
        pw > cw && ph <= ch || //only the width is bigger or
        pw < cw && ph < ch && cw/pw < ch/ph //both width and height is smaller, ratio at width is smaller
        )
        _qpCurrent = _qpSource.scaledToWidth(cw, Qt::TransformationMode::FastTransformation);
    else if (pw > cw && ph > ch && pw/cw <= ph/ch || //both width and high are bigger, ratio at width is bigger or
        ph > ch && pw <= cw || //only the height is bigger or
        pw < cw && ph < ch && cw/pw > ch/ph //both width and height is smaller, ratio at height is smaller
        )
        _qpCurrent = _qpSource.scaledToHeight(ch, Qt::TransformationMode::FastTransformation);

    int x = (cw - _qpCurrent.width())/2, y = (ch - _qpCurrent.height())/2;

    QPainter paint(this);
    paint.drawPixmap(x, y, _qpCurrent);
}

Usage : the same as using a normal label for displaying image wirthout the setScaledContents

img_Result = new QPictureLabel(ui.parent);
layout = new QVBoxLayout(ui.parent);
layout->setContentsMargins(11, 11, 11, 11);
ui.parent->setLayout(layout);
layout->addWidget(img_Result);

//{...}

QPixmap qpImage(qsImagePath);
img_Result->setPixmap(qpImage);

Actually there is a very simple solution for this problem. There are two things you should modify:

1. Set the scaled content to true (mentioned above)

2. Set the label's size policy to ignored

   QLabel lblImage;
   
   lblImage->setPixmap( QPixmap( "big_image.jpg" ) );
   
   lblImage->setScaledContents( true );
   
   lblImage->setSizePolicy( QSizePolicy::Ignored, QSizePolicy::Ignored );
   

If the lblImage is resizing automatically, the image will stretch to the size of the label.