Display image in Qt to fit label size

Question

I already tried several methods on displaying an image on a form, but none of them works how I would like.

I've read many places that the easiest way is to create a label and use that to display the image. I have a label, which size is specified by the layout, but if I load an image into it with a pixmap, the label is resized to the size of the image. If I use img tag as text or css background property, it won't display the whole image. What I would like to do is to load the image and fit into the label, not changing the label's size, but when I resize my window, and by that resizing the label as well, the image should be resized too so it will always fit into it.

If the only method is to get the label's size, and resize the pixmap so it would fit, and handle the resize event (signal), how could I resize the pixmap? I hope I won't need to save the whole thing into a QImage and create a pixmap from it each time.

Also, how can I center it? If it can't fit both the width and the height, I would like the smaller dimension to be centered.

Oh, and I don't want to use sliders to handle overflows.

Answer 1

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

Answer 2

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.

Answer 3

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);

Answer 4

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));