Keeping the aspect ratio of a sub-classed QWidget during resize

Question

I'm creating a new widget, by subclassing the QWidget class. I'd like to be able to set a ratio (for its height and its width) for this widget, which will always be maintained.

For this, I've always searched, using the Qt5 documentation, Google, and Stackoverflow. Obviously, I've found answers: in particular, this one. But, unfortunately, not even one is fully effective:

Setting the sizeIncrement does totally nothing, even if the widget is a window
I tried to overload resizeEvent, but I really don't know how to do this...
If I follow this answer, two things:
1. If the widget is a top-level window, the ratio isn't maintained at all, I can resize it as I want.
2. If I place this widget in a layout, if I just increase both width and height of the window, the ratio is maintained. But as soon as I increase the width or the height to much, the widget is flattened. Instead, I would like that the layout automatically adjust its size to keep the widget's ratio.

So, how could I manage to keep the aspect ratio of a subclassed QWidget?

Here is another post I worked on that works well, at least for `QLabel`s. http://stackoverflow.com/questions/8211982/qt-resizing-a-qlabel-containing-a-qpixmap-while-keeping-its-aspect-ratio — phyatt, May 02 '15 at 18:17
Have you tried reimplementing `QWidget::heightForWidth()`? That allows you to specify an aspect ratio. If that's not enough, create a parent "dummy" QWidget to hold your widget in a 3x3 QGridLayout, with your widget in the middle and QSpacerItem's around the edges (see `QGridLayout::addItem()`). The QSpacerItem's can provide the extra spacing (or be sized down to 0) as needed when resizing. You'd probably need to use `QGridLayout::setColumnStretch()` and `QGridLayout::setRowStretch()`. — Anthony, May 02 '15 at 19:47
Thanks both of you, your solutions work, but the same issue remains: if I try to increase solely the width of the widget, then the aspect ratio isn't maintained anymore. — Spirine, May 02 '15 at 21:03

score 17 · Accepted Answer · edited Jul 20 '17 at 23:24

Create a parent widget (e.g., AspectRatioWidget) in which to place your widget. For the parent widget, subclass QWidget and give it a QBoxLayout. Put your widget into the center, and QSpacerItems on either side. Then in the parent widget's QWidget::resizeEvent adjust the direction and stretches as needed. I've provided an example below. To use, just create an instance of AspectRatioWidget and pass the constructor a pointer to your widget and the desired aspect ratio.

// header
class AspectRatioWidget : public QWidget
{
public:
    AspectRatioWidget(QWidget *widget, float width, float height, QWidget *parent = 0);
    void resizeEvent(QResizeEvent *event);

private:
    QBoxLayout *layout;
    float arWidth; // aspect ratio width
    float arHeight; // aspect ratio height
};

// cpp
AspectRatioWidget::AspectRatioWidget(QWidget *widget, float width, float height, QWidget *parent) :
    QWidget(parent), arWidth(width), arHeight(height)
{
    layout = new QBoxLayout(QBoxLayout::LeftToRight, this);

    // add spacer, then your widget, then spacer
    layout->addItem(new QSpacerItem(0, 0));
    layout->addWidget(widget);
    layout->addItem(new QSpacerItem(0, 0));
}

void AspectRatioWidget::resizeEvent(QResizeEvent *event)
{
    float thisAspectRatio = (float)event->size().width() / event->size().height();
    int widgetStretch, outerStretch;

    if (thisAspectRatio > (arWidth/arHeight)) // too wide
    {
        layout->setDirection(QBoxLayout::LeftToRight);
        widgetStretch = height() * (arWidth/arHeight); // i.e., my width
        outerStretch = (width() - widgetStretch) / 2 + 0.5;
    }
    else // too tall
    {
        layout->setDirection(QBoxLayout::TopToBottom);
        widgetStretch = width() * (arHeight/arWidth); // i.e., my height
        outerStretch = (height() - widgetStretch) / 2 + 0.5;
    }

    layout->setStretch(0, outerStretch);
    layout->setStretch(1, widgetStretch);
    layout->setStretch(2, outerStretch);
}

Thanks! This solution is the only one that worked for me, subclassing QOpenGLWidget. I added `layout->setContentsMargins(0, 0, 0, 0);` in the constructor to remove the margins. — ForeverLearning, Sep 20 '18 at 17:46
@Anothoy Random question, but is there a way of using two of these AspectRatioWidgets in a `QHBoxLayout` with stretches on either side and still have them fill the space? When I use `addStretch` on either side of the aspect ratio widgets, it forces them to their minimum size and they doesnt enlarge/shrink anymore. — 956MB, Nov 27 '21 at 14:41

score 9 · Answer 2 · answered Apr 16 '18 at 07:24

I have rewritten Anthony's code in Python/PySide2:

from PySide2.QtWidgets import QBoxLayout, QSpacerItem, QWidget


class AspectRatioWidget(QWidget):
    def __init__(self, widget, parent):
        super().__init__(parent)
        self.aspect_ratio = widget.size().width() / widget.size().height()
        self.setLayout(QBoxLayout(QBoxLayout.LeftToRight, self))
        #  add spacer, then widget, then spacer
        self.layout().addItem(QSpacerItem(0, 0))
        self.layout().addWidget(widget)
        self.layout().addItem(QSpacerItem(0, 0))

    def resizeEvent(self, e):
        w = e.size().width()
        h = e.size().height()

        if w / h > self.aspect_ratio:  # too wide
            self.layout().setDirection(QBoxLayout.LeftToRight)
            widget_stretch = h * self.aspect_ratio
            outer_stretch = (w - widget_stretch) / 2 + 0.5
        else:  # too tall
            self.layout().setDirection(QBoxLayout.TopToBottom)
            widget_stretch = w / self.aspect_ratio
            outer_stretch = (h - widget_stretch) / 2 + 0.5

        self.layout().setStretch(0, outer_stretch)
        self.layout().setStretch(1, widget_stretch)
        self.layout().setStretch(2, outer_stretch)

Keeping the aspect ratio of a sub-classed QWidget during resize

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