Qt: resizing a QLabel containing a QPixmap while keeping its aspect ratio

Question

I use a QLabel to display the content of a bigger, dynamically changing QPixmap to the user. It would be nice to make this label smaller/larger depending on the space availa

Answer 1

I just use contentsMargin to fix the aspect ratio.

#pragma once

#include 

class AspectRatioLabel : public QLabel
{
public:
    explicit AspectRatioLabel(QWidget* parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags());
    ~AspectRatioLabel();

public slots:
    void setPixmap(const QPixmap& pm);

protected:
    void resizeEvent(QResizeEvent* event) override;

private:
    void updateMargins();

    int pixmapWidth = 0;
    int pixmapHeight = 0;
};

#include "AspectRatioLabel.h"

AspectRatioLabel::AspectRatioLabel(QWidget* parent, Qt::WindowFlags f) : QLabel(parent, f)
{
}

AspectRatioLabel::~AspectRatioLabel()
{
}

void AspectRatioLabel::setPixmap(const QPixmap& pm)
{
    pixmapWidth = pm.width();
    pixmapHeight = pm.height();

    updateMargins();
    QLabel::setPixmap(pm);
}

void AspectRatioLabel::resizeEvent(QResizeEvent* event)
{
    updateMargins();
    QLabel::resizeEvent(event);
}

void AspectRatioLabel::updateMargins()
{
    if (pixmapWidth <= 0 || pixmapHeight <= 0)
        return;

    int w = this->width();
    int h = this->height();

    if (w <= 0 || h <= 0)
        return;

    if (w * pixmapHeight > h * pixmapWidth)
    {
        int m = (w - (pixmapWidth * h / pixmapHeight)) / 2;
        setContentsMargins(m, 0, m, 0);
    }
    else
    {
        int m = (h - (pixmapHeight * w / pixmapWidth)) / 2;
        setContentsMargins(0, m, 0, m);
    }
}

Works perfectly for me so far. You're welcome.