Qt - QLabel won't resize with fixed ratio

Question

Introduction

I am working with the Designer inside the Qt Creator and have a QMainWindow with a QLabel in it. Because the program loads pictures and displays them inside the label i want that the label resizes with a ratio of 1.25 inside setted boundaries when expanding or shrinking the QMainWindow. The label should resize INDEPENDENT, again INDEPENDENT from its content.

What i want:

• Open the main window for first time:
  width: 640, height: 512
• Shrinking the main window:
  label shrinks with constant ratio (640/512) till minimum size (320 x 256)
• Expanding the main window:
  label expands with constant ratio (640/512) till maximum size (1280 x 1024)

1. Approach:

Therefor i...

• added a QLabel(called imageLabel) inside the centralWidget of the QMainWindow
• set the centralWidget's layout to grid layout (QGridLayout)
• sed the following properties to the QLabel:
  • geometry - Can not set the values because of grid layout usages!
  • minimumSize > width: 320, height: 256 (Minimum values)
  • maximumSize > width: 1280, height: 1024 (Maximum values)
  • sizePolicy > Horizontal Policy == Vertical Policy == Expanding

Element structure:

This doesn't work because i can not set an initial size in the 'geometry' section. The label does not scale with fixed ratio although it respects minimum and maximum values.

2. Approach:

Following that answer i set an initial pixmap:

QPixmap p;
ui->imageLabel->setPixmap(p.scaled(640, 512, Qt::KeepAspectRatio));

Which didn't change anything.

3. Approach:

I also applied the other answer's class and promoted it to the widget:

That didn't change anything too.

4. Approach:

I then combined the 2. and 3. approach and set an initial pixmap which...

...didn't change anything.

Here is what it does for the approaches 1. - 4.:

5. Approach

Adding the label of 4. approach into a widget:

Well the label doesn't resize at all:

So, how can get the label to have an initial size of 640 x 512 and scale with fixed ratio between 1280 x 1024 and 320 x 256 ?

Answer 1

A possible solution is to install an eventFilter to the centralwidget so doing the required calculation is set the size.

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->centralWidget->installEventFilter(this);
    ui->imageLabel->setPixmap(QPixmap(":/image.png"));
    ui->imageLabel->setScaledContents(true);
}

bool MainWindow::eventFilter(QObject *watched, QEvent *event)
{
    const float ratio = 1.25;
    if(watched == ui->centralWidget && event->type() == QEvent::Resize
            && ui->centralWidget->width() > 0
            && ui->centralWidget->height() > 0){
        float central_ratio = 1.0*ui->centralWidget->width()/ui->centralWidget->height();        QSize s;
        if(central_ratio > ratio){
            s = QSize(ratio*ui->centralWidget->height(), ui->centralWidget->height());
        }
        else{
            s = QSize(ui->centralWidget->width(), ui->centralWidget->width()/ratio);
        }
        ui->imageLabel->resize(s);
    }
    return QMainWindow::eventFilter(watched, event);
}